US20110159512A1

US20110159512A1 - Polynucleotide sequences of candida dubliniensis and probes for detection

Info

Publication number: US20110159512A1
Application number: US13/061,937
Authority: US
Inventors: Kaustuv Sanyal; Sreedevi Padmanabhan; Jitendra Thakur
Original assignee: Jawaharial Nehru Centre for Advanced Scientific Research
Current assignee: Jawaharial Nehru Centre for Advanced Scientific Research
Priority date: 2008-09-25
Filing date: 2008-11-07
Publication date: 2011-06-30
Also published as: WO2010035276A1; US20140295443A1; US9334535B2

Abstract

The present invention relates to identification of centromeric sequences of Candida dubliniensis and localization of CdCse4p centromeric histone to the identified region. Also the present invention relates to distinguishing Candida dubliniensis from other members of genus Candida.

Description

FIELD OF THE INVENTION

BACKGROUND AND PRIOR ART OF THE INVENTION

Candida is a genus of yeasts. Many species of this genus are endosymbionts of animal hosts including humans. While usually living as commensals, some Candida species have the potential to cause disease. Clinically, the most significant member of the genus is Candida albicans, which can cause infections (called candidiasis or thrush) in humans and other animals, especially in immunocompromised patients. Many Candida species are members of gut flora in animals, including C. albicans in mammalian hosts, whereas others live as endosymbionts in insect hosts.
Among the other important members of this genus Candida dubliniensis is a significant pathogenic fungi. Candida dubliniensis is an organism often associated with AIDS patients but can be associated with immunocompetent patients as well. It is a germ cell-positive yeast of the genus Candida, similar to Candida albicans but it forms a different cluster upon DNA fingerprinting. It appears to be particularly adapted for the mouth but can be found at very low rates in other anatomical sites. Candida dubliniensis is found all around the world. The species was only described in 1995. It is thought to have been previously identified as Candida albicans. Retrospective studies support this, and have given an indication of the prevalence of C. dubliniensis as a pathogen.
This isolate is germ tube positive which accounts for its historic miss-identification as C. albicans. The most useful test for distinguishing C. dubliniensis from C. albicans is to culture at 42° C. Most C. albicans grows well at this temperature, but most C. dubliniensis do not. There are also significant differences in the chlamydiospores between C. albicans and C. dubliniensis although they are otherwise phenotypically very similar.
A study done in Europe of 2,589 isolates that were originally reported as C. albicans revealed that 52 of them (2.0%) were actually C. dubliniensis. Most of these isolates were from oral or faecal specimens from HIV positive patients, though one vaginal and two oral isolates were from healthy volunteers. Another study done in the United States, used 1,251 yeasts previously identified as C. albicans, it found 15 (1.2%) were really C. dubliniensis. Most of these samples were from immunocompromised individuals: AIDS, chemotherapy, or organ transplant patients. The yeast was most often recovered from respiratory, urine and stool specimens. The Memorial Sloan-Kettering Cancer Center also did several studies, both retrospective, and current. In all 974 germ-tube positive yeasts, 22 isolates (2.3%) from 16 patients were C. dubliniensis.
Molecular analysis show that C. dubliniensis is distinct from C. albicans by 13-15 nucleotides in the ribosomal RNA gene sequences. Early reports purported that C. dubliniensis was responsible for, fluconazole-resistant thrush but susceptibility studies reveal that its categorical distribution is similar to C. albicans with isolates ranging from susceptible to resistant.
Previous literature describes that Centromeric DNA sequences in the pathogenic yeast Candida albicans are all different and unique (Sanyal et al, 2004). The Cse4p-containing centromere regions of Candida albicans have unique and different DNA sequences on each of the eight chromosomes. However similar studies have not been carried out in C. dubliniensis.
Amongst the most prevalent methods of distinguishing C. dubliniensis from C. albicans are the compositions and methods for the detection and identification of species of Candida, in particular, to nucleic acid probes that specifically hybridize to the internal transcribed spacer 2 (ITS2) of the ribosomal DNA (rDNA) repeat region of Candida species (such as C. albicans and C. dubliniensis).
Another method of identification includes use of multiplex PCR which uses essentially three factors: (i) the elevated number of copies from the rRNA genes (about 100 copies per genome), (ii) the differences regarding the sizes of the ITS regions and (iii) the elevated variability of these region sequences among the different species of Candida. Thus, this technique is based on the amplification of DNA fragments specific of the internal transcribed spacer regions 1 (ITS-I) and 2 (ITS-2) by multiplex PCR. The methodology uses the combination of two universal primers and seven specific primers for each one of the Candida species studied, in a single PCR reaction, originating two fragments of different sizes for each species (European publication no: EP1888745).
Most techniques used so far distinguish C. dubliniensis from other species by identification of rDNA or RNA sequences of the genome.
The genome of C. dubliniensis has not been sequenced completely and the work to find out more information about its genome is in progress.
However the present invention has been able to assign centromeric functions to the sequence identified and these centromeric sequences are further used to distinguish Candida dubliniensis from other members of the genus based on the localization of histone proteins CdCse4p.
Faithful chromosome segregation during mitosis and meiosis in eukaryotes is performed by a dynamic interaction between spindle microtubules and kinetochores. The kinetochore is a proteinaceous structure that forms on a specific DNA locus on each chromosome, termed as the centromere (CEN). Centromeres have been cloned and characterized in several organisms from yeasts to humans. Interestingly, there is no centromere-specific cis-acting DNA sequence that is conserved across species (1). However, centromeres in all eukaryotes studied to date assemble into specialized chromatin containing a histone H3 variant protein in the CENP-A/Cse4p family. Members of this family are called centromeric histones (CenH3s) and are regarded as possible epigenetic markers of CEN identity (1, 2). The Saccharomyces cerevisiae centromere, the most intensively studied budding yeast centromere, is a well defined, short 125 bp) region (hence called a “point” centromere), and consists of two conserved consensus sequences (Centromere DNA Elements; CDEs), CDEI (8 bp) and CDEIII (25 bp) separated by CDEII, a 78-86 by non-conserved AT-rich (>90%) “spacer”-sequence (3). CDEI is not absolutely necessary for mitotic centromere function (4). Retention of a portion of CDEII is essential for CEN activity, but changes in length or base composition of CDEII cause only partial inactivation (4, 5). The S. cerevisiae CenH3, ScCse4p, has been shown to bind to a single nucleosome containing the non-conserved CDEII and to flanking CDEI and CDEIII regions (6). CDEIII is absolutely essential: centromere function is completely inactivated by deletion of CDEIII, or even by single base substitutions in the central CCG sequence. Centromeres of most other eukaryotes, including the fission yeast Schizosaccharomyces pombe, are much longer and more complex than those of S. cerevisiae and are called “regional” centromeres (3). The centromeres of S. pombe are 40-110 kb in length, and organized into distinct classes of repeats which are further arranged into a large inverted repeat. The non-repetitive central region, also known as the central core (cc), contains a 4-7 kb non-homologous region that is not conserved in all three chromosomes (3). The CenH3 homolog in S. pombe, Cnp1p, binds to the central core and the inner repeats (7). However, the central domain alone cannot assemble centromere chromatin de novo, but requires the cis-acting dg/K repeat present at the outer repeat array to promote de novo centromere assembly (8, 9). Several experiments suggest that unlike in S. cerevisiae, no unique conserved sequence within S. pombe centromeres is sufficient for establishment and maintenance of centromere function, although flanking repeats play a crucial role in establishing heterochromatin that is important for centromere activity (10). Studies in a pathogenic budding yeast, Candida albicans, containing regional centromeres suggest that each of its eight chromosomes contains a different, 3-5 kb, non-conserved DNA sequence that assembles into Cse4p-rich centromeric chromatin (11, 12). C. albicans centromeres partly resemble those of S. pombe but lack any pericentric repeat that is common to all of its eight centromeres (12). Therefore, the mechanisms by which CenH3s confer centromere identity, are deposited at the right location, and are epigenetically propagated for several generations in C. albicans without any centromere-specific DNA sequence remain largely unknown.

OBJECTIVES OF THE INVENTION

The main objective of the present invention is to obtain a polynucleotide sequence. Another main objective of the present invention is to obtain sets of primers for amplification of the polynucleotide sequences of Candida dubliniensis.
Yet another main objective of the present invention is to obtain a process for identification of centromeric sequences of Candida dubliniensis
Still another main objective of the present invention is to obtain a method of distinguishing Candida dubliniensis from Candida albicans.
Still another main objective of the present invention is to obtain a kit for identification of Candida dubliniensis.

STATEMENT OF THE INVENTION

Accordingly, the present invention relates to a polynucleotide sequence having SEQ ID NO 1, 2, 3, 4, 5, 6, 7 or 8; a set of 20 primers having SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27 as forward primers and SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26 and 28 as corresponding reverse primers respectively; a set of 14 primers having SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41 as forward primers and SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42 as corresponding reverse primers respectively; a set of 10 primers having SEQ ID NOS. 43, 45, 47, 49 and 51 as forward primers and SEQ ID NOS. 44, 46, 48, 50 and 52 as corresponding reverse primers respectively; a set of 16 primers having SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67 as forward primers and SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68 as corresponding reverse primers respectively; a set of 10 primers having SEQ ID NOS. 69, 71, 73, 75 and 77 as forward primers and SEQ ID NOS. 70, 72, 74, 76 and 78 as corresponding reverse primers respectively; a set of 16 primers haying SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93 as forward primers and SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94 as corresponding reverse primers respectively; a set of 18 primers having SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111 as forward primers and SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112 as corresponding reverse primers respectively; a set of 14 primers having SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126 as forward primers and SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125 as corresponding reverse primers respectively; a process of identification of centromeric sequences of Candida dubliniensis, said method comprising steps of a) identifying putative Cse4p binding region and b) amplifying the putative Cse4p binding region to identify centromeric sequences of the Candida dubliniensis; a method of distinguishing Candida dubliniensis from Candida albicans in a sample, said method comprising steps of a) isolating DNA from the organism in the sample and b) amplifying the Cse4p binding regions with primers capable of amplifying said regions in the Candida dubliniensis to distinguish it from Candida albicans and a kit for identification of Candida dubliniensis comprising set of primers having SEQ ID NOS. 9 to 126.

BRIEF DESCRIPTION OF ACCOMPANYING SEQUENCE LISTINGS

SEQ ID NOS. 1, 2, 3, 4, 5, 6, 7 and 8: Centromeric polynucleotide sequences for Chromosome 1, 2, 3, 4, 5, 6, 7 and 8 of Candida dubliniensis.
SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27: Forward Primers for Chromosome 1 of Candida dubliniensis.
SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26 and 28: Reverse Primers for Chromosome 1 of Candida dubliniensis.
SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41: Forward Primers for Chromosome 2 of Candida dubliniensis.
SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42: Reverse Primers for Chromosome 2 of Candida dubliniensis.
SEQ ID NOS. 43, 45, 47, 49 and 51: Forward Primers for Chromosome 3 of Candida dubliniensis.
SEQ ID NOS. 44, 46, 48, 50 and 52: Reverse Primers for Chromosome 3 of Candida dubliniensis.
SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67: Forward Primers for Chromosome 4 of Candida dubliniensis.
SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68: Reverse Primers for Chromosome 4 of Candida dubliniensis.
SEQ ID NOS. 69, 71, 73, 75 and 77: Forward Primers for Chromosome 5 of Candida dubliniensis.
SEQ ID NOS. 70, 72, 74, 76 and 78: Reverse Primers for Chromosome 5 of Candida dubliniensis.
SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93: Forward Primers for Chromosome 6 of Candida dubliniensis.
SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94: Reverse Primers for Chromosome 6 of Candida dubliniensis.
SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111: Forward Primers for Chromosome 7 of Candida dubliniensis.
SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112: Reverse Primers for Chromosome 7 of Candida dubliniensis.
SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126: Forward Primers for Chromosome 8 of Candida dubliniensis.
SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125: Reverse Primers for Chromosome 8 of Candida dubliniensis.

BRIEF DESCRIPTION OF ACCOMPANYING FIGURES

FIG. 1: Orthologous Cse4p-rich centromere regions in C. albicans and C. dubliniensis.

FIG. 2: Localization of CdCse4p at the kinetochore of C. dubliniensis.

FIG. 3: Binding of two evolutionarily conserved key kinetochore proteins, CdCse4p (CENP-A homolog) and CdMif2p (CENP-C homolog) to the same regions of different C. dubliniensis chromosomes.

FIG. 4: Comparative analysis of CEN6 region of C. albicans and its orthologous region in C. dubliniensis showing genome rearrangement.

FIG. 5: The centromeric histone in C. dubliniensis, CdCse4p, belongs to the Cse4p/CENP-A family.

FIG. 6: Relative enrichment profiles of CdCse4p in various C. dubliniensis chromosomes.

FIG. 7: The CENP-C homolog in C. dubliniensis (CdMif2p) is co-localized with CdCse4p.

FIG. 8: Relative chromosomal positions of Cse4p-binding regions in C. albicans and C. dubliniensis.

FIG. 9: Conserved blocks in the pericentric regions of various chromosomes of C. dubliniensis and C. albicans.

BRIEF DESCRIPTION OF ACCOMPANYING TABLES

Table 1: Comparison of the amino acid sequence homology of the ORFs flanking the CEN regions in C. albicans and C. dubliniensis
Table 2: List of PCR Primers used for ChIP assays.
Table 2B: List of PCR primers used for Cse4 complementation experiments
Table 3: Sequence coordinates of the Cse4p- binding and the pericentric regions in all the chromosomes of C. albicans and C. dubliniensis
Table 4: List of strains
Table 5: Comparison of mutation rates in Cse4p-binding and other genomic noncoding regions in C. albicans and C. dubliniensis.
Table 6: Homology between the repeats in the pericentric region of C. albicans and C. dubliniensis

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a polynucleotide sequence having SEQ ID NO 1, 2, 3, 4, 5, 6, 7 or 8.
The present invention also relates to a set of 20 primers having SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27 as forward primers and SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26 and 28 as corresponding reverse primers respectively.
In another embodiment of the present invention, the forward and the reverse primers are used for amplification of centromeric region of chromosome 1 of Candida dubliniensis.
The present invention also relates to a set of 14 primers having SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41 as forward primers and SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42 as corresponding reverse primers respectively.
In another embodiment of the present invention, the forward and the reverse primers are used for amplification of centromeric region of chromosome 2 of Candida dubliniensis.
The present invention also relates to a set of 10 primers having. SEQ ID NOS. 43, 45, 47, 49 and 51 as forward primers and SEQ ID NOS. 44, 46, 48, 50 and 52 as corresponding reverse primers respectively.
In another embodiment of the present invention, the forward and the reverse primers are used for amplification of centromeric regions of chromosome 3 of Candida dubliniensis.
The present invention also relates to a set of 16 primers having SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67 as forward primers and SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68 as corresponding reverse primers respectively.
In another embodiment of the present invention, the forward and the reverse primers are used for amplification of centromeric regions of chromosome 4 of Candida dubliniensis.
The present invention also relates to a set of 10 primers having SEQ ID NOS. 69, 71, 73, 75 and 77 as forward primers and SEQ ID NOS. 70, 72, 74, 76 and 78 as corresponding reverse primers respectively.
In another embodiment of the present invention, the forward and the reverse primers are used for amplification of centromeric regions of chromosome 5 of Candida dubliniensis.
The present invention also relates to a set of 16 primers having SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93 as forward primers and SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94 as corresponding reverse primers respectively.
In another embodiment of the present invention, the forward and the reverse primers are used for amplification of centromeric regions of chromosome 6 of Candida dubliniensis.
The present invention also relates to a set of 18 primers having SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111 as forward primers and SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112 as corresponding reverse primers respectively.
In another embodiment of the present invention, the forward and the reverse primers are used for amplification of centromeric regions of chromosome 7 of Candida dubliniensis.
The present invention also relates to a set of 14 primers having SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126 as forward primers and SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125 as corresponding reverse primers respectively.
In another embodiment of the present invention, the forward and the reverse primers are used for amplification of centromeric regions of chromosome 8 of Candida dubliniensis.
The present invention also relates to a process of identification of centromeric sequences of Candida dubliniensis, said method comprising steps of:

- a) identifying putative Cse4p binding region; and
- b) amplifying the putative Cse4p binding region to identify centromeric sequences of the Candida dubliniensis.
  In another embodiment of the present invention, the identification of putative Cse4p biding regions is carried out by sequence analysis and chromatin immunoprecipitation.
  In yet another embodiment of the present invention the amplification of the putative Cse4p binding regions is carried out using any set of forward primer and its corresponding reverse primer selected from a group comprising SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27 and SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26, 28 respectively, for chromosome 1 of Candida dubliniensis; SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41 and SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42 respectively, for chromosome 2 of Candida dubliniensis; SEQ ID NOS. 43, 45, 47, 49 and 51 and SEQ ID NOS. 44, 46, 48, 50 and 52 respectively, for chromosome 3 of Candida dubliniensis; SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67 and SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68 respectively, for chromosome 4 of Candida dubliniensis; SEQ ID NOS. 69, 71, 73, 75 and 77 and SEQ ID NOS. 70, 72, 74, 76 and 78 respectively, for chromosome 5 of Candida dubliniensis; SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93 and SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94 respectively, for chromosome 6 of Candida dubliniensis; SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111 and SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112 respectively, for chromosome 7 of Candida dubliniensis and SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126 and SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125 respectively, for chromosome 8 of Candida dubliniensis or any combination of said primers thereof.
  The present invention also relates to a method of distinguishing Candida dubliniensis from Candida albicans in a sample, said method comprising steps of
- a) isolating DNA from the organism in the sample; and
- b) amplifying the Cse4p binding regions with primers capable of amplifying said regions in the Candida dubliniensis to distinguish it from Candida albicans.
  In another embodiment of the present invention, the identification of putative Cse4p biding regions is carried out by sequence analysis and chromatin immunoprecipitation.
  In yet another embodiment of the present invention, the amplification of the putative Cse4p binding regions is carried out using any set of forward primer and its corresponding reverse primer selected from a group comprising SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27 and SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26, 28 respectively, for chromosome 1 of Candida dubliniensis; SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41 and SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42 respectively, for chromosome 2 of Candida dubliniensis; SEQ ID NOS. 43, 45, 47, 49 and 51 and SEQ ID NOS. 44, 46, 48, 50 and 52 respectively, for chromosome 3 of Candida dubliniensis; SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67 and SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68 respectively, for chromosome 4 of Candida dubliniensis; SEQ ID NOS. 69, 71, 73, 75 and 77 and SEQ ID NOS. 70, 72, 74, 76 and 78 respectively, for chromosome 5 of Candida dubliniensis; SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93 and SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94 respectively, for chromosome 6 of Candida dubliniensis; SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111 and SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112 respectively, for chromosome 7 of Candida dubliniensis and SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126 and SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125 respectively, for chromosome 8 of Candida dubliniensis or any combination of said primers thereof.
  The present invention also relates to a kit for identification of Candida dubliniensis comprising set of primers having SEQ ID NOS. 9 to 126.
  In another embodiment of the present invention, the amplification of the putative Cse4p binding regions is carried out using any set of forward primer and its corresponding reverse primer selected from a group comprising SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27 and SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26, 28 respectively, for chromosome 1 of Candida dubliniensis; SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41 and SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42 respectively, for chromosome 2 of Candida dubliniensis; SEQ ID NOS. 43, 45, 47, 49 and 51 and SEQ ID NOS. 44, 46, 48, 50 and 52 respectively, for chromosome 3 of Candida dubliniensis; SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67 and SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68 respectively, for chromosome 4 of Candida dubliniensis; SEQ ID NOS. 69, 71, 73, 75 and 77 and SEQ ID NOS. 70, 72, 74, 76 and 78 respectively, for chromosome 5 of Candida dubliniensis; SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93 and SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94 respectively, for chromosome 6 of Candida dubliniensis; SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111 and SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112 respectively, for chromosome 7 of Candida dubliniensis and SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126 and SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125 respectively, for chromosome 8 of Candida dubliniensis or any combination of said primers thereof.

The Cse4p-containing centromere regions of Candida albicans have unique and different DNA sequences on each of the eight chromosomes. In closely related yeast, Candida dubliniensis, the centromeric histone, CdCse4p, has been identified and it is shown to be localized at the kinetochore. The putative centromeric regions, orthologous to the C. albicans centromeres, in each of the eight C. dubliniensis chromosomes have been identified by bioinformatics analysis. Chromatin immunoprecipitation followed by polymerase chain reaction using a specific set of primers confirmed that these regions bind CdCse4p in vivo. As in C. albicans, the CdCse4p-associated core centromeric regions are 3-5 kb in length, and show no sequence similarity to one another. Comparative sequence analysis suggests that the Cse4p-rich centromere DNA sequences in these two species have diverged faster than other orthologous intergenic regions, and even faster than our best estimated “neutral” mutation rate. However, the location of the centromere and the relative position of Cse4p-rich centromeric chromatin in the orthologous regions with respect to adjacent open reading frames are conserved in both species, suggesting that centromere identity is not solely determined by DNA sequence. Unlike known point and regional centromeres of other organisms, centromeres in C. albicans and C. dubliniensis have no common centromere-specific sequence motifs or repeats except some of the chromosome-specific pericentric repeats that are found to be similar in these two species. The centromeres of these two Candida species are thus of an intermediate type between point and regional centromeres.
Several lines of evidence suggest that primary DNA sequence may not be the only determinant of CEN identity in regional centromeres. A recent study on several independent clinical isolates of C. albicans reveals that, despite having no centromere specific DNA sequence motifs or repeats common to all of its eight centromeres, centromere sequences remain conserved and their relative chromosomal positions are maintained (12). As a first step toward understanding the importance of cis-acting CEN DNA sequences in centromere function in C. albicans, centromeres of a closely related pathogenic yeast, Candida dubliniensis, which was identified as a less pathogenic independent species in 1995 were identified and characterized. It was thought that CEN DNA comparisons between related Candida species might uncover properties that were not evident from inter-chromosomal comparisons of C. albicans CEN sequences alone. Moreover, functional characterization of centromeres of these two related Candida species may be helpful in understanding the evolution of centromeres. Several studies indicate that both CEN DNA and its associated proteins in animals and plants are rapidly evolving, although the relative position of the centromere is maintained for a long time. The identification and characterization of Cse4p-rich centromere sequences of each of the eight chromosomes of C. dubliniensis was carried out. Comparative genomic analysis of CEN DNA sequences of C. albicans and C. dubliniensis reveals no detectable conservation among Cse4p-associated CEN sequences. Nonetheless, the lengths of Cse4p-enriched DNAs assembled as specialized centromeric chromatin and their relative locations in orthologous regions have been maintained for millions of years. A genome wide analysis also revealed that centromeres are probably the most rapidly evolving genomic loci in C. albicans and C. dubliniensis.
Candida dubliniensis has a total of 8 chromosomes. Chromosomes 1 to 7 are identified based on their respective sizes. The chromosome number 8 has an extensive number of R-DNA repeat sequences. Hence this chromosome is also referred to as Chromosome R.
The invention is further elaborated with the help of following examples. However, these examples should not be construed to limit the scope of the invention.

EXAMPLE 1

Synteny of Centromere-Adjacent Genes Is Maintained In C. albicans And C. dubliniensis

C. albicans and C. dubliniensis diverged about 20 million years ago from a common ancestor (12). Gene synteny (collinearity) is maintained almost throughout the genome in these two organisms. Therefore, potential orthologous CEN regions in C. dubliniensis were examined by identifying open reading frames (ORFs) of C. dubliniensis with homology to CEN-proximal ORFs of C. albicans. C. dubliniensis homologs of C. albicans ORFs that are adjacent to centromere regions were identified by BLAST analysis of the C. dubliniensis genome database available at the Wellcome Trust Sanger Institute website.

Result

The homology of amino acid sequences coded by CEN-adjacent genes in C. albicans and C. dubliniensis ranges from 81% to 99%, as shown in Table 1 below.

TABLE 1

C. albicans	C. dubliniensis	Amino

	C. albicans			Amino		Amino		acid
Chr	ORF	C. dubliniensis	Chromosomal	acid	Chromosomal	acid		homology
No.	No.	ORF No.	coordinates	length	coordinates	length	Orientation	(%)

1	4438	Cd36_06830	1580117-1581640	507	1611890-1613440	516	Direct	88
	4440	Cd36_06810	1559352-1561871	839	1591631-1594162	843	Direct	91
2	1601	Cd36_23540	1923194-1924363	389	1938439-1939608	389	Direct	99
	1604	Cd36_23560	1934775-1931570	916	1947203-1949623	806	Reverse	84
3	2812	Cd36_83930	828667-827105	503	871879-873366	495	Reverse	84
	6923	Cd36_83920	820347-821378	343	865253-866083	276	Direct	90
4	3818	Cd36_44310	1010148-1009312	278	1036396-1037226	276	Reverse	88
	3821	Cd36_44290	1000558-999371	395	1025948-1027126	392	Reverse	81
5	3160	Cd36_51930	467208-466702	168	493689-494072	127	Reverse	95
	4216	Cd36_51940	473741-474247	168	500592-500975	127	Direct	94
6	1096	Cd36_64780	965934-968573	879	934029-936683	884	Direct	84
	2124	Cd36_65100	982460-981390	353	1016599-1017672	357	Reverse	87
7	6522	Cd36_71800	431903-430173	586	439178-440899	573	Reverse	94
	6524	Cd36_71780	423631-422459	390	424821-425993	390	Reverse	99
R	597	Cd36_33630	1759087-1757405	560	1722610-1724292	560	Reverse	97
	600	Cd36_33620	1748818-1745649	1056	1710255-1713449	1064	Reverse	90

The synteny of these genes is maintained in all chromosomes except chromosome 6. FIG. 1 shows orthologous Cse4p-rich centromere regions in C. albicans and C. dubliniensis. Based on BLAST analysis, the putative homologs of C. albicans CEN-adjacent ORFs in C. dubliniensis have been identified. Chromosome numbers are shown on the left (R through 7). The top line for each chromosome denotes C. albicans centromere regions and the bottom line corresponds to the orthologous regions in C. dubliniensis. The dotted and crossed boxes correspond to Cse4p-binding regions in C. albicans and C. dubliniensis respectively. Only one homolog is shown for each chromosome of C. albicans and C. dubliniensis. ORFs and the direction of transcription of corresponding ORFs are shown by open arrows. Only those ORFs which have homologs in both C. albicans and C. dubliniensis are shown. The number on the top of each arrow corresponds to the C. albicans assembly 19 ORF numbers (for example, Orf19.600 has been shown as 600). The length of CEN-containing intergenic regions of C. albicans and orthologous regions in C. dubliniensis are shown. This analysis was done based on Assembly 20 of Candida albicans Genome Database and the present version (16 May, 2007) of the Candida dubliniensis Genome database.
C. albicans CEN6 is flanked by Orf19.1097 and Orf19.2124. Since there is no Orf19.1097 homolog in C. dubliniensis, the C. dubliniensis homolog of Orf19.1096, the gene adjacent to Orf19.1097 in C. albicans were identified. The distance between Orf19.1096 and Orf19.2124 is 12.8 kb in C. albicans as opposed to 80 kb in C. dubliniensis. A systematic analysis of this 80 kb region of C. dubliniensis reveals that two paracentric inversions followed by an insertion between Orf19.1096 homolog and its downstream region occurred in C. dubliniensis at the left arm of the orthologous pericentric region as compared to C. albicans. FIG. 4 shows comparative analysis of CEN6 region of C. albicans and its orthologous region in C. dubliniensis showing genome rearrangement. Chromosomal maps of the chromosome 6 of C. albicans and C. dubliniensis where the red dots represent the CEN regions. Black arrows along with the ORF numbers show the gene arrangement and the direction of transcription. Two paracentric inversions in C. dubliniensis are marked in shaded red and grey boxes. The direction of the shaded boxes (gradation of colors) represents the inversions that have occurred in C. dubliniensis when compared to C. albicans. The green arrows show the breakpoints where the inversions have occurred. The blue region in C. dubliniensis shows the region of insertions of ORFs from other chromosomes. The yellow regions are unaltered. The orange arrow shows the Orf19.1097 in C. albicans and the orange star in the C. dubliniensis map shows that there is a premature termination codon in the Orf19.1097 homolog of C. albicans in C. dubliniensis. Brown bar indicates Cse4p-binding region.

EXAMPLE 2

The Centromeric Histone Protein of C. dubliniensis (CdCse4p) Is Localized At the Kinetochore

CenH3 proteins in the Cse4p/CENP-A family have been shown to be uniquely associated with centromeres in all organisms studied to date (1). Using CaCse4p as the query in a BLAST analysis against the C. dubliniensis genome, the centromeric histone of C. dubliniensis, CdCse4p were identified.

Identification of CdCse4p And CdMif2p

The C. dubliniensis Cse4p was identified by a BLAST search with C. albicans Cse4p (CaCse4p) as the query sequence against the C. dubliniensis genome sequence database. This sequence analysis revealed three protein sequences with high homology to CaCse4p; two are the C. dubliniensis putative histone H3 proteins (Chr RCd36_—32350; Chr1-Cd36_—04010) and the other CdCse4p (Chr 3-Cd36_—80790). The CdCSE4 gene encodes a putative 212 aa-long protein with 100% identity in the C terminal histone fold domain of CaCse4p. A pair wise comparison of the CaCse4p and CdCse4p sequences revealed that they share 97% identity and 1.4% similarity over a 212 aa overlap as shown in FIG. 5.
Using CaMif2p as the query sequence in the BLAST search against the C. dubliniensis genome database, a single hit was retrieved, which was identified as the CENP-C homolog (Cd36_—63360) in C. dubliniensis showing 77% identity and 5% similarity in 516 aa overlap with CaMif2p. FIG. 7 shows the CENP-C homolog in C. dubliniensis (CdMif2p) is co-localized with CdCse4p. (A) Sequence alignment of CaMif2p and CdMif2p showing the conserved CENP-C block (red box) (B) Localization of CdMif2p at various stages of cell cycle in C. dubliniensis. (C) ChIP enrichment profiles of CdMif2p on chromosomes 1 and 3 in the strain CDM1 by determining the intensities of (+Ab) minus (−Ab) signals divided by the total DNA signals and are normalized to a value of 1 for the same obtained using primers for a non-centromeric locus (CdLEU2). The CdMIF2 gene codes for a putative 520 aa-long protein with a conserved CENP-C box required for centromere targeting (11) that is identical in C. albicans and C. dubliniensis as shown in FIG. 5. This histone is found to be highly similar (97% identity over 211 aa) to CaCse4p. CdCse4p codes for a 212-aa-long predicted protein with a C-terminal (aa residues 110-212) histone-fold domain (HFD). The HFD of Cse4p in C. albicans and C. dubliniensis is identical as shown in FIG. 5. FIG. 5 shows the centromeric histone in C. dubliniensis, CdCse4p, belongs to the Cse4p/CENP-A family. A) Phylogenetic tree of the Cse4 protein sequences in yeasts in the radiation format using neighbor-joining method of Molecular Evolutionary Genetics Analysis version 3.1 (MEGA) software showing Cse4 proteins in C. albicans and C. dubliniensis are highly related. Ca-Candida albicans, Cd-Candida dubliniensis, Db-Debaryomyces hansenii, Pa-Pichia angusta, Kl-Kluyveromyces lactis, Cn-Cryptococcus neoformans, Sp-Schizosaccharomyces pombe, Af-Aspergillus fumigatus, Nc-Neurospora crassa, YI-Yarrowia lipolytica, Ag-Ashbya gossypii, Sc-Saccharomyces cerevisiae, Cg-Candida glabrata. B) Pairwise comparison of Cse4p in C. albicans and C. dubliniensis showing homologies in N-terminal region and C-terminal histone fold domain.

EXAMPLE 3

The Centromeric Histone Protein of C. dubliniensis (CdCse4p) Can Functionally Compliment Histone Protein of C. albicans (CaCse4p)

In order to examine whether CdCse4p can functionally complement CaCse4p, CdCSE4 from its native promoter (pAB1CdCSE4) cloned in an ARS2/HIS1 plasmid (pAB1) in a C. albicans strain (CAKS3b) carrying the only full length copy of CaCSE4 under control of the PCK1 promoter was expressed.

Complementation Assay

To examine whether CdCse4p can complement CaCse4p function, a C. albicans strain was constructed, where the first allele of CaCSE4 was disrupted using URA-blaster cassette followed by recycling of URA3 marker, and the second allele was placed under control of the PCK1 promoter. To disrupt the first CaCSE4 allele, a 4.9 kb URA-blaster-based CaCSE4 deletion cassette was released from pDC3 (Sanyal & Carbon, 2002) as Sa/I-SacI fragment and transformed BWP17 selecting for uridine prototrophy. The correct integrant (CAKS1b) was selected by Southern analysis. Thereafter, Ura-strain, obtained by intrachromosomal recombination between hisG repeats resulting in the loss of URA3 marker, was selected on medium containing 5-fluoroorotic acid (5-FOA). The correct revertant (CAKS2b) was identified by PCR analysis. To place the wild type CSE4 allele under regulation of the PCK1 promoter in CAKS2b, pPCK1-CSE4 was linearized (Sanyal & Carbon, 2002) by. EcoRV and used it to transform strain CAKS2b, selecting transformants for uridine prototrophy. The desired integrant (CAKS3b) carrying the only full-length copy of CSE4 under control of the PCK1 promoter was identified by PCR analysis. CAKS3b can grow on succinate medium (where the PCK1 promoter is induced) but is unable to grow on glucose medium (where PCK1 promoter is repressed) as shown in FIG. 2A. To test whether CdCse4p can complement CaCse4p function, both CdCSE4 and CaCSE4 genes were cloned in an ARS2/HIS1 plasmid, pAB1 (Baum et al., 2006). A 2.14-kb fragment carrying CdCSE4 (CdChr3 coordinates 170543-172683) and a 2.13-kb fragment carrying CaCSE4 (CaChr3 coordinates 172252-174384) genes along with their respective promoters and terminators were amplified using FCdCSE4/RCdCSE4 and FCaCSE4/RCaCSE4 primer pairs, respectively, as listed in Table 2 below.

TABLE 2

Primer	Sequence	Chromosomal locations

For CdCEN1

CdCEN1-1(F)	AAGCCCTTTGGATGTTGACTACGC	1593208-1593231
CdCEN1-2(R)	CCATCGACAGGGCCCATGTG	1593417-1593398
CdCEN1-3(F)	TATGATTATACCCCAATCCA	1595086-1595105
CdCEN1-4(R)	AGGATCAGTTACCAATGTTG	1595287-1595268
CdCEN1-3′(F)	CAACAATCAACAATTTCTGCTCCTCATG	1596131-1596158
CdCEN1-4′(R)	AAGTGGGTATCACCTTATTCGCAAATGA	1596368-1596341
CdCEN1-5(F)	CCTTTTTAAACGTGACACGCTCAAA	1597063-1597087
CdCEN1-6(R)	GGAAAAGTTGCGTGAGGAAATGGA	1597302-1597279
CdCEN1-5′(F)	CGGGTGCATCTAAGAAGGGTTTTA	1598062-1598085
CdCEN1-6′(R)	CAATATAACCTTGCACCCGTCAAATACG	1598347-1598320
CdCEN1-7(F)	GTTGCAGTGCATTGTACGAGGTAAGCTC	1599081-1599108
CdCEN1-8″(R)	TGCAACTGATCCGAGACAACTTCAAAC	1599271-1599245
CdCEN1-7′(F)	GATCGCAAGCGAAGCACGAAATGAC	1600481-1600505
CdCEN1-8′(R)	CAATGTCTGTTCGACCACCATTCCC	1600721-1600697
CdCEN1-9(F)	AGAGCGAGCACCTGGTATTCCCAAG	1601290-1601314
CdCEN1-10(R)	CACCCAAAGCCCAGCTTAAATTCC	1601509-1601486
CdCEN1-9′(F)	TTTCAATTTAGCTGACTCCTTACCCTGG	1602167-1602194
CdCEN1-10′(R)	TTTTCGGTGATTTTGCCAAGAAGTTC	1602410-1602385
CdCEN1-11(F)	CAGCATTCATCCGGGTAAAGTGTTG	1603320-1603344
CdCEN1-12(R)	CAACGGATCCAAGGTCACCACATAG	1603543-1603519

Control (Non centromeric locus in chromosome 7)

CdLeu2-1(F)	AACTATCACAGTCTTGCCTGGTGA	119386-119409
CdLeu2-2(R)	ACAGCACCAGTGCCCCATTT	119618-119637

For CdCEN2

CdCEN2-1(F)	CGCGGTCCAAGAAGATAATC	1940515-1940534
CdCEN2-2(R)	CATCATGGGATGTAATTGCT	1940649-1940668
CdCEN2-3(F)	AGTGTAAGTCTTCGGGATAC	1942509-1942528
CdCEN2-4(R)	GTGAGCGAATAGAATAATTG	1942685-1942704
CdCEN2-5(F)	AGCTACATCTATTTTCAATGCACTC	1944606-1944630
CdCEN2-6(R)	AATTGCTCTGAAACAGCCAG	1944877-1944896
CdCEN2-7(F)	TATACCCCCGAATTAACAAGTGCGC	1943700-1943724
CdCEN2-8(R)	CAGTGCAGGTGCTTTCGTTTACCAG	1943847-1943871
CdCEN2-9(F)	CATCAGTTCAATTGATGGGGTTGTTCTG	1945542-1945569
CdCEN2-10(R)	AAACTGGCATAGCTTTTTGCATTATTGCC	1945736-1945764
CdCEN2-11(F)	ATTTCGAGAGGACTTGGTTCGTGC	1946646-1946669
CdCEN2-12(R)	CCGTACCCAAATAAAACTCCCAGC	1946844-1946867
CdCEN2-15(F)	TACAAAGCGGGTGATAAGGA	1947305-1947054
CdCEN2-16(R)	GGCGCAAAAGGAAATAGC	1947234-1947217

For CdCEN3

CdCEN3-1(F)	ACACTGTCTTGTCTTGTGTCTGAAGTCG	865133-865160
CdCEN3-2(R)	TTCTCTGTGTGTGGGCCCTCAGTAC	865293-865317
CdCEN3-3(F)	TCATCCATCATATCACAAATCCTACTG	867274-867300
CdCEN3-4(R)	GTTATTTTGAAAGTTGGGGAGAGGG	867456-867480
CdCEN3-5(F)	CCTACGACATGAACACATCAAACTACTC	869090-869117
CdCEN3-6(R)	TGCTTTTGTTGAAAACTTGCGAAAC	869243-869267
CdCEN3-7(F)	AGGCTAGTCGGTGGTTAACGGTTGTGTG	870638-870665
CdCEN3-8(R)	GACTCGGAATAAACACCATCGCCGATGC	870856-870883
CdCEN3-9(F)	GGTCCAATTAGAATCGGGTCGTTCCATG	872528-872555
CdCEN3-10(R)	CGTCATCCCTTCTATCTCTAACGTG	872683-872707

For CdCEN4

CdCEN4-1(F)	ATCATATCATGCAGCCCAACTCCG	1028245-1028268
CdCEN4-2(R)	CGGACGTAGTGAAACGATTGTTGG	1028410-1028433
CdCEN4-3(F)	ACAATTCCCAGTAAACCATTATAAAAG	1029835-1029861
CdCEN4-4(R)	CATTCATAATCTGATTTGTAGGCTC	1029965-1029989
CdCEN4-3′(F)	TGCTAAACGACCCCCTCAAAA	1030554-1030574
CdCEN4-4′(R)	GTACGACGATCATCAGCAACCAA	1030776-1030798
CdCEN4-5(F)	AATTAATTCGGATAGTTGGGGGAGACCG	1032446-1032473
CdCEN4-6(R)	ATTGAGCTGCTCACTTCACTGCCAC	1032619-1032643
CdCEN4-5′(F)	GCAGCGTTCTTGTGACCGTGAG	1033199-1033220
CdCEN4-6′(R)	TTGAATTGGACAGGGGCTTAGG	1033477-1033498
CdCEN4-7(F)	TGTGGTGGAGGGTCATCCATTTGTTGGTTG	1034406-1034435
CdCEN4-8(R)	GGCGACCCTCATGCACCCTACCAAATAAA	1034609-1034637
CdCEN4-7′(F)	AAGTACGGATGGTTGTTA	1035010-1035028
CdCEN4-8′(R)	TAGTCATTCTGCCATCTCTTAT	1035231-1035252
CdCEN4-9(F)	CCATGAACAAAAGGTTAGGTGGTGCTCC	1036158-1036185
CdCEN4-10(R)	GGGGAGTTGAATGGTGTGGTGTTAC	1036367-1036391

For CdCEN5

CdCEN5-7(F)	TCCAGCGTCAGACATTTTTCCAGT	494058-494081
CdCEN5-8(R)	TGCCCCGCGGTTGACAGT	494213-494230
CdCEN5-1(F)	TGGCCTCTCCCTTACAAAATTTGCCC	495324-495349
CdCEN5-2(R)	GGGAGATGAGGGGTGATTGAGGTAATAG	495504-495531
CdCEN5-3(F)	GCTCCAGTACCAACGAAAACGACTTC	496907-496932
CdCEN5-4(R)	GCATTTGAAAACTGCCAATGTAGTC	497035-497059
CdCEN5-5(F)	GCTGGGATAGTTTAGAGGCAGACTGTG	498944-498971
CdCEN5-6(R)	CCTCAATCACCCCTCATCTCCCTAC	499130-499155
CdCEN5-9(F)	AAGGGCAAGGAACAAGTCACAAGT	500673-500696
CdCEN5-10(R)	TATCAGCGCCGGTTTTAGCAC	500941-500961

For CdCEN6

CdCEN6-15(F)	GTGCCAACTTTCTCCTGAT	1002806-1002824
CdCEN6-16(R)	AGCGATTATTAAGTCTATGTGG	1002985-1002964
CdCEN6-13(F)	GAAGCAGCGACCCAACAGATAA	1003044-1003065
CdCEN6-14(R)	TTGAGCGAAATTGGGTAGAGTC	1003262-1003283
CdCEN6-5(F)	TGTCCATTCCCCAAACTTCATACGGACCAC	1004039-1004068
CdCEN6-6(R)	GAATGCTGGAAGGACTTGAGAAATG	1004175-1004199
CdCEN6-5′(F)	GAAACCAATAACAAGGAAAGAGTA	1005046-1005069
CdCEN6-6′(R)	CAATGGGAAAAAGAAATCAGTAG	1005313-1005335
CdCEN6-7(F)	GACGAGAGCATGTACTCAACTACGTGTC	1006472-1006499
CdCEN6-8(R)	GAATCTTGATTGAAATGCGAGGAAC	1006668-1006692
CdCEN6-9(F)	CATCCAATAACATTGATTTACTACTTTTAG	1008985-1009014
CdCEN6-10(R)	TTTTTTTTTCTCAAAGATTTAGCAG	1009115-1009139
CdCEN6-9′(F)	TGTACGATCAACCCAGAGTGC	1009504-1009524
CdCEN6-10′(R)	ACATGCCATTACCAACAACAGTC	1009749-1009771
CdCEN6-3(F)	TAGCTGTATTAAAAAATTCTGGCCGCATA	1015917-1015945
CdCEN6-4(R)	TCTGACAAAAAACCTCGTATGACCC	1016066-1016042

For CdCEN7

CdCEN7-1(F)	CTAGAGCTATGTTGTGACAGTCCACC	427615-427640
CdCEN7-2(R)	CTTCTGGAATTGAGCCAATCCCTAG	427777-427801
CdCEN7-3(F)	CTAGCTATTCAAGCATCCGTAGGCAGTC	429103-429130
CdCEN7-4(R)	CCCATACCCGGGTGGTGTAGTATAA	429228-429252
CdCEN7-5(F)	GTAGGCGCTACATATGAACTTCGTGC	436328-436354
CdCEN7-6(R)	AGATAATGTCTGAATGTCATTCGGG	436479-436504
CdCEN7-9′(F)	TCCAATGGGTGCTAAGATGAA	434047-434068
CdCEN7-10′(R)	TCCCGCCTGATTTTTGAA	434292-434310
CDCEN7-7(F)	TTATTTGATAGCCTAATTTCACCTGATG	438005-438031
CdCEN7-8(R)	ATTAACTGACTTTGAACCAGCAATG	438205-438230
CdCEN7-9(F)	AACGGTCACCTGATGAATAGAGTGGC	432732-432758
CdCEN7-10(R)	GACTGAAGCGTCCATACTTGGGATC	432956-432981
CdCEN7-11(F)	CCCAGAAGTATCCACTAGGGAACTTG	435240-435268
CdCEN7-12(R)	TTGTTCTGGTCAATGGTACAGCAAC	435365-435390
CdCEN7-13(F)	CACGCAACTAGAATGGCATGAATATATG	439500-439527
CdCEN7-14(R)	AGATCCGGTGTCTGTCTTATTGCTC	439630-439654
CdCEN7-15(F)	CCTGCGTTGTAATCATTTGTTGTC	440443-440466
CdCEN7-16(R)	TTACTCCGCCTTTGATCCCTATTT	440640-440617

For CdCENR

CdCENR-1(R)	ATTAAGGAGCTTCGTGAGGCTGTCG	1723671-1723647
CdCENR-2(F)	CATTTCCTTCAAAGGCACCGGGATG	1723429-1723453
CdCENR-3(R)	ACGTTGCTTACTGGTGGCTATGCGG	1721710-1721686
CdCENR-4(F)	AAGCTTTTATTGCGGTGAACTGGGG	1721461-1721485
CdCENR-5(R)	ACATATAATAGCCTACCACACGCCTTGC	1719373-1719346
CdCENR-6(F)	TGACATTGTGGAAAGTTAATCGCGG	1719202-1719226
CdCENR-7(R)	TGAAATTGGAGACTAAGTGTTGCATTCG	1717531-1717504
CdCENR-8(F)	ACAGTTTCCACACAACTCAGCAAGACA	1717330-1717356
CdCENR-9(R)	TTTGCCGGGATAAGCTTTTATTGCG	1715642-1715618
CdCENR-10(F)	TTTCAGGACACCAGAAGATGGCCAC	1715409-1715433
CdCENR-9′(F)	CCCCCGCCGTGAAAAACA	1713200-1713217
CdCENR-10′(R)	CTACAAACGCCACACCCGAAACT	1713426-1713404
CdCENR-11(R)	ACCTCAACATCGACACAGTCGCACC	1712709-1712185
CdCENR-12(F)	AGCAGAAACCTCGATGTTTGAGCCG	1712487-1712511

TABLE 2B

Primer	Sequence

FCaCse4

RCaCse4	TGCTCTAGACCAAAATCCCTCTTTCTGTATTTG

FCdCse4	CCCGAGCTCCAAGTGTATTTTTCATCTTTGGTAG

RCdCse4	CCCAAGCTTCTATTTTGCCACCAAAACCCATCTT

These amplified CdCSE4 and CaCSE4 sequences were digested with SacI/HindIII and SacI/XbaI, respectively, and cloned into corresponding sites of pAB1 to get pAB1CdCSE4 and pAB1CdCSE4. Subsequently CAKS3b was transformed with pAB1, pAB1CaCSE4 or pAB1CdCSE4 and transformants were selected for histidine prototrophy on succinate medium followed by streaking on succinate as well as glucose containing media.

Result

The ability of the strain CAKS3b carrying pAB1CdCSE4 to grow as good as the same strain carrying a control plasmid pAB1CaCSE4 on glucose medium (where endogenous CaCSE4 expression is suppressed) suggests that CdCse4p can complement CaCse4p function and hence codes for the centromeric histone in C. dubliniensis (FIG. 2B).
FIG. 2 shows localization of CdCse4p at the kinetochore of C. dubliniensis. (A) The C. albicans strain CAKS3b was streaked on media containing succinate and glucose and incubated at 30° C. for 3 days. (B) CAKS3b is transformed with pAB1, pAB1CaCSE4 or pAB1CdCSE4. These transformants were streaked on plates containing complete media lacking histidine with succinate or glucose as the carbon source. (C) C. dubliniensis strain Cd36 was grown in YPD and fixed. Fixed cells were stained with DAPI (a-d), anti-Ca/CdCse4p (e-h) and anti-tubulin (i-l) antibodies. The intense red dot-like CdCse4p signals were observed in unbudded (e) and at different stages of budded cells (f-h). Corresponding spindle structures are shown by co-immunostaining with anti-tubulin antibodies (i-l). Arrows indicate the position of spindle pole bodies in large-budded cells at anaphase. (Bar=10 μm).

EXAMPLE 4

Subcellular Localization of CdCse4p In C. dubliniensis

The subcellular localization of CdCse4p in C. dubliniensis strain Cd36 was further examined by indirect immunofluorescence.

Indirect Immunofluorescence

Intracellular CdCse4p or CdMif2p were visualized by indirect immunofluorescence microscopy as described previously. Asynchronously grown cells of Cd36 or CDM1 were fixed with 37% formaldehyde at room temperature for an hour. Antibodies were diluted as follows: 1:30 for anti-α-tubulin (YOL1/34) (Abcam); 1:500 for affinity purified rabbit anti-Ca/CdCse4p and rabbit anti-Protein A (Sigma); 1:500 for Alexa fluor 488 goat anti-rat IgG (Invitrogen) and 1:500 for Alexa fluor 568 goat anti-rabbit IgG (Invitrogen). The positions of nuclei of the cells were determined by staining with 4′, 6-diamidino-2-phenylindole (DAPI) as described previously. Cells were examined at 100× magnification on a confocal laser scanning microscope (LSM 510 META, Carl Zeiss). Using LSM 5 Image Examiner, digital images were captured. Images were processed by Adobe PhotoShop software.

Result

Indirect immunofluorescence microscopy using affinity purified polyclonal anti-Ca/CdCse4p antibodies (against aa1-18 of CaCse4p/CdCse4p) revealed bright dot-like signals in all cells. The dots always co-localized with nuclei stained with DAPI (FIG. 2C). Each bright dot-like signal represents a cluster of 16 centromeres. Unbudded G1 cells exhibited one dot per cell, while large-budded cells at later stages of the cell cycle exhibited two dots that co-segregated with the DAPI-stained nuclei in daughter cells (FIG. 2C). The localization patterns of CdCse4p appear to be identical to those of CaCse4p in C. albicans at corresponding stages of the cell cycle. Co-immunostaining of fixed Cd36 cells with anti-tubulin and anti-CdCse4p antibodies showed that CdCse4p signals are localized close to the spindle pole bodies, analogous to typical localization patterns of kinetochore proteins in S. cerevisiae and C. albicans (FIG. 2C). Together, these results strongly suggest that CdCse4p is the authentic centromeric histone of C. dubliniensis.

EXAMPLE 5

Centromeric Chromatin On Various C. dubliniensis Chromosomes Is Restricted To A 3-5 kb Region

Standard chromatin immunoprecipitation (ChIP) assays with anti-Ca/CdCse4p antibodies to assay for enrichment of CdCse4p on putative CEN regions (orthologous to C. albicans CENs) in C. dubliniensis strain Cd36.

Chromatin Immunoprecipitation (ChIP) Assay And Sequence Analysis

Chromatin immunoprecipitation (ChIP) by anti-CdCse4 antibodies followed by PCR analysis was done as described previously (9, 11). This suggests that the predicted centromeric regions of all chromosomes of C. dubliniensis are enriched in centromeric specific histone (CdCse4p) binding. Asynchronously grown culture of Cd36 was crosslinked with formaldehyde and sonicated to get chromatin fragments of an average size of 300-500 bp. The fragments were Immunoprecipitated with anti-Ca/CdCse4p antibodies and checked by PCR. PCR reaction was set up using 10 pmol of both forward and reverse primers (MWG Biotech & Ocimum Biosolutions), 5 μl of 10× Taq buffer (Sigma), 5 μl of 2.5 mM dNTPs mix, 2 μl of DNA template and 0.3 μl of Taq polymerase (Sigma) in 50 μl reaction volume. PCR amplification was carried out using PCR machine (BIORAD) with the following conditions: 1 min at 94° C. (denaturation), 30 s at 45° C. -55° C. (annealing temperature is variable with the primers used) and 1 min at 72° C. (extension). A final extension of 4 min was given at 72° C. PCR with total DNA (1:10 dilution) and ±antibody ChIP DNA fractions were performed using 1/25 th of the template. The boundaries of the CEN regions on each chromosome of C. dubliniensis were mapped using semi-quantitative ChIP-PCR in strain Cd36. Sequence-specific PCR primers were designed at approximately 1 kb sequence intervals that spans the putative CEN region of each chromosome of C. dubliniensis (Table 2 above). CdLEU2 PCR primers were used as an internal control in all PCR reactions. PCR amplification was performed and the PCR products were resolved on 1.5% agarose gels and band intensities were quantified using Quantity One 1-D Analysis Software (BioRad). Enrichment values equal (+Ab) minus (−Ab) signals divided by the total DNA signal and were normalized to a value of 1 for LEU2. The PCR primers used in this study are listed in Table 2 above. Similarly, a ChIP assay to determine occupancy of TAP tagged CdMif2p was performed using the strain CDM1 with anti-Protein A antibodies. All other conditions were identical as it was described above for CdCse4p ChIP antibodies.

Result

The immunoprecipitated DNA sample was analyzed by PCR using a specific set of primers designed from the putative CEN sequences (Table 2 above). These regions are, indeed, found to be associated with CdCse4p as shown in FIG. 3. This ChIP-PCR analysis precisely localized the boundaries of CdCse4p-binding to a 3-5 kb region on each chromosome (FIG. 3).
FIG. 3 shows two evolutionarily conserved key kinetochore proteins, CdCse4p (CENP-A homolog) and CdMif2p (CENP-C homolog) bind to the same regions of different C. dubliniensis chromosomes. Standard ChIP assays were performed on strains Cd36 and CDM1 (CdMif2-TAP-tagged strain) using anti-Ca/CdCse4p or anti-Protein A antibodies and analyzed with specific primers corresponding to putative centromere regions of C. dubliniensis to PCR amplify DNA fragments (150 to 300 bp) located at specific intervals as indicated (Table 2 above). Graphs showing relative enrichment of CdCse4p (blue lines) and CdMif2p (red lines) that mark the boundaries of centromeric chromatin in various C. dubliniensis chromosomes. PCR was performed on total, immunoprecipitated (+Ab), and beads only control (−Ab) ChIP DNA fractions (see Supporting FIGS. 6 and 7). The coordinates of primer locations are based on the present version (16 May, 2007) of the Candida dubliniensis genome database. The coordinates are listed in Table 3 below. Enrichment values are calculated by determining the intensities of (+Ab) minus (−Ab) signals divided by the total DNA signals and are normalized to a value of 1 for the same obtained using primers for a noricentromeric locus (CdLEU2) and plotted. The chromosomal coordinates are marked along X-axis while the enrichment values are marked along
Y-axis. Black arrows show the location and arrowheads indicate the direction of transcription.

TABLE 3

Chr		C. albicans	C. dubliniensis
No.	Regions	coordinates	coordinates

R	Region from left ORF	1748819-1750873	1713450-1716138
	Cse4 binding region	1750874-1755348	1716139-1720954
	Region from right ORF	1755349-1757404	1720955-1722609
1	Region from left ORF	1561872-1564187	1594163-1596130
	Cse4 binding region	1564188-1567117	1596131-1600697
	Region from right ORF	1567118-1580116	1600698-1611889
2	Region from left ORF	1924364-1928514	1939609-1943699
	Cse4 binding region	1928515-1931474	1943700-1946867
	Region from right ORF	1931475-1931569	1946868-1947202
3	Region from left ORF	821379-823848	866084-867273
	Cse4 binding region	823849-826997	867274-870883
	Region from right ORF	826998-827104	870884-871878
4	Region from left ORF	1000559-1002628	1027127-1029834
	Cse4 binding region	1002629-1006266	1029835-1034637
	Region from right ORF	1006267-1009311	1034638-1036395
5	Region from left ORF	467209-469044	494073-495323
	Cse4 binding region	469045-472074	495324-499155
	Region from right ORF	472075-473740	499156-500591
6	Region from left ORF	975879-976872	993828-1003043
	Cse4 binding region	976873-980625	1003044-1006692
	Region from right ORF	980626-981389	1006693-1009568
7	Region from left ORF	423632-426037	425994-435239
	Cse4 binding region	426038-428938	435240-438230
	Region from right ORF	428939-430172	438231-439177

However, as mentioned earlier, the homologs of two genes adjacent to the CEN6 region in C. albicans are 80 kb apart in chromosome 6 of C. dubliniensis due to chromosome rearrangement (FIG. 4).
Since other CEN regions of C. dubliniensis are present in ORF-free regions that are greater than 3 kb, first all the intergenic regions, 3 kb or longer were identified, to find CEN6 in this 80 kb region. The ChIP-PCR analysis using specific primers from such regions delimited Cse4p-binding to a 3.6 kb region that is adjacent to the C. albicans Orf19.2124 homolog in C. dubliniensis (FIG. 3 and FIG. 6; not all ChIP data are shown). FIG. 6 shows relative enrichment profiles of CdCse4p in various C. dubliniensis chromosomes. CdCse4p-associated chromosome regions were enriched by ChIP using anti-Ca/CdCse4p antibodies. Specific primers corresponding to putative centromere regions of C. dubliniensis were used to PCR amplify DNA fragments (150 to 300 bp) located at specific intervals as indicated (Table 2). PCR was performed on total, immunoprecipitated (+Ab), and beads only control (−Ab) DNA fractions. Reverse images of ethidium bromide stained PCR products resolved on 1.5% agarose gels are aligned with respect to their chromosomal map position of each CEN region. The coordinates of primer locations are based on the present version (16 May, 2007) of the Candida dubliniensis genome database. Enrichment values are calculated by determining the intensities of (+Ab) minus (−Ab) signals divided by the total DNA signals and are normalized to a value of 1 for the same obtained using primers for a non-centromeric locus (CdLEU2). The intensity of each band was determined by using Quantity One 1-D Analysis Software (Bio-Rad, USA). Panels show the CdCse4p enrichment profiles on C. dubliniensis chromosomes at corresponding regions as indicated. Black arrows and grey arrows correspond to complete and incomplete ORFs, respectively, and indicate the direction of transcription.
Thus, CdCse4p-rich CEN regions- and determined the boundaries of centromeric chromatin in all eight chromosomes in C. dubliniensis were successfully identified. It was also found that the relative distance of Cse4p-rich centromeric chromatin from orthologous neighboring ORFs is similar in both species in most cases (FIG. 1).

EXAMPLE 6

The Evolutionarily Conserved Kinetochore Protein CENP-C Homolog In C. dubliniensis, CdMif2p Binds Preferentially To CdCse4p-associated DNA

Proteins in the CENP-C family are shown to be associated with kinetochores in a large number of species. Using CaMif2p as the query sequence, the CENP-C homolog (CdMif2p) in C. dubliniensis was identified.

Homology Detection And Mutation Rate Measurement

For homology detection, Sigma (version 1.1.3) and DIALIGN (version 2.2.1), to align ORF-free DNA sequences were used. Default parameters were used for both programs, but Sigma was given an auxiliary file of intergenic sequences from which to estimate a background model. Orthologous genes were aligned (at amino-acid level) with T-Coffee. Instances of the following seven codons where the first two positions were conserved in both species were examined: GTn (valine), TCn (serine), CCn (proline), ACn (threonine), GCn (alanine), CGn (arginine), GGn (glycine) (n=any nucleotide). Third position mutations here do not change the amino acid. (Leucine was ignored because of a variant codon in these species). A naïve count of mutation rates in the third position yields 0.27. Taken into consideration genome-wide bias for each codon, an upper-bound mutation rate of 0.42 was obtained.
For this analysis Sigma (version 1.1.3) (4) and DIALIGN 2 (5), to align ORF-free centromeric and other intergenic sequences were used. Default parameters were used for both programs, but Sigma was given an auxiliary file of intergenic sequence from which to estimate a background model. For protein-coding sequence, WU-BLAST 2.0 (tblastn) querying each annotated coding region of C. albicans against the chromosome sequences of C. dubliniensis was run. Parameters used were “filter=seg matrix=blosum62 hspsepQmax=1000 hspsepSmax=2000”. Hits with a summed P-value of 1e-30 or less were identified as potential orthologs. Criteria for ortholog assignment were sequence similarity and synteny (requiring at least two common syntenous immediate neighbors out of four). This led to 2653 high-confidence predictions. These orthologous genes were aligned (at amino-acid level) with T-Coffee (6). Then the following seven amino acids were considered, when conserved, and coded by the indicated codons, in both species: GTn (valine), TCn (serine), CCn (proline), ACn (threonine), GCn (alanine), CGn (arginine), GGn (glycine) (n=any nucleotide). Other synonymous codons, if any, were ignored. Leucine was ignored because of a variant codon, CTG, that codes for serine in these species. A naïve count of mutation rates in the third position yields 0.27. This was improved on by considering the genome-wide bias for each codon, as follows: let the third-position conservation probability be q. Then if a third position nucleotide in C. albicans is b, in C. dubliniensis it stays b with probability q, and mutates with probability (1-q). If it mutates, it was assumed that the probability of the new nucleotide is drawn from the known codon bias. For each amino acid A, the individual mutation rate, P(b₂/b₁,A) for third-position codon changing from b₁in C. albicans to b₂in C. dubliniensis was measured (the results are mathematically identical for evolution from a common ancestor), and solved for q; the weighted average of q for all amino acids and all pairs of observed third-position nucleotides b₁and b2 were then taken This works out to q=0.58, giving a mutation rate of 0.42. (Technically, this mutation rate is a slight overestimate, because a mutated b2 from a distribution was drawn that includes b₁; but it is a credible upper bound.)

Results

CdMif2p shows 77% identity and 5% similarity in 516 aa overlap. The CdMif2p codes for a 520-aa-long predicted protein in which the CENP-C box (aa residues 275-297) is 100% identical in C. albicans and C. dubliniensis. FIG. 7 shows the CENP-C homolog in C. dubliniensis (CdMif2p) is co-localized with CdCse4p. (A) Sequence alignment of CaMif2p and CdMif2p showing the conserved CENP-C block (red box) (B) Localization of CdMif2p at various stages of cell cycle in C. dubliniensis. (C) ChIP enrichment profiles of CdMif2p on chromosomes 1 and 3 in the strain CDM1 by determining the intensities of (+Ab) minus (−Ab) signals divided by the total DNA signals and are normalized to a value of 1 for the same obtained using primers for a non-centromeric locus (CdLEU2).

EXAMPLE 7

Construction of CDM1 Carrying C-terminally TAP-tagged CdMIF2

A strain (CDM1) to express CdMif2p with a C-terminal tandem affinity purification (TAP) tag from its native promoter in the background of one wild-type copy of CdMIF2 was constructed.
Strains, media and transformation procedures. The Candida dubliniensis and C. albicans strains used in this study are listed in Table 4.

TABLE 4

Yeast strains	Genotype	Source

Candida dubliniensis

Cd36	Clinical isolate	10
CdUM4B	ura3D1::FRT/ura3D2::FRT	8
CdM1	ura3D1::FRT/ura3D2::FRT	This study
	MIF2/MIF2-TAP (URA3)

Candida albicans

BWP17	Δura3::imm434/Δura3::imm434	11
	Δhis1::hisG/Δhis1::hisG Δarg4::hisG/
	Δarg4::hisG
CAKS1b	Δura3::imm434/Δura3::imm434	This study
	Δhis1::hisG/Δhis1::hisG Δarg4::hisG/
	Δarg4::hisG CSE4/
	cse4::hisG:URA:hisG
CAKS2b	Δura3::imm434/Δura3::imm434	This study
	Δhis1::hisG/Δhis1::hisG Δarg4::hisG/
	Δarg4::hisG CSE4/cse4::hisG
CAKS3b	Δura3::imm434/Δura3::imm434	This study
	Δhis1::hisG/Δhis1::hisG Δarg4::hisG/
	Δarg4::hisG cse4::PCK1pr-
	CSE4(URA3)/cse4::hisG

These strains were grown yeast extract/peptone/dextrose (YPD), yeast extract/peptone/succinate (YPS), or supplemented synthetic/dextrose (SD) minimal media at 30° C. as described. C. albicans and C. dubliniensis cells were transformed by standard techniques.

CdMIF2 downstream sequence (from +1634 to +2198 with respect to the start codon of CdMIF2) was PCR amplified with primer pair CdM3 (CGG GGT ACC GAT TGC AAG AAG TAC TAC ATA AGA GAG) and CdM4 (GCC CGA GCT CGC AGG TAA AAT TGT TCT TGA GGA GCC G) thereby introducing KpnI and SacI restriction sites (underlined). The resulting PCR amplified fragment was digested with KpnI and SacI and cloned into corresponding sites of pUC19 to generate pCDM1. TAP cassette along with CaURA3 gene was released from plasmid pPK335 (7) as BamHI-KpnI fragment and cloned into corresponding sites of pCDM1 to generate pCDM2. Subsequently CdMIF2 RF sequence from +1090 to +1548 was PCR amplified using primer pair CdM1 (ACG CGT CGA CCC CCC ACT GAT TAC GAT TAT GAA TCT GAT CC) and CdM2 (CAT GCC ATG GCC CAA TTC GTA TCG ATT TCT TCT GGT TIC) and cloned into pCDM2 as NcoI-SalI fragment to get pCDM3. Finally, a 2 kb amplicon was PCR amplified by the primer pair CdM1 and CdM4 using pCDM3 as the template. This PCR fragment was used to transform CdUM4B strain (8). The correct Ura+ transformant (CDM1) was identified by PCR analysis.

Result

The subcellular localization patterns using polyclonal anti-Protein A antibodies in C. dubliniensis strain (CDM1) at various stages of cell cycle is very similar to those observed for CdCse4p (FIG. 7). Binding of TAP tagged CdMif2p in the strain CDM1 was analyzed by standard ChIP assays using anti-Protein A antibodies This experiment suggests that CdMif2p binds to the same 3 kb CdCse4p-rich region of two different chromosomes (Chromosome 1 and 3) in C. dubliniensis. Binding of two different evolutionarily conserved kinetochore proteins CdCse4p and CdMif2p at the same regions strongly implies that these regions are centromeric. (FIG. 3 and FIG. 7).

EXAMPLE 8

Comparative Sequence Analysis Between C. albicans And C. dubliniensis Reveals That Cse4p-rich Centromere Regions Are the Most Rapidly Evolving Loci of the Chromosome

Pairwise alignment of CdCse4p-rich sequences on different chromosomes with one another reveals no homology. To compare orthologous CEN regions of C. albicans


Cse4p-	Cse4p-binding
binding	(shuffled)	Pericentric	Intergenic

Total bases	26836	26836	40280	593782
Aligned	12440	11650	27684	530847
(DIALIGN2)	(46%)	(43%)	(68%)	(89%)
Mutated	7624	7201	10229	154473
(DIALIGN2)	(61%)	(62%)	(36%)	(29%)
Aligned	0	0	15015	334363
(Sigma)			(37%)	(56%)
Mutated	0	0	3323	57548
(Sigma)			(22%)	(17%)

and C. dubliniensis, pairwise alignments using Sigma and DIALIGN2 were performed. These programs assemble global alignments from significant gapless local alignments. Sigma detects no homology in Cse4p-binding regions. DIALIGN2, with default parameters, reports a little homology; but when nonorthologous sequence were compared, (namely, CEN sequences from non-matching chromosomes), it reports almost identical results (Table 5).

Table 5

In other words, it finds no homology beyond what it would with the “null hypothesis” of unrelated sequence. Similar results were obtained with other sequence alignment programs. It is concluded that there is no significant homology in the orthologous Cse4p-containing CEN regions in C. albicans and C. dubliniensis, even though the CEN regions are flanked by orthologous, syntenous ORFs. However, neighboring (pericentric) ORF-free regions, located between the Cse4pbinding regions and CEN-adjacent ORFs, do exhibit a higher degree of homology compared to Cse4p-rich regions. Mutation rates were counted only in aligned blocks (ignoring insertions and deletions); DIALIGN2 aligns 68% of these regions, with a mutation rate of 36%, while Sigma aligns 38% of the regions, with a mutation rate of 22% in aligned regions. Much of the conservation occurs towards the outer ends of these regions, that is, near the bounding ORFs.
To estimate a “neutral” DNA mutation rate, 2,653 putative gene orthologs of C. albicans in C. dubliniensis were identified. For homology detection, Sigma (version 1.1.3) and DIALIGN (version 2.2.1), to align ORF-free DNA sequences were used. Default parameters were used for both programs, but Sigma was given an auxiliary file of intergenic sequences from which to estimate a background model. Orthologous genes were aligned (at amino-acid level) with T-Coffee. Instances of the following seven codons where the first two positions were conserved in both species were examined: GTn (valine), TCn (serine), CCn (proline), ACn (threonine), GCn (alanine), CGn (arginine), GGn (glycine) (n=any nucleotide). Third position mutations here do not change the amino acid. (Leucine was ignored because of a variant codon in these species). A naïve count of mutation rates in the third position yields 0.27. Taken into consideration genome-wide bias for each codon, an upper-bound mutation rate of 0.42 was obtained.
The genes with T-Coffee were aligned, and the synonymous mutation rates using seven codons that are “fully degenerate” in the third position was measured (the first two bases determine the coded amino acid). A naïve count of the third-position mutation rate yields 27%. Correcting for genome-wide codon biases yields 42%, an upper-boundary estimate for the “neutral” rate of DNA mutation between these two yeasts (see Materials and Methods). This rate corresponds to a pairwise conservation , rate (“proximity”) q=0.58, or a proximity to a common ancestor of 0.76. Tests on synthetic DNA sequence (as reported in 21) suggest that Sigma would easily align such sequence; therefore, it appears that CaCse4p-binding sequences (but not pericentric regions) have diverged faster than expected from the neutral point-mutation rate in these yeasts.
309 homologous intergenic regions were also identified in these species that were between 1000 and 5000 by long (comparable in length with the Cse4p-binding regions). These regions were aligned with Sigma and DIALIGN2, and measured mutation rates in aligned regions only (ignoring insertions and deletions). Sigma aligned 56% of the input intergenic sequence, with a mutation rate of 17%; DIALIGN2 aligned 89% of the input sequence, with a mutation rate of 29%. This rate is less than our estimated neutral mutation rate of 42%, suggesting constraints on the evolution of intergenic DNA sequences. Although pericentric regions evolve slower than the neutral rate determined above, they have a smaller fraction of conserved blocks and a greater mutation rate than intergenic sequences.
Interestingly, despite the rapid divergence of CEN DNA sequences, the relative position of the CEN on each chromosome is conserved in all cases. FIG. 8 shows relative chromosomal positions of Cse4p-binding regions in C. albicans and C. dubliniensis. Red oval shows Cse4p-binding region.
The relative location of the Cse4p-rich centromeric chromatin in the ORF-free region is also similar in both species (FIG. 7). Although no homology was found among Cse4p-binding regions in matching chromosomes, some of the ORF-free pericentric regions in matching chromosomes have repeated segments, both within the same species and across the two species (FIG. 9).
FIG. 9 shows conserved blocks in the pericentric regions of various chromosomes of C. dubliniensis and C. albicans. The cyan dotted blocks represent the Cse4p-binding regions. DNA sequence stretches of various chromosomes having significant similarities (ClustalW scores above 80) are shown by colored arrows as indicated. The numbers on each chromosome represent their coordinates in respective genome database. The direction of the arrows represents the orientation of repeats. A BLAST search was done to identify the repeats flanking the CEN region against the C. dubliniensis genome database with C. albicans CEN flanking repeats as the query sequences (10). The inverted repeats were observed in the chromosomes R, 1 and 5 of C. albicans and C. dubliniensis (Table 6). The LTRs such as epsilon, zeta, episemon) are also shown.
TABLE 6

% homology

Chr Coordinates in between the inverted

No. Repeat C. dubliniensis repeats^¶

R IRR 1720958-1721270 (D) 100

IRR 1716158-1715822 (R)

1 IR1 1595932-1595989 (D) 96

IR1 1602853-1602907 (R)

5 IR5 493690-494369 (D) 99

IR5 500277-500974 (R)

These results strongly suggest that factors other than Cse4pbinding DNA sequences determine centromere identity in these species. The role of pericentric regions in determining centromere identity remains unclear.

Result

Thus, the core CdCse4p-rich centromeric DNA sequences of all eight chromosomes of C. dubliniensis. Two important evolutionarily conserved kinetochore proteins, CdCse4p and CdMif2p are shown to be bound to these regions. Each of these CEN regions has unique and different DNA sequence composition without any strong sequence motifs or centromere-specific repeats that are common to all the eight centromeres, and has A-T content similar to that of the overall genome. In these respects they are remarkably similar to CEN regions of C. albicans (11, 12). Though genes flanking corresponding CENs in these species are syntenous, the Cse4p-binding regions show no significant sequence homology. They appear to have diverged faster than other intergenic sequence of similar length, and even faster than our best estimated neutral mutation rate for ORFs.
A study, based on computational analysis of centromere DNA sequences and kinetochore proteins of several organisms, indicates that point centromeres have probably derived from regional centromeres and appeared only once during evolution. The core Cse4p-rich regions of C. albicans and C. dubliniensis are intermediate in length between the point S. cerevisiae-like centromeres and the regional S. pombe centromeres. The characteristic features of point and regional yeast centromeres are the presence of consensus DNA sequence elements and repeats, respectively, organized around a nonhomologous core CenH3-rich region (CDEII and central core of S. cerevisiae and S. pombe, respectively). Both C. albicans and C. dubliniensis centromeres lack such conserved elements or repeats around their non-conserved core centromere regions.
Based on these features, it is proposed that these Candida species possess centromeres of an “intermediate” type between point and regional centromeres. On rare occasions, functional neocentromeres form at non-native loci in some organisms. However, neocentromere activation occurs only when the native centromere locus becomes non-functional. Therefore, native centromere sequences may have components that cause them to be preferred in forming functional centromeres. Despite sequence divergence, the location of the Cse4p-rich regions in orthologous regions of C. albicans and C. dubliniensis has been maintained for millions of years. Homology was also observed in orthologous pericentric regions in a pair-wise chromosome-specific analysis in these two species. Moreover, several short stretches of DNA sequences are found to be common in pericentric regions of some, but not all, C. albicans and C. dubliniensis chromosomes. Both in budding and fission yeasts, pericentric regions contain conserved elements that are important for CEN function. In the absence of any highly specific sequence motifs or repeats in these regions, it is possible that specific histone modifications at more conserved pericentric regions facilitate the formation of a specialized three-dimensional common structural scaffold that favors centromere formation in these Candida species. It is an enigma that, despite their conserved function and conserved neighboring orthologous regions, core centromeres evolve so rapidly in these closely related species. Satellite repeats, that constitute most of the Arabidopsis and Orzya centromeres, have been shown to be evolving rapidly. However, because of their repetitive nature, these plant centromeres are subject to several events such as mutation, recombination, deletion and translocation that may contribute to rapid change in centromere sequence. In the absence of any such highly repetitive sequences at core centromere regions of C. albicans and C. dubliniensis, such accelerated evolution is particularly striking. It is important to mention that a very recent report based on comparison of chromosome III of three closely related species of Saccharomyces paradoxus suggests that centromere seems to be the fastest evolving part in the chromosome. One possible mechanism for rapid evolution is error-prone replication of CEN DNA followed by inefficient repair. In fact, pausing of replication forks at the centromeres has been reported in S. cerevisiae. If a similar situation exists in C. albicans and C. dubliniensis, it is possible that core CEN regions are replicated by error-prone DNA polymerases, a situation similar to translesion DNA synthesis. Several studies reveal that centromeres function in a highly species-specific manner. Henikoff and colleagues proposed that rapid evolution of centromeric DNA and associated proteins may act as a driving force of speciation (1). The consequence of the rapid change in centromere sequence that was observed in these two closely related Candida species may contribute to generation of functional incompatibility of centromeres to facilitate speciation. To understand the mechanisms of centromere formation in the absence of specific DNA sequence cues, it will be important to identify more genetic and epigenetic factors that may contribute to the formation of specialized centromeric chromatin architecture.

LIST OF SUPPORTING REFERENCES

1. Thompson J-D, Higgins D-G, Gibson T-J (1994) CLUSTAL W: improving the sensitivity of progressive multiple sequence alignment through sequence weighting, position-specific gap penalties and weight matrix choice. Nucleic Acids Res 22:4673-4680.
2. Kumar S, Tamura K, Nei M (2004) MEGA3: integrated software for Molecular Evolutionary Genetics Analysis and sequence alignment. Brief Bioinform 5:150-163.
3. Gouet P, Courcelle E, Stuart D-I, Metoz F (1999) ESPript: analysis of multiple sequence alignments in PostScript. Bioinformatics 15:305-308.
4. Siddharthan R (2006) Sigma: multiple alignment of weakly-conserved non-coding DNA sequence. BMC Bioinformatics 7:143.
5. Morgenstern B (1999) DIALIGN2: improvement of the segment-to-segment approach to multiple sequence alignment. Bioinformatics 15:211-218.
6. Notredame C, Higgins D, Heringa J (2000) T-Coffee: A novel method for sequence alignments. J Mol Biol 302:205-217.
7. Corvey C et al. (2005) Carbon Source-dependent assembly of the Snf1p kinase complex in Candida albicans. J Biol Chem 280:25323-25330.
8. Staib P, Moran G-P, Sullivan D-J, Coleman D-C, Morschhauser J (2001) Isogenic strain construction and gene targeting in Candida dubliniensis. J Bacteriol 183:2859-2865.
9. Sanyal K, Baum M, Carbon J (2004) Centromeric DNA sequences in the pathogenic yeast Candida albicans are all different and unique. Proc Natl Acad Sci U S A 101:11374-11379.
10. Sullivan D-J, Westemeng T-J, Haynes K-A, Bennett D-E, Coleman D-C (1995) Candida dubliniensis sp. nov.: phenotypic and molecular characterization of a novel species associated with oral candidosis in HIV-infected individuals. Microbiology 141:1507-1521.
11. Wilson R-B, Davis D & Mitchell A-P (1999) Rapid hypothesis testing with Candida albicans through gene disruption with short homology regions. J Bacteriol 181:1868-1874.
12. Mishra P-K, Baum M, Carbon J (2007) Centromere size and position in Candida albicans are evolutionarily conserved independent of DNA sequence heterogeneity. Mol Genet Genomics 278:455-465.

Claims

1. A polynucleotide sequence consisting of SEQ ID NO 1, 2, 3, 4, 5, 6, 7 or 8.

2.-25. (canceled)

26. A set of polynucleotide primers comprising forward and reverse primers that hybridize to a centromeric region of Candida dubliniensis selected from the group consisting of Chromosome 1, Chromosome 2, Chromosome 3, Chromosome 4, Chromosome 5, Chromosome 6, Chromosome 7 and Chromosome R.

27. A set of 20 primers as claimed in claim 26, wherein the forward and the reverse primers are used for amplification of centromeric region of chromosome 1 of Candida dubliniensis.

28. A set of 20 primers according to claim 27 consisting of SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27 as forward primers and SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26 and 28 as corresponding reverse primers respectively.

29. A set of 14 primers as claimed in claim 26, wherein the forward and the reverse primers are used for amplification of centromeric region of chromosome 2 of Candida dubliniensis.

30. A set of 14 primers according to claim 29 consisting of SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41 as forward primers and SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42 as corresponding reverse primers respectively.

31. A set of 10 primers as claimed in claim 26, wherein the forward and the reverse primers are used for amplification of centromeric regions of chromosome 3 of Candida dubliniensis.

32. A set of 10 primers according to claim 31 consisting of SEQ ID NOS. 43, 45, 47, 49 and 51 as forward primers and SEQ ID NOS. 44, 46, 48, 50 and 52 as corresponding reverse primers respectively.

33. A set of 16 primers as claimed in claim 26, wherein the forward and the reverse primers are used for amplification of centromeric regions of chromosome 4 of Candida dubliniensis.

34. A set of 16 primers according to claim 33 consisting of SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67 as forward primers and SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68 as corresponding reverse primers respectively.

35. A set of 10 primers as claimed in claim 26, wherein the forward and the reverse primers are used for amplification of centromeric regions of chromosome 5 of Candida dubliniensis.

36. A set of 10 primers according to claim 35 consisting of SEQ ID NOS. 69, 71, 73, 75 and 77 as forward primers and SEQ ID NOS. 70, 72, 74, 76 and 78 as corresponding reverse primers respectively.

37. A set of 16 primers as claimed in claim 26, wherein the forward and the reverse primers are used for amplification of centromeric regions of chromosome 6 of Candida dubliniensis.

38. A set of 16 primers according to claim 37 consisting of SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93 as forward primers and SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94 as corresponding reverse primers respectively.

39. A set of 18 primers as claimed in claim 26, wherein the forward and the reverse primers are used for amplification of centromeric regions of chromosome 7 of Candida dubliniensis.

40. A set of 18 primers according to claim 39 consisting of SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111 as forward primers and SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112 as corresponding reverse primers respectively.

41. A set of 14 primers as claimed in claim 26, wherein the forward and the reverse primers are used for amplification of centromeric regions of chromosome R of Candida dubliniensis.

42. A set of 14 primers according to claim 41 consisting of SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126 as forward primers and SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125 as corresponding reverse primers respectively.

43. A process of identification of centromeric sequences of Candida dubliniensis, said method comprising steps of:

a) identifying putative Cse4p binding region; and

b) amplifying the putative Cse4p binding region to identify centromeric sequences of the Candida dubliniensis.

44. The process as claimed in claim 43, wherein the identification of putative Cse4p biding regions is carried out by sequence analysis and chromatin immunoprecipitation.

45. The process as claimed in claim 43, wherein the amplification of the putative Cse4p binding regions is carried out using any set of a forward primer selected from the group consisting of SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, respectively, for chromosome 1 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42, respectively, for chromosome 2 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 43, 45, 47, 49 and 51 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 44, 46, 48, 50 and 52, respectively, for chromosome 3 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68, respectively, for chromosome 4 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 69, 71, 73, 75 and 77 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 70, 72, 74, 76 and 78, respectively, for chromosome 5 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94, respectively, for chromosome 6 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112, respectively, for chromosome 7 of Candida dubliniensis and a forward primer selected from the group consisting of SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125, respectively, for chromosome R of Candida dubliniensis; or any combination of said primers thereof.

46. A method of distinguishing Candida dubliniensis from Candida albicans in a sample, said method comprising steps of

a) isolating DNA from the organism in the sample; and

b) amplifying the Cse4p binding regions with primers capable of amplifying said regions in the Candida dubliniensis to distinguish it from Candida albicans.

47. The method as claimed in claim 46, wherein the identification of putative Cse4p biding regions is carried out by sequence analysis and chromatin immunoprecipitation.

48. The method as claimed in claim 46, wherein the amplification of the putative Cse4p binding regions is carried out using any set of a forward primer selected from the group consisting of SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, respectively, for chromosome 1 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42, respectively, for chromosome 2 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 43, 45, 47, 49 and 51 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 44, 46, 48, 50 and 52, respectively, for chromosome 3 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68, respectively, for chromosome 4 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 69, 71, 73, 75 and 77 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 70, 72, 74, 76 and 78, respectively, for chromosome 5 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94, respectively, for chromosome 6 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112, respectively, for chromosome 7 of Candida dubliniensis and a forward primer selected from the group consisting of SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125, respectively, for chromosome R of Candida dubliniensis; or any combination of said primers thereof.

49. A kit for identification of Candida dubliniensis comprising a set of primers having SEQ ID NOS. 9 to 126.

50. The kit as claimed in claim 49, wherein the amplification of the putative Cse4p binding regions is carried out using any set of a forward primer selected from the group consisting of SEQ ID NOS. 9, 11, 13, 15, 17, 19, 21, 23, 25 and 27 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, respectively, for chromosome 1 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 29, 31, 33, 35, 37, 39 and 41 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 30, 32, 34, 36, 38, 40 and 42, respectively, for chromosome 2 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 43, 45, 47, 49 and 51 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 44, 46, 48, 50 and 52, respectively, for chromosome 3 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 53, 55, 57, 59, 61, 63, 65 and 67 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 54, 56, 58, 60, 62, 64, 66 and 68, respectively, for chromosome 4 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 69, 71, 73, 75 and 77 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 70, 72, 74, 76 and 78, respectively, for chromosome 5 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 79, 81, 83, 85, 87, 89, 91 and 93 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 80, 82, 84, 86, 88, 90, 92 and 94, respectively, for chromosome 6 of Candida dubliniensis; a forward primer selected from the group consisting of SEQ ID NOS. 95, 97, 99, 101, 103, 105, 107, 109 and 111 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 96, 98, 100, 102, 104, 106, 108, 110 and 112, respectively, for chromosome 7 of Candida dubliniensis and a forward primer selected from the group consisting of SEQ ID NOS. 114, 116, 118, 120, 122, 123 and 126 and its corresponding reverse primer selected from the group consisting of SEQ ID NOS. 113, 115, 117, 119, 121, 124 and 125, respectively, for chromosome R of Candida dubliniensis; or any combination of said primers thereof.