EP1007719A1 - Methods of making an rnp particle having nucleotide integrase activity - Google Patents
Methods of making an rnp particle having nucleotide integrase activityInfo
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- EP1007719A1 EP1007719A1 EP98924888A EP98924888A EP1007719A1 EP 1007719 A1 EP1007719 A1 EP 1007719A1 EP 98924888 A EP98924888 A EP 98924888A EP 98924888 A EP98924888 A EP 98924888A EP 1007719 A1 EP1007719 A1 EP 1007719A1
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- rna
- sequence
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- C—CHEMISTRY; METALLURGY
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- C12N9/00—Enzymes; Proenzymes; Compositions thereof; Processes for preparing, activating, inhibiting, separating or purifying enzymes
- C12N9/14—Hydrolases (3)
- C12N9/16—Hydrolases (3) acting on ester bonds (3.1)
- C12N9/22—Ribonucleases RNAses, DNAses
Definitions
- Nucleotide integrases are molecular complexes that are capable of cleaving nucleic acid substrates at specific recognition sites and of concomitantly inserting nucleic acid molecules into the nucleic acid substrate at the cleavage site. Thus, nucleotide integrases are useful tools, particularly for genome mapping, genetic engineering and disrupting the synthesis of gene products. Structurally, nucleotide integrases are ribonucleoprotein (RNP) particles that comprise an excised, group II intron RNA and a group II intron-encoded protein, which is bound to the group II intron RNA.
- RNP ribonucleoprotein
- nucleotide integrases are made by isolating RNP particles that have nucleotide integrase activity from source organisms which comprise a DNA molecule that encodes both the RNA and protein subunits of the nucleotide integrase.
- source organisms which comprise a DNA molecule that encodes both the RNA and protein subunits of the nucleotide integrase.
- the source organisms are mutagenized.
- the mutagenesis is a laborious, multistep process.
- this process yields limited quantities of the nucleotide integrase.
- nucleotide integrases which are not laborious and which permit the nucleotide integrase to be readily modified from the wild type. Methods which yield at least microgram quantities of substantially pure nucleotide integrases are especially desirable.
- the present invention provides new, improved, and easily manipulable methods for making nucleotide integrases.
- the nucleotide integrase is prepared by introducing a DNA molecule which comprises a group II intron DNA sequence into a host cell.
- the DNA molecule further comprises a sequence which encodes a tag that facilitates isolation of RNP particles having nucleotide integrase activity from the host cell.
- the tag sequence is linked to the open reading frame (ORF) sequence of the group II intron DNA.
- the group II intron DNA sequence is then expressed in the host cell such that RNP particles having nucleotide integrase activity are formed in the cell.
- Such RNP particles comprise an excised group II intron RNA molecule and a group II intron-encoded protein, both of which are encoded by the introduced DNA molecule. Thereafter, the RNP particles having nucleotide integrase activity are isolated from the cell.
- the nucleotide integrase is prepared by combining in vitro an excised, group II intron RNA, referred to hereinafter as "exogenous RNA", with a group II intron-encoded protein.
- exogenous RNA is prepared by in vitro transcription of a DNA molecule which comprises a group II intron sequence.
- the group II intron-encoded protein is made by introducing into a host cell a DNA molecule that comprises at least the open reading frame sequence of a group II intron and then expressing the open reading frame sequence in the host cell.
- the DNA molecule may comprise the open reading frame sequence operably linked to a promoter, preferably an inducible promoter.
- the cell is fractionated and the protein is recovered and combined in vitro with the exogenous RNA to provide RNP particles having nucleotide integrase activity.
- the DNA molecule may comprise a group II intron sequence that encodes both a group II intron RNA as well as a group II intron encoded protein.
- the DNA molecule is then expressed in the host cell to provide RNP particles that comprise the group II intron-encoded protein bound to the group II intron RNA.
- the RNP particles comprising the group II intron-encoded protein and the group II intron RNA are isolated from the cell and treated with a nuclease to remove the RNA and to provide the group II-intron encoded protein.
- the group II intron-encoded protein is then combined in vitro with the exogenous RNA to provide RNP particles having nucleotide integrase activity.
- the present invention also relates to isolated RNP particles having nucleotide integrase activity.
- Figure 1 depicts the interaction at the target site between the EBS1 and EBS2 sequences of the group II intron RNA 2 of the S. cerevisiae mitochondrial COX1 gene, hereinafter referred to as the "aI2" RNA, with the IBS1 and IBS2 sequences of a nucleic acid substrate.
- the cleavage site is represented by an arrow.
- Figure 2 is a schematic representation of the domains in three representative group II- intron encoded proteins, namely the protein which is encoded by the ORF sequence of the group II intron 2 of the S. cerevisiae mitochondrial CO ⁇ 7 gene, the group II intron 2 of the M. polymorpha mitochondrial COX1 gene, and the group II intron 1 of the N. tabacum chloroplast trnK gene. ___
- Figure 3 is the plasmid map of pETLtrA19.
- Figure 4 shows the nucleotide sequence of the 2.8 kb Hindlll fragment that is present in pETLtrA19 and that includes the Ll.ltrB intron D ⁇ A sequence and portions of the nucleotide sequence of the flanking exons ItrBEl and ltrBE2, SEQ. ID. NO. 1., the nucleotide sequence of the LtrA open reading frame, SEQ. ID. NO. 2, and the amino acid sequence of the LtrA protein, SEQ. ID. NO 3.
- Figure 5 is the plasmid map of plasmid pETLtrAl-1.
- Figure 6 is a schematic representation of the inserts in pLE12, pETLtrA19 and pETLtrAl-1.
- Figure 7A is the sequence of the sense strand of a double-stranded D ⁇ A substrate
- SEQ. ID. ⁇ . 4 which is cleaved by R ⁇ P particles that comprise a wild-type excised
- Figure 7B is the sequence of the sense strand of a double stranded D ⁇ A substrate which is cleaved by R ⁇ P particles that comprise an excised
- Ll.ltrB intron R ⁇ A having a modified EBS1 sequence and an LtrA protein.
- Figure 8a is a schematic depiction of the substrate which is cleaved by R ⁇ P particles comprising the wild-type Ll.ltrB intron R ⁇ A and the LtrA protein
- Figure 8b shows the IBS1 and IBS2 sequences of the substrate and the cleavage sites of the double-stranded D ⁇ A substrate which is cleaved by these R ⁇ P particles.
- nucleotide integrases are endonucleases that are capable of cleaving nucleic acid substrates at specific recognition sites and of concomitantly inserting nucleic acid molecules into the substrate at the cleavage site.
- nucleotide integrases are ribonucleoprotein (RNP) particles that comprise an excised, group II intron RNA and a group II intron-encoded protein, which is bound to the excised group II intron RNA.
- RNP ribonucleoprotein
- Excised group II intron RNA refers to the RNA that is, or that is derived from, an in vitro or in vivo transcript of the group II intron DNA and that lacks flanking exon sequences at the 5' end and the 3' end of the intron sequence.
- the excised, group II intron RNA typically has six domains and a characteristic secondary and tertiary structure, which is shown in Saldahana et al., 1993, Federation of the American Society of Experimental Biology Journal, Vol 7 pi 5-24, which is specifically incorporated herein by reference.
- Domain IV of the group II intron RNA contains the open reading frame ("ORF") nucleotide sequence which encodes the group II intron encoded protein.
- the excised group II intron RNA also has two sequences in domain I which are capable of hybridizing with two sequences in the target site of the intended nucleic acid substrate.
- the first sequence referred to hereinafter as the "EBS1” sequence
- the second sequence referred to hereinafter as the "EBS2” sequence
- IBS2 is capable of hybridizing with a sequence, hereinafter referred to as the "IBS2" sequence, which is upstream of the IBS1 sequence.
- the excised group II intron RNA has a wild-type sequence, i.e. a sequence which is identical to the sequence of a group II intron RNA that is found in nature, or the excised group II intron RNA has a modified sequence, i.e. a sequence which is different from the sequence of group II intron RNA molecules that are found in nature.
- the EBS1 sequence typically is complementary to a sequence of about 5-7 nucleotides, hereinafter referred to as the "first set" , which is located at the 3' end of the exon that is joined to the 5' end of the intron in the gene.
- the EBS2 sequence of the wild-type group II intron RNA typically is complementary to a sequence of about 5-7 nucleotides in the 5' exon, hereinafter referred to as the "second set" , which is upstream, typically immediately upstream, of the first set.
- the EBS1 and EBS2 sequences of a wild-type group II intron RNA can usually be predicted by finding sequences in domain I of the intron that are complementary to the first set and second set of nucleotides in the 5' exon.
- EBS1 comprises 7 nucleotides, is located at position 3132-3138 (numbered according to Mills et al., 1996, J. Bact, 178, 3531-3538), and has the sequence GUUGUGG.
- EBS2 of the wild-type Ll.ltrB intron RNA comprises 6 nucleotides, is located at positions 3076-3081 and has the sequence AUGUGU.
- EBS1 comprises 6 nucleotides, is located at position 426-431 (numbered according to Bonitz et al, 1980, J. Biol. Chem.: 255, 11927-11941), and has the sequence CGUUGA.
- EBS2 of the wild-type all RNA comprises 6 nucleotides, is located at positions 376-381 and has the sequence ACAAUU. In the wild-type group II intron RNA 2 of the S.
- EBS1 comprises 6 nucleotides, is located at position 2985-2990 (numbered according to Bonitz et al., 1980, J. Biol. Chem.: 255, 11927-11941) and has the sequence AGAAGA.
- EBS2 of the wild-type aI2 RNA comprises 7 nucleotides, is located at positions 2935-29410, and has the sequence UCAUUAA.
- the interaction between EBS1 and EBS2 of the wild-type aI2 RNA with its intended substrate is depicted in Figure 1.
- the excised group II intron RNA may also have a sequence different from a group II intron RNA that is found in nature, and thus be a modified, excised group II intron RNA.
- Modified excised group II intron RNA molecules include, for example, group II intron RNA molecules that have nucleotide base changes or additional nucleotides in the internal loop regions of the group II intron RNA, preferably the internal loop region of domain IV and group II intron RNA molecules that have nucleotide base changes in the sequences of EBS1 and/or EBS2.
- Nucleotide integrases in which the group II intron RNA has nucleotide base changes in the sequences of EBS1 or EBS2, as compared to the wild type typically have altered specificity for the intended nucleic acid substrate.
- the group II intron-encoded protein has an X domain, a reverse transcriptase domain, and, preferably, a Zn domain.
- the X domain of the protein has a maturase activity.
- the Zn domain of the protein has Zn 2+ finger-like motifs.
- a group II intron-encoded protein includes modified group II intron-encoded proteins that have additional amino acids at the N terminus, or C terminus, or alterations in the internal regions of the protein as well as wild-type group II intron-encoded proteins. The domains of three representative group II intron-encoded proteins are depicted in Figure 2.
- the RNP particles having nucleotide integrase activity cleave single-stranded RNA molecules, single-stranded DNA molecules, and double-stranded DNA molecules.
- the RNP particles having nucleotide integrase activity also insert the group II intron RNA subunit of the RNP particle into the cleavage site.
- RNP particles having nucleotide integrase activity both cleave nucleic acid substrates and insert nucleic acid molecules into the cleavage site.
- the nucleotide integrase inserts the group II intron RNA into the first strand, i.e., the strand that contains the IBS1 and IBS2 sequences, of the cleaved DNA substrate and, preferably, a cDNA molecule into the second strand of the cleaved DNA substrate.
- the excised group II intron RNA subunit of the nucleotide integrase catalyzes cleavage of the single-stranded-substrates and the first strand of the double- stranded DNA substrate.
- the cleavage that is catalyzed by the excised group II intron RNA also results in the insertion, either partially or completely, of the excised group II intron RNA into the cleavage site, i.e. between nucleotide +1, which is immediately downstream of the cleavage site, and nucleotide -1, which is immediately upstream of the cleavage site.
- the group II intron- encoded protein subunit catalyzes cleavage of the second strand of the double-stranded DNA substrate.
- the second strand of the double stranded DNA substrate is cut at a position from about 9 to about 11 base pairs downstream of the cleavage site in the first strand, i.e.
- the RNP particle having nucleotide integrase activity is active under conditions that are similar to physiological conditions.
- the EBS1 and EBS2 sequences of the group II intron RNA of the nucleotide integrase have at least 90% complementarity, preferably full complementarity, with the IBS1 and IBS2 sequences, respectively, of the intended substrate.
- nucleotide base changes be made in the non-complementary EBS sequences.
- the nucleotide delta which immediately precedes the first nucleotide of EBS 1 be complementary to the nucleotide at +1 in the target site.
- the group II intron RNA is modified to contain a delta nucleotide which is complementary to the nucleotide at +1 on the sense strand of the substrate.
- the target site has a sequence that is recognized by the group II intron-encoded protein of the nucleotide integrase.
- cleavage of a double-stranded DNA substrate is achieved with a nucleotide integrase comprising a wild-type Ll.ltrB RNA and LtrA protein if the first strand of the substrate contains the sequence, 5'-TCGATCGTGAACACATCCATAACC'3', SEQ.ID.NO. . which represents the sequence from -23 to +1 in the target site of the first strand.
- RNP particles having nucleotide integrase activity are made by introducing an isolated DNA molecule which comprises a group II intron DNA sequence into a host cell.
- the DNA molecule further comprises an IBS1 sequence and an IBS2 sequence just upstream of the 5' end of the group II intron DNA sequence to allow splicing of the group II intron RNA from a transcript of the group II intron DNA sequence.
- Suitable DNA molecules include, for example, viral vectors, plasmids, and linear DNA molecules.
- the group II intron DNA sequence is expressed in the host cell such that excised RNA molecules encoded by the introduced group II intron DNA sequence and protein molecules encoded by introduced group II intron DNA sequence are formed in the cell.
- the excised group II intron RNA and group II intron-encoded protein are combined within the host cell to produce an RNP particle having nucleotide integrase activity.
- the introduced DNA molecule also comprises a promoter, more preferably an inducible promoter, operably linked to the group II intron DNA sequence.
- the DNA molecule further comprises a sequence which encodes a tag to facilitate isolation of the RNP particles having nucleotide integrase activity, such as, for example, an affinity tag and/or an epitope tag.
- the tag sequences are at the 5' or 3' end of the open reading frame sequence.
- Suitable tag sequences include, for example, sequences which encode a series of histidine residues, the Herpes simplex glycoprotein D, i.e., the HSV antigen, or glutathione S-transferase.
- An especially suitable tag is a sequence which encodes the intein from the S. cerevisiae VMA1 gene linked to the chitin binding domain from Bacillus circulans.
- the introduced DNA molecule also comprises nucleotide sequences that encode a replication origin and a selectable marker.
- the introduced DNA molecule comprises sequences that encode molecules that modulate expression, such as for example T7 lysozyme.
- the DNA molecule comprising the group II intron sequence is introduced into the host cell by conventional methods, such as, by cloning the DNA molecule into a vector and by introducing the vector into the host cell by conventional methods, such as electroporation or by CaCl 2 -mediated transformation procedures.
- the method used to introduce the DNA molecule depends on the particular host cell used. Suitable host cells are those which are capable of expressing the group II intron DNA sequence. Suitable host cells include, for example, heterologous or homologous bacterial cells, yeast cells, mammalian cells, and plant cells. In those instances where the host cell genome and the group II intron DNA sequence use different genetic codes, it is preferred that the group II intron DNA sequence be modified to comprise codons that correspond to the genetic code of the host cell.
- the group II intron DNA sequence typically, is modified by using a DNA synthesizer or by in vitro site directed mutagenesis, such as by PCR mutagenesis, to prepare a group II intron DNA sequence with different codons.
- DNA sequences that encode the tRNA molecules which correspond to the genetic code of the group II intron are introduced into the host cell.
- DNA molecules which comprise sequences that encode factors that assist in RNA or protein folding, or that inhibit RNA or protein degradation are also introduced into the cell.
- transformed host cell means a host cell that has been genetically engineered to contain and express additional DNA, primarily heterologous DNA, and is not limited to cells which are cancerous. Then the RNP particles having nucleotide integrase activity are isolated from the transformed host cells.
- the RNP particles having nucleotide integrase activity are isolated, preferably by lysing the transformed cells, such as by mechanically and/or enzymatically disrupting the cell membranes of the transformed cell. Then the cell lysate is fractionated into an insoluble fraction and soluble fraction. Preferably, an RNP particle preparation is isolated from the soluble fraction.
- the RNP particle preparations include the RNP particles having nucleotide integrase activity as well as ribosomes, mRNA and tRNA molecules. Suitable methods for isolating RNP particle preparations include, for example, centrifugation of the soluble fraction through a sucrose cushion.
- the RNP particles are further purified from the RNP particle preparation or from the soluble fraction by, for example, separation on a sucrose gradient, or a gel filtration column, or by other types of chromatography.
- the RNP particles having nucleotide integrase activity are purified from the particle preparation by affinity chromatography on a matrix which recognizes and binds to the tag.
- NiNTA SuperflowTH from Qiagen, Chatsworth CA, is suitable for isolating RNP particles having nucleotide integrase activity when the group II intron-encoded protein has a histidine tag.
- the a system which employs a chitin column and an intein and chitin binding domain tag on the group II intron-encoded protein results in the production of RNP particles that are substantially pure, i.e., the intron encoded protein represents at least 95% of the protein in the RNP particles eluted from the column.
- the latter system is particularly suitable for isolating RNP particles having nucleotide integrase activity.
- the nucleotide integrase is formed by combining an isolated exogenous RNA with an isolated group II intron-encoded protein in vitro to provide a reconstituted RNP particle having nucleotide integrase activity.
- the exogenous RNA is made by in vitro transcription of the group II intron DNA.
- the exogenous RNA may be made by in vitro transcription of the group II intron DNA only, i.e. the transcript lacks flanking exon sequences.
- the exogenous RNA is made by in vitro transcription of the group II intron DNA and the DNA of all, or portions, of the flanking exons to produce an unprocessed transcript which contains the group II intron RNA and the RNA encoded by the flanking exons or portions thereof. Then the exogenous RNA is spliced from the unprocessed transcript.
- the purified group II intron-encoded protein is prepared by introducing into a host cell an isolated DNA molecule that comprises at least the open reading frame sequence of a group II intron.
- the DNA molecule may comprise a group II intron ORF sequence operably linked to an inducible promoter.
- the DNA molecule may comprise a group II intron DNA sequence.
- the introduced DNA molecule also comprises a sequence at the 5' or 3' end of the group II intron ORF sequence which, when expressed in the host cell, provides an affinity tag or epitope on the N-terminus or C-terminus of the group II intron-encoded protein.
- the DNA molecule may comprise at the 5' or 3' end of the ORF, for example, a sequence which encodes a series of histidine residues, or the HSV antigen, glutathione-S-transferase, or an intein linked to a chitin binding domain.
- the DNA molecule also comprises nucleotide sequences that encode a replication origin and a selectable marker.
- the ORF sequence is then expressed in the host cell, preferably by adding a molecule which induces expression, to provide a host cell that contains RNP particles comprising the group II intron-encoded protein associated with endogenous nucleic acids, particularly endogenous RNA molecules. Then the transformed cell is lysed, and preferably fractionated into a soluble fraction and an insoluble fraction. The RNP particles comprising the protein and the endogenous RNA are then isolated, preferably from the soluble fraction, preferably by using methods such as affinity chromatography.
- the RNP particles are then incubated with the exogenous RNA, preferably in a buffer, to allow the exogenous RNA to displace the associated RNA molecules and to form RNP particles having nucleotide integrase activity.
- the RNP particles are treated with a nuclease to remove the RNA that is associated with the group II intron encoded protein prior to incubation of the protein preparation with the exogenous RNA.
- the RNP particles may be treated with the nuclease by adding the nuclease to the soluble fraction. Alternatively, the RNP particles may be treated with the nuclease after isolation of the RNP particles from the soluble fraction.
- RNP particles comprising a group II intron- encoded protein associated with an excised group II intron RNA that encodes the protein are produced.
- the group II intron-encoded protein is not associated with an excised, group II intron RNA that encodes the protein.
- the RNP particles that are produced when a splicing-defective group II intron DNA sequence is introduced and expressed in a host cell comprise other types of RNA molecules, such as for example, unspliced group II intron RNA molecules that encode the protein, ribosomal RNA molecules, mRNA molecules, tRNA molecules or other nucleic acids.
- the transformed cell is lysed, and preferably fractionated into a soluble fraction and an insoluble fraction.
- the RNP particles comprising the protein are then isolated, preferably from the soluble fraction, preferably by using methods such as affinity chromatography.
- the isolated RNP particles are then treated with a nuclease that degrades all of the endogenous RNA molecules.
- the RNP particles are treated with a nuclease which can be chemically inactivated, such as for example, micrococcal nuclease.
- the group II intron-encoded protein preparation is then combined with the exogenous RNA, preferably in a buffer, to allow formation of RNP particles having nucleotide integrase activity
- RNP particles having nucleotide integrase activity are substantially pure, i.e., at least 95% of the protein in the final RNP particle preparation is the group II intron-encoded protein.
- the present methods also offer the further advantage of permitting the sequences of the RNA component and the protein component of the nucleotide integrase to be readily modified.
- the nucleotide integrases are modified by introducing nucleotide base changes, deletions, or additions into the group II intron RNA by PCR mutagenesis of the group II intron.
- the following examples of methods for preparing a group II intron-encoded protein and for preparing nucleotide integrases are included for purposes of illustration and are not intended to limit the scope of the invention.
- RNP particles having nucleotide integrase activity and comprising an excised RNA that is encoded by the Ll.ltrB intron of a lactococcal cojugative element pRSOl of Lactococcus lactis and the protein encoded by the ORF of the Ll.ltrB intron were prepared by transforming cells of the BLR(DE3) strain of the bacterium Escherichia coli, which has the recA genotype, with the plasmid pETLtrA19.
- Plasmid pETLtrA19 which is schematically depicted in Figure 3, comprises the DNA sequence for the group II intron Ll.ltrB from Lactococcus lactis, shown as a thick line, positioned between portions of the flanking exons ItrBEl and /trBE2, shown as open boxes.
- pETLtrA19 also comprises the DNA sequence for the T7 RNA polymerase promoter and the T7 transcription terminator. The sequences are oriented in the plasmid in such a manner that the ORF sequence, SEQ. ID. NO. 2, within the Ll.ltrB intron is under the control of the T7 RNA polymerase promoter.
- the ORF of the Ll.ltrB intron shown as an arrow box, encodes the protein LtrA.
- the sequence of the Ll.ltrB intron and the flanking exon sequences present in pETLtrA19 are shown in Figure 4 and SEQ. ID. NO. 1.
- Vertical lines in Figure 4 denote the junctions between the intron and the flanking sequences.
- the amino acid sequence of the LtrA protein, SEQ. ID. NO. 3 is shown under the ORF sequence, SEQ. ID. NO. 2, in Figure 4.
- EBS 1 and EBS2 include nucleotides 457 through 463 (EBS1), nucleotides 401 through 406 (EBS2a) , and nucleotides 367 through 372 (EBS2b). Domain IV is encoded by nucleotide 705 to 2572.
- pETLtrA19 was prepared first by digesting pLE12, which was obtained from Dr. Gary Dunny from the University of Minnesota, with Hindlll and isolating the restriction fragments on a 1% agarose gel.
- isopropyl-beta- D-thiogalactoside was added to the culture to a final concentration of 1 mM and incubation was continued for 3 hours. Then the entire culture was harvested by centrifugation at 2,200 x g, 4°C, for 5 minutes. The bacterial pellet was washed with 150 mM NaCl and finally resuspended in 1/20 volume of the original culture in 50 mM Tris, pH 7.5, 1 mM EDTA, 1 mM DTT, and 10% (v/v) glycerol (Buffer A)and 2 mg/ml lysozyme. Bacteria were frozen at -70°C.
- lysate To produce a lysate the bacteria were thawed and frozen at -70°C three times. Then 4 volumes of 500 mM KCl, 50 mM CaCl 2 , 25 mM Tris, pH 7. 5, and 5 mM DTT (HKCTD) were added to the lysate and the mixture was sonicated until no longer viscous, i.e. for about 5 seconds or longer. The lysate was fractionated into a soluble fraction and insoluble fraction by centrifugation at 14,000 x g, 4°C, for 15 minutes.
- the result is a preparation of partially-purified RNP particles that comprise the excised Ll.ltrB intron RNA and the LtrA protein
- the yield of RNP particles was 25 to 50 O.D. 260 units ( ⁇ 16 ⁇ g protein) per 100 ml culture, with 1 O.D. 260 units of RNPs containing 0.3 to 3 ⁇ g LtrA protein.
- the partially-purified RNPs were further purified by an additional centrifugation through a 1.85 M sucrose cushion, as described above.
- RNP particles having nucleotide integrase activity and comprising the LtrA protein and the excised Ll.ltrB intron RNA were prepared as described in example 1 except the plasmid pETLtrA19 was used to transform cells of the BL21(DE3) strain of E. coli. The transformed cells were fractionated into a soluble fraction and an insoluble fraction as described in Example 1 to provide a preparation of RNP particles having nucleotide integrase activity
- RNP particles having nucleotide integrase activity and comprising the LtrA protein and the excised Ll.ltrB intron RNA were prepared by transforming cells of the E. coli strains BLR(DE3) with pETLtrA19 as described in Example 1 except that the transformed E. coli were grown in SOB medium and shaken at 300 rpm during the 3 hour incubation. The transformed cells were fractionated into a soluble fraction and an insoluble fraction as described in Example 1 to provide a preparation of RNP particles having nucleotide integrase activity
- Example 4 RNP particles having nucleotide integrase and comprising the LtrA protein and the excised Ll.ltrB intron RNA were prepared as described above in sample 1 except that the plasmid pETLtrA19 was used to transform cells of the E. coli strain BL21(DE3). The cells were also transformed with plasmid pOM62 which is based on the plasmid pACYC184 and has an approximately 150 bp insert of the argU(dnaY) gene at the EcoRI site. The argil gene encodes the tRNA for the rare arginine codons AGA and AGG. The LtrA gene contains 17 of the rare arginine codons. The transformed cells were grown in SOB medium and fractionated into a soluble fraction and an insoluble fraction as described in Example 1 to provide a preparation of RNP particles having nucleotide integrase activity.
- RNP particles having nucleotide integrase and comprising the excised Ll.ltrB intron RNA and the LtrA protein were prepared by transforming host cells as described above in Example 1 except that the LtrA ORF was tagged at the C-terminus with a His 6 affinity tag and an epitope derived from the Herpes simplex virus glycoprotein D. The tag is used to facilitate isolation of the RNP particles.
- the plasmid adding the tags was made in two steps by using PCR. In the first step, a fragment containing exon 1 and the LtrA ORF was amplified using primers LtrAexl.Xba having the sequence 5' TCACCTCATCTAGACATTTTCTCC 3', SEQ. ID. NO.
- the resulting plasmid plntermediate-C fuses the 3' end of the LtrA ORF to an HSV tag and His 6 purification tag, both of which are present on the vector pET-27b(+).
- intron sequences 3' to the ORF and exon 2 are amplified using pLE12 as a template and the 5' primer LtrAConZnl, having the sequence 5'CACAAGTGATCATTTACGAACG 3', SEQ. ID. No. 7 and the 3' primer LtrAex2, which has the sequence 5'TTGGGATCCTCATAAGCTTT GCCGC 3', SEQ. ID. NO. 8.
- the PCR product is cut with Bell and BamWl, the resulting fragment filled in, gel purified and cloned into plntermediate-C, which has been cleaved with Bpu ⁇ ⁇ Q2 ⁇ and filled in.
- the resulting plasmid is designated pC-hisLtrA19.
- Cells of the BLR(DE3) strain of E. coli were transformed as described in example 1 with plntermediate-C and cultured at 37°C for 3 hours in SOB medium as described in example 3.
- the cells were also fractionated into a soluble fraction, which contains RNP particles having nucleotide integrase activity, and an insoluble fraction as described in example 1.
- the RNP particles were further purified as described in example 1.
- RNP particles having nucleotide integrase activity and comprising an excised Ll.ltrB intron RNA and the LtrA protein were prepared by transforming host cells as described above in example 1 except that the LtrA ORF was tagged at the N-terminus with a His 6 affinity tag and the epitope tag XPRESSTM which was obtained from Invitrogen, San Diego, CA. The tag is used to facilitate isolation of the RNP particles.
- the plasmid adding the tags was made in two steps by using PCR. In the first step, a fragment was made in two steps by using PCR mutagenesis.
- the LtrA ORF and 3' exon were amplified and BamHl sites were appended to both the 5' an 3' end of the LtrA ORF using pLE12 as a substrate and the following pair: 5' primer N-LtrA 5', having the sequence 5'CAAAGGATCCGATGAAACCA ACAATGGCAA 3', SEQ. ID. NO. 9; and the 3' primer LtrAex2, SEQ. ID. NO 8.
- the PCR product was cut with E ⁇ mHl and the resulting restriction fragment was gel purified and cloned into the BamHl site of plasmid pRSETB obtained from Invitrogen, San Diego, CA.
- the resulting plasmid plntermediate- ⁇ fuses the ⁇ terminus of the LtrA ⁇ RF to a His 6 purification tag, and adds an XPRESSTM epitope tag from the vector.
- the 5' exon and Ll.ltrB intron sequences 5' to the ORF were amplified using pLE12 as a substrate and the 5' primer ⁇ deLTR5, having the sequence 5'AGTGGCTTCCATATGCTTGGTCATCACCTCATC 3', SEQ. ID. No. 10 and 3' primer NdeLTR3', which has the sequence 5'
- the PCR product was cut with Nde I, the fragment gel purified and cloned into plntermediate-N, which had also been cut with Nde I. Plasmids were screened for the orientation of the insert, and those oriented such that the 5' exon was proximal to the T7 promoter were used to transform the host cells.
- the resulting plasmid pFinal-N expresses a message under the control of the T7 polymerase promoter which comprises the El and E2 portions of the exons 1 LtrBEl and LtrBEl, and the LtrA ORF fused at the 5'end with an His 6 purification tag and the XPRESSTM epitope tag.
- Cells of the BLR(DE3) strain of E. coli were transformed as described in example 1 with plntermediate-N and cultured at 37°C f or 3 hours in SOB medium as described in example 3.
- the cells were also fractionated into a soluble fraction, which contains RNP particles having nucleotide integrase activity, and an insoluble fraction as described in example 1.
- the RNP particles were further purified as described in example 1.
- RNP particles having nucleotide integrase activity and comprising an excised Ll.ltrB intron RNA and the LtrA protein were prepared as described by transforming host cells as described above in example 1 except that the LtrA ORF was tagged at the C-terminus with an intein from Saccharomyces cerevisiae VMA1 gene and the chitin binding domain (CBD) from Bacillus circulans .
- the tag was used to facilitate purification of the RNP particles and was added using components of the ImpactTM purification system obtained from New England Biolabs, Beverly, MA.
- a plasmid adding the tags was made in two steps by using PCR.
- the LtrA ORF was amplified by PCR using pETLtrA19 as template and using 5' primer LtrAexpr, 5'-AAACCTCCATATGAAACCAACAATG-3', SEQ. ID. NO. and 3' primer ltrimpact: 5 AACTTCCCGGGCTTGTGTTTATGAATCAC-3',
- PCR product was cut with Nde and Smal and cloned into pCYB2, obtained from New England Biolabs, Beverly, MA, and cleaved with the same enzymes. Colonies were screened for inserts and two independent colonies with the desired insert were retained to yield pLHPInt21 and pLHPInt22.
- pLHPInt21 was cleaved with Pstl, the overhangs repaired with T4 DNA polymerase in the presence of 0.2 mM dNTPs. The DNA was then phenol extracted, ethanol precipitated and then partially digested with Pml I.
- the approximately 1580 bp Pm ⁇ l- Pst I fragment was cloned into pETLtrA19 digested with Pml I.
- the clones with correct insert were screened and one oriented such that the intein is fused to the C terminus of the LtrA ORF was called pLIlInt.
- the resulting construct expresses the
- Ll.ltrB intron and fuses the LtrA ORF with the sequences that encode VMAI intein and CBD.
- the cultures were harvested, washed with 150 mM NaCl lOmM Tris-HCl (pH 7.5), and repelleted and stored in 50 ml of Buffer I (20 mM Tris-HCl (pH8.0), 0.5 M NaCl, 0.1 mM EDTA, 0.1% NP-40).
- Buffer I 20 mM Tris-HCl (pH8.0), 0.5 M NaCl, 0.1 mM EDTA, 0.1% NP-40).
- the cells were broken by sonicating for 1 minute 3 times in_a Bronson sonicator at setting 7.
- the lysate was cleared by centrifugation at 12,000 x g for 30 minutes.
- the cleared lysate was loaded on a chitin affinity column equilibrated with Buffer I.
- the RNP particles comprising a tagged protein are retained on the column.
- RNP particles having nucleotide integrase activity and comprising the LtrA protein and an excised Ll.ltrB intron RNA having an altered EBSl sequence were prepared as described above in example 1 except that the cells were transformed and the RNP particles were made using pLIl-EBSl/-6C.
- the pLIl-EBSl/-6C construct which has a single nucleotide change G to C at position 6 in the EBSl (G3137C as based on Mills et al, 1996) sequence of the wild-type intron and a complementary change in the 5' exon at position -6 relative to the 5' splice site to permit splicing was constructed via two PCR steps.
- pETLtrA19 was subjected to PCR with primers OP2, 5'- GGATCGAGATCTCGATCCCG, SEQ. ID. NO. and IP11 : 5'CGCACGT TATCGATGTGTTCAC, SEQ. ID. NO. to introduce the single nucleotide change in the exon, and with primers IP4, 5'-TTATGGTTGTCGACTTATCTGTTATC, SEQ.ID.NO. . and OP1, OP1 : 5'-CTTCGAATACCGGTTCATAG, SEQ. ID. NO. to introduce the single nucleotide change in EBSl.
- the single nucleotide change in the IP4 primer introduces a Sail site in the EBSl sequence, which was subsequently used to identify the desired clones.
- the second PCR step was performed using the above two PCR products as Primers and pETLtrA19 DNA linearized with Bglll and BamHl as the template.
- the second PCR product was reamplified with flanking primers OP2 and OP1 using Pfu polymerase from Stratagene and digested with Eglll and BsrGl to yield a 554-bp fragment that was cloned between the BsrGl and Rglll sites of pETLtrA19.
- EXAMPLE 9 A partially-purified preparation of the LtrA protein, which is encoded by the ORF of the Ll.ltrB intron, using plasmid pETLtrAl-1 was prepared. Plasmid pETLtrAl-1 is _a_ derivative of pETLtrA19 and lacks exon 1 and the intron sequences upstream of the LtrA ORF. Accordingly, the LtrA ORF is directly downstream of the phage T7 promoter following the Shine-Dalgarno sequence in the plasmid. The plasmid map of pETLtrAl-1 is shown in Figure 5. pETLtrAl-1 was made by using the polymerase chain reaction to amplify the LtrA
- RNP particles having nucleotide integrase activity and comprising an excised, Ll.ltrB intron RNA which lacks the ORF and an LtrA protein was prepared by mixing an in vitro- synthesized intron RNA with an LtrA protein preparation that was made by digesting the RNP particles prepared as described above in example 1 with micrococcal nuclease (MN).
- MN micrococcal nuclease
- 1.0 O.D 260 of the RNP particle preparation were resuspended in 40 ⁇ l of 10 mM Tris, HC1, pH 7.5, 10 mM MgCl 2> 2.5 mM CaCl 2 , 5 mM DTT and incubated with 12 or 36 units of MN from Pharmacia for 10 minutes at 22° C , after which the MN was inactivated by addition of EGTA to 7.5 mM.
- the group II intron RNA was generated by in vitro transcription of pLI2- ⁇ ORF.
- pLI2- ⁇ ORF which has a large deletion in the intron ORF, was derived from pLI2 by inverse PCR with primers ⁇ ORFa: 5'- GGGGGGGCTAGCACGCGTCGCCACGTAATAAATATCTG GACG, SEQ. ID.
- RNA was linearized with BamHl and transcribed with phage T3 RNA polymerase, and the in v/tro-synthesized RNA (30 to 50 ⁇ g) was spliced for 60 min at 42°C in 100 ⁇ l of 1 M NH 4 C1, 100 mM MgCl 2 , and 50 mM Tris-HCl (pH 7.5). Prior to reconstitution, the RNA was heated to 85 to 90°C for 2 minutes, then stored on ice.
- RNP particles having nucleotide integrase and comprising the LtrA protein and an excised Ll.ltrB intron RNA having a kanamycin resistance gene inserted in domain IV in place of the LtrA ORF were prepared as described above in example 10 except that the RNA component was made using which replaces amino acids 39-573 of the LtrA ORF with a kan ⁇ gene, was constructed by cloning the 1,252-bp Sail fragment containing the kan ⁇ gene from pUK4K (Pharmacia, Piscataway, NJ) into the Mlul site of pL12-ORF by blunt-end ligation after filling in both the Sail and Mlul sites with Klenow polymerase (Life Technologies, Gaithersburg, MD)
- RNP particles lacking nucleotide integrase activity were prepared as described in Example 1 from cells of the BLR(DE3) strain of E. coli that had been transformed with plasmid pETl la, which lacks a group II intron. Accordingly, is these RNP particles do not comprise excised, group II RNA or group II intron-encoded proteins and therefore, do not have nucleotide integrase activity. Comparative Example B.
- RNP particles lacking nucleotide integrase activity were prepared as described in Example 1 from cells of the BLR(DE3) strain of E. coli that had been transformed with plasmid pETLtrA19FS, which comprises the sequence of an LtrA ORF having a frame shift 372 base pairs downstream from the initiation codon of the LtrA ORF. frame. Accordingly, the RNP particles contain a truncated LtrA protein, i.e. an LtrA protein lacking the Zn domain and, therefore, do not have nucleotide integrase activity. Characterization of the RNP particles of Examples 1 and 2. ,
- RNP particles containing the group II intron-encoded protein LtrA can be prepared by expression of the group II intron Ll.ltrB in a heterologous host cell.
- the reverse transcriptase activities of the RNP particles of examples 1 and 2 and the RNP particles of comparative examples A and B were assayed by incubating each of the RNP particle preparations with a poly(rA) template and oligo (dT, 8 ) as a primer.
- the RNP particles of examples 1 and 2 exhibited reverse transcriptase activity, while the RNP particles of comparative examples A and B exhibited no reverse transcriptase activity.
- the methods described in examples 1 and 2 are useful for preparing RNP particles that have reverse transcriptase activity.
- the reverse transcriptase activity that is present in nucleotide integrases allows incorporation of a cDNA molecule into the cleavage site of the double stranded DNA which is cut by the nucleotide integrase. Characterizing the Distribution and Yield of the LtrA Protein
- a portion of the insoluble fraction and soluble fraction of the lysates from the cells transformed and cultured according to the methods described in examples 1, 2, 3, 4 and 9 were subjected to SDS polyacrylamide gel electrophoresis. Following electrophoresis, the SDS gels were stained with Coomassie blue to compare the yield of the LtrA protein and the distribution of the 70 kDa LtrA protein prepared by the methods of examples 1, 2, 3, 4 and 9. As shown on the gel, more of the LtrA protein was found in the soluble fraction when the transformed BLR (DE3) cells were grown in SOB medium and shaken at 300 rpm than when the transformed BLR cells were grown in LB medium and shaken at 200 rpm.
- the total amount of LtrA protein produced by the transformed BLR cells that is the amount of LtrA in both the soluble and insoluble fractions, increased when, as described in example 4, a plasmid comprising the Ll.ltrB intron and a plasmid comprising argU(dnaY) gene were both introduced into the host cells, the LtrA protein which was expressed in cells transformed with a plasmid which lacks the 5' segment of the Ll.ltrB. intron, as described in example 9, was significantly more insoluble than the LtrA protein which was expressed in cells transformed with a plasmid that contained the 5 'segment of the intron as well as the LtrAORF.
- a portion of the insoluble fraction and soluble fractions of the lysates from the cells transformed and cultured according to the methods described in examples 5 and 6 and in comparative examples A and B were subjected to electrophoresis on duplicate SDS- polyacrylamide gels. One of the gels was stained with Coomassie blue and the proteins on the duplicate were transferred to nitrocellulose paper by Western blotting.
- a primary antibody to the HSV antigen and an alkaline phosphatase-labeled anti-mouse IgG secondary antibody were used in an enzyme-linked immunoassay to identify proteins carrying the HSV epitope or the XpressTM tag.
- the anti-HSV antibody and the anti-XpressTM tag antibody bound to a protein having a molecular weight of approximately 70 kDa, which is close to the calculated molecular weight of the LtrA protein.
- the HSV tagged LtrA protein and the XpressTM tagged LtrA protein were found in the soluble and insoluble fractions from cells transformed with pIntermediateC and pIntermediateN but not in the soluble fractions and insoluble fractions of cells transformed with pET27b(+) and pRSETB.
- the methods of examples 5 and 6 are useful for preparing an RNP particle comprising a tagged group II intron encoded protein.
- a portion of the RNP particle preparation of example 7 and comparative examples A and B were subjected to SDS polyacrylamide gel electrophoresis, which was subsequently stained with Coomassie Blue.
- a band of approximately 70 kDa, which corresponds to the predicted molecular weight of the LtrA protein was seen in the lanes containing aliquots of the RNP particles of Example 7 and was absent from the lanes containing the RNP particles prepared from comparative examples A and B.
- the quantity of LtrA protein in RNP particles in the eluant from the chitin column was estimated to be approximately 0.5 mg/liter of start culture.
- the LtrA protein in these RNP particles was approximately 95% pure. Accordingly, the method of claim 7 is highly preferred for making large amounts of highly purified RNP particles having nucleotide integrase activity.
- Nucleotide integrases are useful for cleaving RNA substrate, single-stranded DNA substrates and one or both strands of a double-stranded DNA substrate, catalyzing the attachment of the excised, group II intron RNA molecule to the RNA substrate, the single- stranded DNA substrate, and to the first strand, i.e. the strand that contains the IBS1 and IBS2 sequence, of the double-stranded DNA substrate. Nucleotide integrases also catalyze the formation of a cDNA molecule on the second strand, i.e. the strand that is complementary to the first strand, of a cleaved double-stranded DNA substrate.
- the nucleotide integrases are useful analytical tools for determining the location of a defined sequence in a double- stranded DNA substrate.
- the simultaneous insertion of the nucleic acid molecule into the first strand of DNA permits tagging of the cleavage site of the first strand with a radiolabeled molecule.
- the automatic attachment of an RNA molecule onto one strand of the DNA substrate permits identification of the cleavage site through hybridization studies that use a probe that is complementary to the attached RNA molecule.
- An attached RNA molecule that is tagged with a molecule such as biotin also enables the cleaved DNA to be affinity purified.
- cleavage of RNA molecules, single stranded DNA molecules, and one or both strands of a double stranded DNA molecule and the concomitant insertion of a nucleotide sequence into the cleavage site permits incorporation of new genetic information or a genetic marker into the cleavage site, as well as disruption of the cleaved gene.
- the nucleotide integrases are also useful for rendering the substrate DNA nonfunctional or for changing the characteristics of the RNA and protein encoded by the substrate DNA.
- RNP particles having nucleotide integrase activity can be used to cleave nucleic acid substrates at a wide range of temperatures, good results are obtained at a reaction temperature from about 30°C to about 42°C, preferably from about 30° to about 37°C.
- a suitable reaction medium contains a monovalent cation such as Na + or K + , and a divalent cation, preferably a magnesium or manganese ion, more preferably a magnesium ion, at a concentration that is less than 100 mM and greater than 1 mM.
- the divalent cation is at a concentration of about 5 to about 20 mM.
- the preferred pH for the medium is from about 6.0-8.5, more preferably about 7.5-8.0.
- the LtrA protein is also useful for transcribing RNA molecules.
- Cleavage of Double Stranded DNA Substrates A. Cleaving a Double-Stranded DNA Substrate with the RNP Particles of Example 1
- the substrate which is cleaved by the nucleotide integrase, which comprises the excised Ll.ltrB intron RNA and the LtrA protein, is schematically depicted in Figure 8(a).
- the IBS1 and IBS2 sequence of the substrate is shown in figure 8(b).
- the IBS1 and IBS2 sequences which are complementary to the EBS sequences of the Lltr.B intron RNA are present in exon 1 of the ItrB gene.
- the RNP particles prepared according to the method of example 1 cleaved the sense strand of the substrate at position 0, which is the exon 1 and exon 2 junction, and cleaved the antisense strand at +9.
- RNP particles prepared as described in examples 10 and 11 were able to efficiently cleave the double-stranded DNA substrate and to either partially or fully integrate the intron RNA subunit of the nucleotide integrase into the cleavage site.
- RNP particles that comprise LtrA protein and an Ll.ltrB intron RNA which lacks an ORF sequence have complete nucleotide integrase activity.
- RNP particles that comprise an LtrA protein and an LltrB intron RNA in which the ORF has been replaced with a sequence encoding a different gene product also have complete nucleotide ⁇ integrase activity
- 0.025 O.D. 250 units of the RNP particle preparations of example 8 were reacted with 125 fmoles (150,000 cpm) of the 129 base pair internally-labeled double-stranded DNA substrate which comprises the sequence depicted in Figure 7 A for 20 minutes.
- 0.025 O.D. 260 units of the RNP particle preparations of example 8 were reacted with 125 fmoles (150,000 cpm) of a 129 base pair internally-labeled double-stranded DNA substrate which comprises a modified exon 1 and wild-type exon 2 of the Ll.ltrB gene for 20 minutes.
- the sequence of the first strand of the 129 base pair substrate, in which the nucleotide at position -6 relative to the putative cleavage site in the wild-type exon 1 is changed from a C to a G is underlined in Figure 7B.
- the putative cleavage sites in the first strand of the substrates shown in Figure 7A and 7B are depicted by a vertical line.
- the products were glyoxalated and analyzed in a 1% agarose gel.
- Endonuclease assays were also conducted to confirm that cleavage occurred between nucleotides -1 an +1 in the first strand of the substrate and at position +9 in the second strand of the substrate, and also to confirm that a nucleic acid molecule had been inserted into the cleavage site.
- the RNP particles prepared as described in example 8 were able to efficiently cleave the double-stranded DNA substrate shown in Figure 7b and to either partially or fully integrate the intron RNA subunit of the RNP particles into the cleavage site.
- the EBSl sequence of the modified Ll.ltrB intron in the RNP particles prepared as described in example 8 is complementary to the IBS1 sequence of the substrate shown in Figure 7b.
- the EBSl sequence of the modified Ll.ltrB intron in the RNP particles prepared as described in example 8 is not complementary to the IBS1 sequence of the substrate shown in Figure 7a.
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