Bacteriophage P2 integrase: another possible tool for site-specific recombination in eukaryotic cells.

AIMS: To investigate if the site-specific tyrosine integrase (Int) from phage P2 has features that would make it interesting for use of gene transfer into eukaryotic cells. These include the possibility of promoting recombination with a nonphage sequence, abolishing the requirement for the bacterial DNA-binding and -bending protein integration host factor (IHF), and localization to the nucleus of eukaryotic cells. METHODS AND RESULTS: We show that the Int protein catalyzes site-specific recombination using a human sequence in Escherichia coli and in vitro although not as efficiently as with the wild-type bacterial sequence, and that insertion of high mobility group recognition boxes in the phage attachment site substrate abolish the requirement of IHF and allows efficient recombination in vitro in a eukaryotic cell extract. Furthermore, we show by fluorescence that the Int protein contains a functional intrinsic nuclear localization signal, localizing it to the nucleus in both HeLa and 293 cells. CONCLUSIONS: We conclude that P2 Int may be a potential tool for site-specific integration of genes into the human chromosome. SIGNIFICANCE AND IMPACT OF THE STUDY: The study implies the possibility of using multiple prokaryotic Int proteins with different specific integration sites in human cells for future gene therapy programmes.

Detection of homologous recombination among bacteriophage P2 relatives.

Sequencing of five late genes from 18 isolates of P2-like bacteriophages showed that these are at least 96% identical to the genes of phage P2. A maximum-parsimony phylogenetic analysis of these genes showed excess homoplasy of a magnitude three to six times higher than that expected. Examination of the distribution of the number of homoplasies at parsimoniously informative sites and incompatibility matrices of such sites revealed a pattern typical for extensive recombination. It has been shown that phage P2 probably incorporated some functionally complete genes or gene modules by recombination with other phages or with different hosts, but homologous recombination within genes has previously not been shown. In this paper we demonstrate that homologous recombination between P2-like bacteriophages occurs randomly at multiple breakpoints in five late genes. The rate of recombination is high but, since some phages were sampled decades apart and in different parts of the world, this has to be viewed on an evolutionary time scale. The applicability of different methods used for detection of recombination breakpoints and estimation of rates of recombination in bacteriophages is discussed.

The two active-site tyrosine residues of the a protein play non-equivalent roles during initiation of rolling circle replication of bacteriophage p2.

The A protein of bacteriophage P2 initiates rolling circle DNA replication by a single-stranded cut at the origin. Two well-conserved tyrosine residues, interspaced by three amino acid residues, are required for the cleavage-joining activity of the protein. The functional relationship between these tyrosine residues was investigated by site-directed mutagenesis. We found that the two tyrosine residues located in the presumed catalytic site of P2 A play non-equivalent functional roles. Tyrosine residue 454 is superior in nicking single-stranded DNA compared to tyrosine residue 450, while both could promote joining at equal efficiency. Specific peptide-oligonucleotide adducts after cleavage reaction and protease digestion could be observed for both tyrosine residues. We propose that tyrosine 454 initiates replication and that tyrosine 450 is able to cleave the DNA only when tyrosine 454 is covalently joined to DNA, thereby reinitiating replication. Also, the involvement of divalent cations in the catalytic activity of P2 A was investigated. While the cleavage reaction was strongly discriminating between different divalent cations, primarily prefering magnesium, the joining reaction showed the same efficiency independently of what divalent cation was provided. This phenomenon could reflect conformational changes of the protein upon binding to DNA. Finally, we found that a large part of the C terminus but not the N terminus is dispensable for initiation of replication both in vivo and in vitro.

Characterization of the developmental switch region of bacteriophage P2 Hy dis.

In this work, the DNA sequence of the transcriptional switch that affects the development of the P2 Hy dis bacteriophage was determined. The switch contains two face-to-face-located promoters and two repressors, Cox and C. The locations of the Pc and Pe promoters were determined by primer extension analysis. The P2 Hy dis homolog of the P2 multifunctional Cox protein was shown to be able to substitute for P2 Cox in repression of the P2 Pc promoter, excision of the P2 prophage, and activation of the satellite phage P4 PLL promoter. A directly repeated sequence, flanking the--35 region of the Pe promoter, was found to be important for C repressor binding as well as for repression. The P4 E protein was shown to derepress the developmental switch of P2 Hy dis in a plasmid-based derepression assay.

The multifunctional bacteriophage P2 cox protein requires oligomerization for biological activity.

The Cox protein of bacteriophage P2 is a multifunctional protein of 91 amino acids. It is directly involved in the site-specific recombination event leading to excision of P2 DNA out of the host chromosome. In this context, it functions as an architectural protein in the formation of the excisome. Cox is also a transcriptional repressor of the P2 Pc promoter, thereby ensuring lytic growth. Finally it promotes derepression of prophage P4, a nonrelated defective satellite phage, by activating the P4 P(LL) promoter that controls P4 DNA replication. In this case it binds upstream of the P(LL) promoter, which normally is activated by the P4 Delta protein. In this work we have analyzed the native form of the Cox protein in vivo, using a bacteriophage lambda cI-based oligomerization assay system, and in vitro, using gel filtration, cross-linking agents, and gel retardation assays. We found that P2 Cox has a strong oligomerization function in vivo as well as in vitro. The in vitro analysis indicates that its native form is a tetramer that can self-associate to octamers. Furthermore we show that oligomerization is necessary for the biological activity by characterizing different cox mutants and that oligomerization is mediated by the C-terminal region.

Interacting interfaces of the P4 antirepressor E and the P2 immunity repressor C.

Antirepressors have been identified as proteins interacting with transcriptional repressors leading to expression of the repressed genes. The defective satellite phage/plasmid P4 has the capacity to derepress the unrelated prophage P2 after infection, thereby getting access to the late functions of the helper that are required for P4 lytic growth. The derepression of prophage P2 is mediated by the P4 E protein that function as an antirepressor by binding to the P2 immunity repressor C. A P2 mutant, sos, has been isolated that is insensitive to the action of the P4 E protein. In the present study, we show that sos is a point mutation in the P2 immunity repressor gene C and that it makes P4 E unable to turn the transcriptional switch of P2 from the lysogenic state to the lytic mode in a two plasmid reporter system. Furthermore, the interaction between C and E, when analysed in the yeast two-hybrid system, is blocked by the sos mutation. An analysis of C mutants indicates that the dimerization function of C is located in the C-terminal part of the protein and the dimerization defective mutants are unable to bind to their operator DNA. The sos mutation does not affect the capacity of the protein to dimerize. Using the yeast two-hybrid system, compensatory E mutants have been isolated that can interact with Sos, but they are unable to turn the transcriptional switch controlled by the Sos repressor. However, one point mutation in the E protein is shown to be unable to turn the transcriptional switch controlled by the wild-type C repressor.

The interaction of bacteriophage P2 B protein with Escherichia coli DnaB helicase.

Bacteriophage P2 requires several host proteins for lytic replication, including helicase DnaB but not the helicase loader, DnaC. Some genetic studies have suggested that the loading is done by a phage-encoded protein, P2 B. However, a P2 minichromosome containing only the P2 initiator gene A and a marker gene can be established as a plasmid without requiring the P2 B gene. Here we demonstrate that P2 B associates with DnaB. This was done by using the yeast two-hybrid system in vivo and was confirmed in vitro, where (35)S-labeled P2 B bound specifically to DnaB adsorbed to Q Sepharose beads and monoclonal antibodies directed against the His-tagged P2 B protein were shown to coprecipitate the DnaB protein. Finally, P2 B was shown to stabilize the opening of a reporter origin, a reaction that is facilitated by the inactivation of DnaB. In this respect, P2 B was comparable to lambda P protein, which is known to be capable of binding and inactivating the helicase while acting as a helicase loader. Even though P2 B has little similarity to other known or predicted helicase loaders, we suggest that P2 B is required for efficient loading of DnaB and that this role, although dispensable for P2 plasmid replication, becomes essential for P2 lytic replication.

The transcriptional switch of bacteriophage WPhi, a P2-related but heteroimmune coliphage.

Phage WPhi is a member of the nonlambdoid P2 family of temperate phages. The DNA sequence of the whole early-control region and the int and attP region of phage WPhi has been determined. The phage integration site was located at 88.6 min of the Escherichia coli K-12 map, where a 47-nucleotide sequence was found to be identical in the host and phage genomes. The WPhi Int protein belongs to the Int family of site-specific recombinases, and it seems to have the same arm binding recognition sequence as P2 Int, but the core sequence differs. The transcriptional switch contains two face-to-face promoters, Pe and Pc, and two repressors, C and Cox, controlling Pe and Pc, respectively. The early Pe promoter was found to be much stronger than the Pc promoter. Furthermore, the Pe transcript was shown to interfere with Pc transcription. By site-directed mutagenesis, the binding site of the immunity repressor was located to two direct repeats spanning the Pe promoter. A point mutation in one or the other repeat does not affect repression by C, but when it is included in both, C has no effect on the Pe promoter. The Cox repressor efficiently blocks expression from the Pc promoter, but its DNA recognition sequence was not evident. Most members of the P2 family of phages are able to function as helpers for satellite phage P4, which lacks genes encoding structural proteins and packaging and lysis functions. In this work it is shown that P4 E, known to function as an antirepressor by binding to P2 C, also turns the transcriptional switch of WPhi from the lysogenic to the lytic mode. However, in contrast to P2 Cox, WPhi Cox is unable to activate the P4 Pll promoter.

The E protein of satellite phage P4 acts as an anti-repressor by binding to the C protein of helper phage P2.

Temperate phage P2 has the capacity to function as a helper for the defective, unrelated, satellite phage P4. In the absence of a helper, P4 can either lysogenize its host or establish itself as a plasmid. For lytic growth, P4 requires the structural genes, packaging and lysis functions of the helper. P4 can get access to the late genes of prophage P2 by derepression, which is mediated by the P4 E protein. E has been hypothesized to function as an anti-repressor. To locate possible epitopes interacting with E, an epitope display library was screened against E, and the most frequent sequence found had some identities to a region within P2 C. Using the yeast two-hybrid system, a clear activation of a reporter gene was found, strongly supporting an interaction between E and C. The P2 C repressor is believed to act as a dimer, which is confirmed in this work using in vivo dimerization studies. The E protein was also found to form dimers in vivo. The E protein only affects dimerization of C marginally, but the presence of E enhances multimeric forms of C. Furthermore, binding of the C protein to its operator is inhibited by E in vitro, indicating that the anti-repressor function of E is mediated by the formation of multimeric complexes of E and C that interfere with the binding of C to its operator.

Derepression of prophage P2 by satellite phage P4: cloning of the P4 epsilon gene and identification of its product.

Escherichia coli phage P4 lacks all of the genetic information necessary for capsid, tail, and lysis functions. P4 is therefore dependent on a helper phage, such as P2, for lytic propagation. During P4 superinfection of a P2 lysogen, the P2 prophage is derepressed by the action of the P4-encoded epsilon gene. We have cloned the epsilon gene and identified the 10-kDa E protein. The epsilon gene product is the only P4 protein required to derepress prophage P2, which leads to in situ P2 DNA replication. A two-plasmid derepression assay system has been developed to examine the derepression activity of E. The reporter plasmid contains the two face-to-face promoters, Pe and Pc, involved in the lysis-lysogeny transcriptional switch of phage P2 and the immunity repressor C. The Pe promoter is coupled to a cat reporter gene. In the construct, the C repressor is transcribed from the Pc promoter and represses the Pe promoter, which mimics the in situ-repressed P2 prophage. The E protein is supplied in trans from a compatible plasmid in which the epsilon gene is under the control of the T7 promoter. We show here that in the two-plasmid assay system, induction of the E protein derepresses the Pe promoter. The ash9 mutation, which is located upstream of the epsilon gene, enhances the E-mediated derepression of the Pe promoter. The purified E protein shows no specific DNA binding activity, and the implications of this are discussed.

A novel mechanism of virus-virus interactions: bacteriophage P2 Tin protein inhibits phage T4 DNA synthesis by poisoning the T4 single-stranded DNA binding protein, gp32.

P2 prophages have been known to inhibit DNA replication and growth of T-even phages. We show here that this inhibition is due to poisoning of the T-even single-stranded DNA binding protein gp32 by the product of the nonessential P2 tin gene. Synthesis of Tin protein from a gene cloned in a multicopy plasmid is necessary and sufficient to completely prevent de novo DNA replication and growth of wild-type T2 or T4 phage. We isolated more than 20 independent mutants that render T-even phages resistant to poisoning by the P2 Tin protein. In all of these mutants, which we call asp, Asp codon 163 of gene 32 is changed to a Gly or Asn codon. The mutant alleles are recessive; i.e., when wild-type and asp mutants coinfect the same host cells, most DNA replication is poisoned by P2 Tin protein. To explain our results, we propose that the P2 Tin protein interacts with T-even gp32 at position 163 and distorts the helical filament of gene 32 protein on single-stranded DNA. Thereby Tin protein inhibits either assembly or function, or both, of the T4 replisome. The inhibition of late gene expression by P2 Tin protein may be an indirect consequence of inhibition of DNA replication.

Functional characterization of the P2 A initiator protein and its DNA cleavage site.

The A protein of bacteriophage P2 initiates DNA replication by a single-stranded cut at the origin, and the DNA replication proceeds unidirectionally by a modified rolling circle type of replication. The P2 A protein belongs to a family of proteins involved in the initiation of rolling circle DNA replication, and the prototype for this family is the well-characterized A protein of phage phi X174. One of the common motifs of this family contains two conserved tyrosine residues, which have been shown to be able to alternate in catalyzing the cleavage as well as joining reactions in the phi X174 A protein. We investigated the role of the conserved tyrosine residues in P2 A protein by in vitro mutagenesis. Only one of the two conserved tyrosine residues was found to be involved in the cleavage reaction. The tyrosine residue dispensable for cleavage and ligation is, however, required at some other stage of the P2 growth cycle, since viable recombinants containing this mutation could not be obtained. The sequence requirements for cleavage of the target site were analyzed with a set of oligonucleotides having single base alterations in the nick region, and the results indicate that only five core nucleotides need to be conserved for efficient cleavage.

Bacteriophage P2: genes involved in baseplate assembly.

The sequences of two previously defined tail genes, V and J, of the temperate bacteriophage P2, and those of two new essential tail genes, W and I, were determined. Their order is the late gene promoter, VWJI, followed by the tail fiber genes H and G, and a transcription terminator. The V gene product is the small spike at the tip of the tail, and the J gene product lies at the edge of the baseplate. The W gene product may be homologous to the product of gene 25 of T4 phage, which is part of the T4 baseplate. A temperature-sensitive mutation in gene V affects satellite phage P4 production more than it affects the production of P2 helper phage. P4 mutations that partially compensate for this defect of gene V lie in the P4 capsid size determination gene, sid.

Ternary complex formation on leaderless phage mRNA.

The phage Lambda PRM promoter-derived cI mRNA and phage P2 gene V mRNA are transcribed beginning with the A residue of the AUG start codon. Using lacZ fusion analysis we have assessed the effects of alterations in the immediate downstream coding region on the translational efficiency of these mRNAs. Mutations, including deletions of the putative downstream box of either cI or gene V mRNAs, showed no significant reduction in expression of the different lacZ fusions. Primer extension inhibition analysis suggests a role of ribosomal protein S1 in cI mRNA recognition.

Interruption of the ferredoxin (flavodoxin) NADP+ oxidoreductase gene of Escherichia coli does not affect anaerobic growth but increases sensitivity to paraquat.

Ferredoxin (flavodoxin) NADP+ oxidoreductase participates in methionine biosynthesis and in the function of two anaerobic enzymes, pyruvate formate-lyase and ribonucleotide reductase. We prepared insertion mutants of Escherichia coli lacking a functional enzyme. They do not require methionine and they grow well anaerobically, but they show increased sensitivity to paraquat.

Studies of bacteriophage P2 DNA replication: localization of the cleavage site of the A protein.

Bacteriophage P2 replicates via a modified rolling circle-type of mechanism, where the P2 A protein acts as an initiator of the replication by inducing a single-stranded cut at the origin of replication (ori). The exact location of the cut induced by the A protein in vivo is determined in this report by: (i) restriction analysis; (ii) DNA sequence analysis; and (iii) primer extensions. It is located 89.2% from the left end of the P2 genome, which is within the coding part of the A gene, in a region devoid of secondary structures. The A gene has been cloned into an expression vector, and the A protein has been purified. The purified A protein does not bind to double-stranded ori containing DNA, but it cleaves single-stranded ori containing DNA, which indicates that a special DNA structure and/or protein is required to make the ori accessible for the A protein.

Regulation of int gene expression in bacteriophage P2.

The int gene of bacteriophage P2 is the only viral gene necessary for the integration of P2 into the Escherichia coli host chromosome. This gene is situated between the phage attachment site, attP, and the repressor C gene, and is cotranscribed with C from the Pc promoter, towards attP. The Pc promoter is negatively controlled by the cox gene, which is the first gene of the early operon. In vitro recombination assays have indicated that in P2 an overproduction of Int is deleterious to the integrative process. We report here that the level of int expression is affected by several different mechanisms after transcriptional initiation. First, a partial transcription termination signal located between the int and C genes reduces the the transcriptional readthrough by about 30%. Second, the ribosome binding site and AUG codon of the int gene are located in a putative stem-loop structure, which may inhibit the initiation of translation. The nip1 mutation (a G to A substitution at the 22nd coding nucleotide of int which results in an increased efficiency of excision) is shown to relieve this inhibition, possible through the formation of an alternative mRNA secondary structure. However, the third and probably most important control of int expression in P2 seems to be that of posttranscriptional autoregulation. The binding site of the Int protein on int gene mRNA is shown to extend into the ribosome binding site of int, supporting our earlier proposed model of competitive binding between Int and ribosomes.

The Cox protein is a modulator of directionality in bacteriophage P2 site-specific recombination.

The P2 Cox protein is known to repress the Pc promoter, which controls the expression of the P2 immunity repressor C. It has also been shown that Cox can activate the late promoter PLL of the unrelated phage P4. By this process, a P2 phage infecting a P4 lysogen is capable of inducing replication of the P4 genome, an example of viral transactivation. In this report, we present evidence that Cox is also directly involved in both prophage excision and phage integration. While purified Cox, in addition to P2 Int and Escherichia coli integration host factor, was required for attR x attL (excisive) recombination in vitro, it was inhibitory to attP x attB (integrative) recombination. The same amounts of Int and integration host factor which mediated optimal excisive recombination in vitro also mediated optimal integrative recombination. We quantified and compared the relative efficiencies of attB, attR, and attL in recombination with attP and discuss the functional implications of the results. DNase I protection experiments revealed an extended 70-bp Cox-protected region on the right arm of attP, centered at about +60 bp from the center of the core sequence. Gel shift assays suggest that there are two Cox binding sites within this region. Together, these data support the theory that in vivo, P2 can exert control over the direction of recombination by either expressing Int alone or Int and Cox together.

Studies of bacteriophage P2 DNA replication. The DNA sequence of the cis-acting gene A and ori region and construction of a P2 mini-chromosome.

A self-replicating plasmid was constructed from the 76.7 to 91.6% region of bacteriophage P2, which contains the P2 origin or replication (ori) and the genetically defined replication genes B and A. The sequence of the 76.7 to 80.2% has been determined previously, and the sequence of the 80.2 to 91.6% region is now reported. The sequenced region contained gene A, which predicts a 761 amino acid residue polypeptide known to induce a single-strand cut at or near ori, and ori, which has been located by electron microscopy to about 89% from the left end of the phage genome. Analysis of plasmid-encoded proteins in minicells indicated that the A gene product was about 78 kilodaltons. Five previously unknown open reading frames (orf-80, orf-81, orf-82, orf-83 and orf-91) were discovered. They have been cloned and their respective products were identified. The products of orf-80, orf-82 and gene A were found to be lethal to the host when overexpressed. The predicted amino acid sequences of the orf-82 and orf-83 gene products were similar to two early gene products of phage 186; the orf-91 product resembled the hypothetical protein of orfd of retron Ec67. Similarities between the products of P2 gene A, 186 gene A and orf2 and orf3 of Ec67 were also found. A P2 mini-chromosome has been constructed that contains only the P2 A gene and the beta-lactamase gene as a marker.