The c1 genes of P1 and P7

Interactions between Viral Regulatory Proteins Ensure an MOI-Independent Probability of Lysogeny during Infection by Bacteriophage P1

Phage P1 is a temperate phage which makes the lytic or lysogenic decision upon infecting bacteria. During the lytic cycle, progeny phages are produced and the cell lyses, and in the lysogenic cycle, P1 DNA exists as a low-copy-number plasmid and replicates autonomously. Previous studies at the bulk level showed that P1 lysogenization was independent of multiplicity of infection (MOI; the number of phages infecting a cell), whereas lysogenization probability of the paradigmatic phage λ increases with MOI. However, the mechanism underlying the P1 behavior is unclear. In this work, using a fluorescent reporter system, we demonstrated this P1 MOI-independent lysogenic response at the single-cell level. We further observed that the activity of the major repressor of lytic functions (C1) is a determining factor for the final cell fate. Specifically, the repression activity of P1, which arises from a combination of C1, the anti-repressor Coi, and the corepressor Lxc, remains constant for different MOI, which results in the MOI-independent lysogenic response. Additionally, by increasing the distance between phages that infect a single cell, we were able to engineer a λ-like, MOI-dependent lysogenization upon P1 infection. This suggests that the large separation of coinfecting phages attenuates the effective communication between them, allowing them to make decisions independently of each other. Our work establishes a highly quantitative framework to describe P1 lysogeny establishment. This system plays an important role in disseminating antibiotic resistance by P1-like plasmids and provides an alternative to the lifestyle of phage λ.

Characterization of a P1-like bacteriophage carrying an SHV-2 extended-spectrum β-lactamase from an Escherichia coli strain

P1 bacteriophages lysogenize bacteria as independent plasmid-like elements. We describe here a P1-like bacteriophage, RCS47, carrying a blaSHV-2 gene, isolated from a clinical strain of Escherichia coli from phylogroup B1, and we report the prevalence of P1-like prophages in natural E. coli isolates. We found that 70% of the sequence of RCS47, a 115-kb circular molecule, was common to the reference P1 bacteriophage under GenBank accession no. AF234172.1, with the shared sequences being 99% identical. RCS47 had acquired two main foreign DNA fragments: a 9,636-bp fragment mobilized by two IS26 elements containing a blaSHV-2 gene, and an 8,544-bp fragment mobilized by two IS5 elements containing an operon encoding a dimethyl sulfoxide reductase. The reference P1 prophage plasmid replication gene belonged to the IncY incompatibility group, whereas that of RCS47 was from an unknown group. The lytic capacity of RCS47 and blaSHV-2 gene transduction, through the lysogenization of RCS47 in the recipient E. coli strains, were not demonstrated. The prevalence of P1-like prophages in various animal and human E. coli strain collections, as determined by the PCR detection of repL, the lytic replication gene, was 12.6%. No differences in the prevalences of these prophages were found between extended-spectrum β-lactamase (ESBL)-producing and non-ESBL-producing strains (P = 0.69), but this prevalence was lower in phylogroup B2 than in the other phylogroups (P = 0.008), suggesting epistatic interactions between P1 family phages and the genetic background of E. coli strains. P1-like phages are part of the mobile elements that carry antibiotic resistance. The high prevalence of P1-like prophages suggests their role may be underestimated.

DNA recombination with a heterospecific Cre homolog identified from comparison of the pac-c1 regions of P1-related phages

Sequencing of the 7 kb immC region from four P1-related phages identified a novel DNA recombinase that exhibits many Cre-like characteristics, including recombination in mammalian cells, but which has a distinctly different DNA specificity. DNA sequence comparison to the P1 immC region showed that all phages had related DNA terminase, C1 repressor and DNA recombinase genes. Although these genes from phages P7, phi(w39) and p15B were highly similar to those from P1, those of phage D6 showed significant divergence. Moreover, the D6 sequence showed evidence of DNA deletion and substitution in this region relative to the other phages. Characterization of the D6 site-specific DNA recombinase (Dre) showed that it was a tyrosine recombinase closely related to the P1 Cre recombinase, but that it had a distinct DNA specificity for a 32 bp DNA site (rox). Cre and Dre are heterospecific: Cre did not catalyze recombination at rox sites and Dre did not catalyze recombination at lox sites. Like Cre, Dre catalyzed both integrative and excisive recombination and required no other phage-encoded proteins for recombination. Dre-mediated recombination in mammalian cells showed that, like Cre, no host bacterial proteins are required for efficient Dre-mediated site-specific DNA recombination.

Genome of bacteriophage P1

P1 is a bacteriophage of Escherichia coli and other enteric bacteria. It lysogenizes its hosts as a circular, low-copy-number plasmid. We have determined the complete nucleotide sequences of two strains of a P1 thermoinducible mutant, P1 c1-100. The P1 genome (93,601 bp) contains at least 117 genes, of which almost two-thirds had not been sequenced previously and 49 have no homologs in other organisms. Protein-coding genes occupy 92% of the genome and are organized in 45 operons, of which four are decisive for the choice between lysis and lysogeny. Four others ensure plasmid maintenance. The majority of the remaining 37 operons are involved in lytic development. Seventeen operons are transcribed from sigma(70) promoters directly controlled by the master phage repressor C1. Late operons are transcribed from promoters recognized by the E. coli RNA polymerase holoenzyme in the presence of the Lpa protein, the product of a C1-controlled P1 gene. Three species of P1-encoded tRNAs provide differential controls of translation, and a P1-encoded DNA methyltransferase with putative bifunctionality influences transcription, replication, and DNA packaging. The genome is particularly rich in Chi recombinogenic sites. The base content and distribution in P1 DNA indicate that replication of P1 from its plasmid origin had more impact on the base compositional asymmetries of the P1 genome than replication from the lytic origin of replication.

Conserved sequence patterns in phages Mu and lambda DNA

The genetic maps of bacteriophages Mu and lambda can be aligned with respect to the functions of their genes. We were interested to ascertain whether the congruence of gene order is reflected at the nucleotide sequence level. A sliding window analysis of sequences from the early regions of both phages revealed a substantial degree of similarity. Equally high scores, however, were found when the early region of Mu was compared to the late region of lambda and in self-comparisons of either Mu or lambda. Hence, the similarity is due to a common pattern of nucleotides rather than to sequence similarities between functionally related genes. Employing degenerated scoring matrices we could show that primarily adenine and thymine residues contribute to the high scores and that a specific clustering of these residues is the basis for the conserved pattern. Since such a similarity was not observed with control sequences of other phages. Escherichia coli or eukaryotic viruses, the data support the notion that Mu and lambda have diverged from a common phage module. In general, our approach could offer a simple and sensitive way to trace distant relationships.

The tripartite immunity system of phages P1 and P7

Prophages P1 and P7 exist as unit copy DNA plasmids in the bacterial cell. Maintenance of the prophage state requires the continuous expression of two repressors: (i) C1 is a protein which negatively regulates the expression of lytic genes including the C1 inactivator gene coi, and (ii) C4 is an antisense RNA which specifically inhibits the synthesis of an anti-repressor Ant. In addition, C1 repression is strengthened by lxc encoding an auxiliary repressor protein. The repressors C1, C4 and Lxc are components of a tripartite immunity system of the two phages. Here, the mode of action of these regulatory components including their antagonists Coi and Ant is described.

The antirepressor of phage P1. Isolation and interaction with the C1 repressor of P1 and P7

Two antirepressor proteins, Ant1 and Ant2, of molecular weight 42 and 32 kDa, respectively, are encoded by P1 as a single open reading frame, with the smaller protein initiating at an in-frame start codon. Another open reading frame, icd, 5' upstream of and overlapping ant1 is required for ant1 expression. Using appropriate ant gene-carrying plasmids we have overproduced and purified Ant1/2 in the form of a protein complex and Ant2 as a single protein. Sequence analysis confirmed the N-terminal amino acids predicted from the DNA sequence of ant1/ant2, except that the N-terminal methionine is missing in the Ant2 protein. Under appropriate conditions the C1 repressors of phages P1 and P7 specifically co-precipitate with the Ant1/2 complex but not with Ant2 protein alone. The results suggest that the antirepressor may exert its C1-inactivating function by a direct protein-protein interaction.

Bacteriophage P1 genes involved in the recognition and cleavage of the phage packaging site (pac)

The packaging of bacteriophage P1 DNA is initiated by cleavage of the viral DNA at a specific site, designated pac. The proteins necessary for that cleavage, and the genes that encode those proteins, are described in this report. By sequencing wild-type P1 DNA and DNA derived from various P1 amber mutants that are deficient in pac cleavage, two distinct genes, referred to as pacA and pacB, were identified. These genes appear to be coordinately transcribed with an upstream P1 gene that encodes a regulator of late P1 gene expression (gene 10). pacA is located upstream from pacB and contains the 161 base-pair pac cleavage site. The predicted sizes of the PacA and PacB proteins are 45 kDa and 56 kDa, respectively. These proteins have been identified on SDS-polyacrylamide gels using extracts derived from Escherichia coli cells that express these genes under the control of a bacteriophage T7 promoter. Extracts prepared from cells expressing both PacA and PacB are proficient for site-specific cleavage of the P1 packaging site, whereas those lacking either protein are not. However, the two defective extracts can complement each other to restore functional pac cleavage activity. Thus, PacA and PacB are two essential bacteriophage proteins required for recognition and cleavage of the P1 packaging site. PacB extracts also contain a second P1 protein that is encoded within the pacB gene. We have identified this protein on SDS-polyacrylamide gels and have shown that it is translated in the same reading frame as is PacB. Its role, if any, in pac cleavage is yet to be determined.

Bacteriophage P1 Bof protein is an indirect positive effector of transcription of the phage bac-1 ban gene in some circumstances and a direct negative effector in other circumstances

Previous genetic studies have suggested that the Bof protein of bacteriophage P1 can act as both a negative and a positive regulator of phage gene expression: in bof-1 prophages, the ref gene and a putative phage ssb gene are derepressed, but expression of an operator-semiconstitutive variant of the phage ban gene (bac-1) is markedly reduced. An explanation of this apparent duality is suggested by recent reports that Bof is a corepressor of genes that are regulated by the phage C1 repressor, including the autoregulated c1 gene itself. Here we show, by means of operon fusions to lacZ, that the balance points between Bof-mediated decreases in c1 expression and Bof-mediated increases in C1 efficacy are different among various C1-regulated genes. Thus, expression of Bof by P1 prophages affects some genes (e.g., bac-1 ban) positively, and others (e.g., ref) negatively. Even at bac-1 ban, where the positive indirect effect of Bof is physiologically dominant, Bof can be seen to act as a corepressor if C1 is supplied from a nonautoregulated (ptac-c1) source, eliminating the effect of Bof on C1 synthesis.

The ImmC region of phage P1 codes for a gene whose product promotes lytic growth

The ImmC region of the temperate bacteriophage P1 contains c1, a gene that codes for a repressor of lytic growth. Located in the region upstream of c1 are four open reading frames capable of coding for low-molecular-weight proteins. The efficiency of lysogeny by P1+Cm was found to be reduced by almost 10(5)-fold when the host cells carry this region of ImmC on a multicopy plasmid. The sequences responsible for interfering with lysogen formation were localized to one of the small open reading frames (orf4) within ImmC. Insertions and deletions within orf4 suppress the virulent phenotype of P1virC mutants when introduced into the phage by recombination. These virC-suppressed mutant phage were found to be incapable of lytic growth unless the product of orf4 is provided in trans. The presence of orf4 was observed to interfere with repression by the c1 protein of ImmC-encoded promoters fused to lacZ. For this reason, we suggest that orf4 corresponds to coi, a gene previously proposed to code for an inactivator of c1-mediated repression.

The bof gene of bacteriophage P1: DNA sequence and evidence for roles in regulation of phage c1 and ref genes

The C1 repressor of bacteriophage P1 acts via 14 or more distinct operators. This repressor represses its own synthesis as well as the synthesis of other gene products. Previously, mutation of an auxiliary regulatory gene, bof, has been shown to increase expression of some C1-regulated P1 genes (e.g., ref) but to decrease expression of others (e.g., ban). In this study the bof gene was isolated on the basis of its ability to depress stimulation of Escherichia coli chromosomal recombination by the P1 ref gene, if and only if a source of C1 was present. C1 alone, but not Bof alone, was partially effective. The bofDNA sequence encodes an 82-codon reading frame that begins with a TTG codon and includes the sites of the bof-1(Am) mutation and a bof::Tn5 null mutation. Expression of ref::lacZ and cl::lacZ fusion genes was partially repressed in trans by a P1 bof-1 prophage or by plasmid-encoded C1 alone, which was in agreement with effects on Ref-stimulated recombination and with previous indirect evidence for c1 autoregulation. Repression of both fusion genes by plasmid-encoded C1 plus Bof or by a P1 bof+ prophage was more complete. When the C1 source also included a 0.7-kilobase region upstream from C1 which encodes the coi gene, repression of both c1::lacZ and ref::lacZ by C1 alone or by C1 plus Bof was much less effective, as if Coi interfered with C1 repressor function.