Identification of psiB genes of plasmids F and R6-5. Molecular basis for psiB enhanced expression in plasmid R6-5

The F plasmid conjutome: the repertoire of E. coli proteins translocated through an F-encoded type IV secretion system

Bacterial conjugation systems pose a major threat to human health through their widespread dissemination of mobile genetic elements (MGEs) carrying cargoes of antibiotic resistance genes. Using the Cre Recombinase Assay for Translocation (CRAfT), we recently reported that the IncFV pED208 conjugation system also translocates at least 16 plasmid-encoded proteins to recipient bacteria. Here, we deployed a high-throughput CRAfT screen to identify the repertoire of chromosomally encoded protein substrates of the pED208 system. We identified 32 substrates encoded by the Escherichia coli W3110 genome with functions associated with (i) DNA/nucleotide metabolism, (ii) stress tolerance/physiology, (iii) transcriptional regulation, or (iv) toxin inhibition. The respective gene deletions did not impact pED208 transfer proficiencies, nor did Group 1 (DNA/nucleotide metabolism) mutations detectably alter the SOS response elicited in new transconjugants upon acquisition of pED208. However, MC4100(pED208) donor cells intrinsically exhibit significantly higher SOS activation than plasmid-free MC4100 cells, and this plasmid carriage-induced stress response is further elevated in donor cells deleted of several Group 1 genes. Among 10 characterized substrates, we gained evidence of C-terminal or internal translocation signals that could function independently or synergistically for optimal protein transfer. Remarkably, nearly all tested proteins were also translocated through the IncN pKM101 and IncP RP4 conjugation systems. This repertoire of E. coli protein substrates, here termed the F plasmid "conjutome," is thus characterized by functions of potential benefit to new transconjugants, diverse TSs, and the capacity for promiscuous transfer through heterologous conjugation systems.

IMPORTANCE

Conjugation systems comprise a major subfamily of the type IV secretion systems (T4SSs) and are the progenitors of a second large T4SS subfamily dedicated to translocation of protein effectors. This study examined the capacity of conjugation machines to function as protein translocators. Using a high-throughput reporter screen, we determined that 32 chromosomally encoded proteins are delivered through an F plasmid conjugation system. The translocated proteins potentially enhance the establishment of the co-transferred F plasmid or mitigate mating-induced stresses. Translocation signals located C-terminally or internally conferred substrate recognition by the F system and, remarkably, many substrates also were translocated through heterologous conjugation systems. Our findings highlight the plasticity of conjugation systems in their capacities to co-translocate DNA and many protein substrates.

Adaptive evolution of plasmid and chromosome contributes to the fitness of a blaNDM-bearing cointegrate plasmid in Escherichia coli

Large cointegrate plasmids recruit genetic features of their parental plasmids and serve as important vectors in the spread of antibiotic resistance. They are now frequently found in clinical settings, raising the issue of how to limit their further transmission. Here, we conducted evolutionary research of a large blaNDM-positive cointegrate within Escherichia coli C600, and discovered that adaptive evolution of chromosome and plasmid jointly improved bacterial fitness, which was manifested as enhanced survival ability for in vivo and in vitro pairwise competition, biofilm formation, and gut colonization ability. From the plasmid aspect, large-scale DNA fragment loss is observed in an evolved clone. Although the evolved plasmid imposes a negligible fitness cost on host bacteria, its conjugation frequency is greatly reduced, and the deficiency of anti-SOS gene psiB is found responsible for the impaired horizontal transferability rather than the reduced fitness cost. These findings unveil an evolutionary strategy in which the plasmid horizontal transferability and fitness cost are balanced. From the chromosome perspective, all evolved clones exhibit parallel mutations in the transcriptional regulatory stringent starvation Protein A gene sspA. Through a sspA knockout mutant, transcriptome analysis, in vitro transcriptional activity assay, RT-qPCR, motility test, and scanning electron microscopy techniques, we demonstrated that the mutation in sspA reduces its transcriptional inhibitory capacity, thereby improving bacterial fitness, biofilm formation ability, and gut colonization ability by promoting bacterial flagella synthesis. These findings expand our knowledge of how cointegrate plasmids adapt to new bacterial hosts.

Ligand-displaying Escherichia coli cells and minicells for programmable delivery of toxic payloads via type IV secretion systems

IMPORTANCE

The rapid emergence of drug-resistant bacteria and current low rate of antibiotic discovery emphasize the urgent need for alternative antibacterial strategies. We engineered Escherichia coli to conjugatively transfer plasmids to specific E. coli and Pseudomonas aeruginosa recipient cells through the surface display of cognate nanobody/antigen (Nb/Ag) pairs. We further engineered mobilizable plasmids to carry CRISPR/Cas9 systems (pCrispr) for the selective killing of recipient cells harboring CRISPR/Cas9 target sequences. In the assembled programmed delivery system (PDS), Nb-displaying E. coli donors with different conjugation systems and mobilizable pCrispr plasmids suppressed the growth of Ag-displaying recipient cells to significantly greater extents than unpaired recipients. We also showed that anucleate minicells armed with conjugation machines and pCrispr plasmids were highly effective in killing E. coli recipients. Together, our findings suggest that bacteria or minicells armed with PDSs may prove highly effective as an adjunct or alternative to antibiotics for antimicrobial intervention.

Ligand-Displaying E. coli Cells and Minicells for Programmable Delivery of Toxic Payloads via Type IV Secretion Systems

Bacterial type IV secretion systems (T4SSs) are highly versatile macromolecular translocators and offer great potential for deployment as delivery systems for therapeutic intervention. One major T4SS subfamily, the conjugation machines, are well-adapted for delivery of DNA cargoes of interest to other bacteria or eukaryotic cells, but generally exhibit modest transfer frequencies and lack specificity for target cells. Here, we tested the efficacy of a surface-displayed nanobody/antigen (Nb/Ag) pairing system to enhance the conjugative transfer of IncN (pKM101), IncF (F/pOX38), or IncP (RP4) plasmids, or of mobilizable plasmids including those encoding CRISPR/Cas9 systems (pCrispr), to targeted recipient cells. Escherichia coli donors displaying Nb's transferred plasmids to E. coli and Pseudomonas aeruginosa recipients displaying the cognate Ag's at significantly higher frequencies than to recipients lacking Ag's. Nb/Ag pairing functionally substituted for the surface adhesin activities of F-encoded TraN and pKM101-encoded Pep, although not conjugative pili or VirB5-like adhesins. Nb/Ag pairing further elevated the killing effects accompanying delivery of pCrispr plasmids to E. coli and P. aeruginosa transconjugants bearing CRISPR/Cas9 target sequences. Finally, we determined that anucleate E. coli minicells, which are clinically safer delivery vectors than intact cells, transferred self-transmissible and mobilizable plasmids to E. coli and P. aeruginosa cells. Minicell-mediated mobilization of pCrispr plasmids to E. coli recipients elicited significant killing of transconjugants, although Nb/Ag pairing did not enhance conjugation frequencies or killing. Together, our findings establish the potential for deployment of bacteria or minicells as Programmed Delivery Systems (PDSs) for suppression of targeted bacterial species in infection settings.

IMPORTANCE

The rapid emergence of drug-resistant bacteria and current low rate of antibiotic discovery emphasize an urgent need for alternative antibacterial strategies. We engineered Escherichia coli to conjugatively transfer plasmids to specific E. coli and Pseudomonas aeruginosa recipient cells through surface display of cognate nanobody/antigen (Nb/Ag) pairs. We further engineered mobilizable plasmids to carry CRISPR/Cas9 systems (pCrispr) for selective killing of recipient cells harboring CRISPR/Cas9 target sequences. In the assembled Programmed Delivery System (PDS), Nb-displaying E. coli donors with different conjugation systems and mobilizable pCrispr plasmids suppressed growth of Ag-displaying recipient cells to significantly greater extents than unpaired recipients. We also showed that anucleate minicells armed with conjugation machines and pCrispr plasmids were highly effective in killing of E. coli recipients. Together, our findings suggest that bacteria or minicells armed with PDSs may prove highly effective as an adjunct or alternative to antibiotics for antimicrobial intervention.

Real-time visualisation of the intracellular dynamics of conjugative plasmid transfer

Conjugation is a contact-dependent mechanism for the transfer of plasmid DNA between bacterial cells, which contributes to the dissemination of antibiotic resistance. Here, we use live-cell microscopy to visualise the intracellular dynamics of conjugative transfer of F-plasmid in E. coli, in real time. We show that the transfer of plasmid in single-stranded form (ssDNA) and its subsequent conversion into double-stranded DNA (dsDNA) are fast and efficient processes that occur with specific timing and subcellular localisation. Notably, the ssDNA-to-dsDNA conversion determines the timing of plasmid-encoded protein production. The leading region that first enters the recipient cell carries single-stranded promoters that allow the early and transient synthesis of leading proteins immediately upon entry of the ssDNA plasmid. The subsequent conversion into dsDNA turns off leading gene expression, and activates the expression of other plasmid genes under the control of conventional double-stranded promoters. This molecular strategy allows for the timely production of factors sequentially involved in establishing, maintaining and disseminating the plasmid.

Protein Transfer through an F Plasmid-Encoded Type IV Secretion System Suppresses the Mating-Induced SOS Response

Bacterial type IV secretion systems (T4SSs) mediate the conjugative transfer of mobile genetic elements (MGEs) and their cargoes of antibiotic resistance and virulence genes. Here, we report that the pED208-encoded T4SS (Tra_pED208) translocates not only this F plasmid but several plasmid-encoded proteins, including ParA, ParB1, single-stranded DNA-binding protein SSB, ParB2, PsiB, and PsiA, to recipient cells. Conjugative protein translocation through the Tra_pED208 T4SS required engagement of the pED208 relaxosome with the TraD substrate receptor or coupling protein. T4SSs translocate MGEs as single-stranded DNA intermediates (T-strands), which triggers the SOS response in recipient cells. Transfer of pED208 deleted of psiB or ssb, which, respectively, encode the SOS inhibitor protein PsiB and single-stranded DNA-binding protein SSB, elicited a significantly stronger SOS response than pED208 or mutant plasmids deleted of psiA, parA, parB1, or parB2. Conversely, translocation of PsiB or SSB, but not PsiA, through the Tra_pED208 T4SS suppressed the mating-induced SOS response. Our findings expand the repertoire of known substrates of conjugation systems to include proteins with functions associated with plasmid maintenance. Furthermore, for this and other F-encoded Tra systems, docking of the DNA substrate with the TraD receptor appears to serve as a critical activating signal for protein translocation. Finally, the observed effects of PsiB and SSB on suppression of the mating-induced SOS response establishes a novel biological function for conjugative protein translocation and suggests the potential for interbacterial protein translocation to manifest in diverse outcomes influencing bacterial communication, physiology, and evolution.

Conjugative DNA transfer induces the bacterial SOS response and promotes antibiotic resistance development through integron activation

Conjugation is one mechanism for intra- and inter-species horizontal gene transfer among bacteria. Conjugative elements have been instrumental in many bacterial species to face the threat of antibiotics, by allowing them to evolve and adapt to these hostile conditions. Conjugative plasmids are transferred to plasmidless recipient cells as single-stranded DNA. We used lacZ and gfp fusions to address whether conjugation induces the SOS response and the integron integrase. The SOS response controls a series of genes responsible for DNA damage repair, which can lead to recombination and mutagenesis. In this manuscript, we show that conjugative transfer of ssDNA induces the bacterial SOS stress response, unless an anti-SOS factor is present to alleviate this response. We also show that integron integrases are up-regulated during this process, resulting in increased cassette rearrangements. Moreover, the data we obtained using broad and narrow host range plasmids strongly suggests that plasmid transfer, even abortive, can trigger chromosomal gene rearrangements and transcriptional switches in the recipient cell. Our results highlight the importance of environments concentrating disparate bacterial communities as reactors for extensive genetic adaptation of bacteria.

Bacteria exchange DNA in their natural environments. The process called conjugation consists of DNA transfer by cell contact from one bacterium to another. Conjugative circular plasmids have been identified as shuttles and reservoirs for adaptive genes. It is now established that such lateral gene transfer plays an essential role, especially for the antibiotic resistance development and dissemination among bacteria. Moreover, integrons, platforms of mobile gene cassettes, have been instrumental in this phenomenon, through their successful association with conjugative resistance plasmids. We demonstrate in this study that the conjugative transfer of plasmids triggers a bacterial stress response—the SOS response—in recipient cells and can impact the cassette content of integrons. The SOS response is already known to induce various genome modifications. Human and animal pathogens cohabit with environmental bacteria, in niches which will favor DNA exchange. SOS induction during conjugation is thus most probably able to impact a wide range of genomes. Bacterial SOS response could then be a suitable target for co-treatment of infections in order to prevent exchange of antibiotic resistance/adaptation genes.

An SOS inhibitor that binds to free RecA protein: the PsiB protein

The process of bacterial conjugation involves the transfer of a conjugative plasmid as a single strand. The potentially deleterious SOS response, which is normally triggered by the appearance of single-stranded DNA, is suppressed in the recipient cell by a conjugative plasmid system centered on the product of the psiB gene. The F plasmid PsiB protein inhibits all activities of the RecA protein, including DNA binding, DNA strand exchange, and LexA protein cleavage. The proteins known to negatively regulate recombinases, such as RecA or Rad51, generally work at the level of dismantling the nucleoprotein filament. However, PsiB binds to RecA protein that is free in solution. The RecA-PsiB complex impedes formation of RecA nucleoprotein filaments on DNA.

Overexpression of the LexA-regulated tisAB RNA in E. coli inhibits SOS functions; implications for regulation of the SOS response

The DNA damage induced SOS response in Escherichia coli is initiated by cleavage of the LexA repressor through activation of RecA. Here we demonstrate that overexpression of the SOS-inducible tisAB gene inhibits several SOS functions in vivo. Wild-type E. coli overexpressing tisAB showed the same UV sensitivity as a lexA mutant carrying a noncleavable version of the LexA protein unable to induce the SOS response. Immunoblotting confirmed that tisAB overexpression leads to higher levels of LexA repressor and northern experiments demonstrated delayed and reduced induction of recA mRNA. In addition, induction of prophage λ and UV-induced filamentation was inhibited by tisAB overexpression. The tisAB gene contains antisense sequences to the SOS-inducible dinD gene (16 nt) and the uxaA gene (20 nt), the latter encoding a dehydratase essential for galacturonate catabolism. Cleavage of uxaA mRNA at the antisense sequence was dependent on tisAB RNA expression. We showed that overexpression of tisAB is less able to confer UV sensitivity to the uxaA dinD double mutant as compared to wild-type, indicating that the dinD and uxaA transcripts modulate the anti-SOS response of tisAB. These data shed new light on the complexity of SOS regulation in which the uxaA gene could link sugar metabolism to the SOS response via antisense regulation of the tisAB gene.

The SOS Regulatory Network

All organisms possess a diverse set of genetic programs that are used to alter cellular physiology in response to environmental cues. The gram-negative bacterium, Escherichia coli, mounts what is known as the "SOS response" following DNA damage, replication fork arrest, and a myriad of other environmental stresses. For over 50 years, E. coli has served as the paradigm for our understanding of the transcriptional, and physiological changes that occur following DNA damage (400). In this chapter, we summarize the current view of the SOS response and discuss how this genetic circuit is regulated. In addition to examining the E. coli SOS response, we also include a discussion of the SOS regulatory networks in other bacteria to provide a broader perspective on how prokaryotes respond to DNA damage.

Yeast Nop15p is an RNA-binding protein required for pre-rRNA processing and cytokinesis

Nop15p is an essential protein that contains an RNA recognition motif (RRM) and localizes to the nucleoplasm and nucleolus. Cells depleted of Nop15p failed to synthesize the 25S and 5.8S rRNA components of the 60S ribosomal subunit, and exonucleolytic 5' processing of 5.8S rRNA was strongly inhibited. Pre-rRNAs co-precipitated with tagged Nop15p confirmed its association with early pre-60S particles and Nop15p bound a pre-rRNA transcript in vitro. Nop15p-depleted cells show an unusually abrupt growth arrest prior to substantial depletion of ribosomal subunits. Following cell synchronization in mitosis, Nop15p-depleted cells undergo nuclear division with wild-type kinetics, activate the mitotic exit network and disassemble their mitotic spindle. However, they uniformly arrest at cytokinesis and fail to assemble a contractile actin ring at the bud neck. In dividing wild-type cells, segregation of nucleolar proteins to the daughter nuclei occurs after separation of the nucleoplasm. In these late mitotic cells, Nop15p was partially delocalized from the nucleolus to the nucleoplasm, consistent with a specific function in cell division in addition to its role in ribosome synthesis.

At the birth of molecular radiation biology

Rational thinking builds on feelings, too. This article starts with a tribute to Richard Setlow, an eminent scientist; it retraces as well some studies in molecular genetics that helped to understand basic questions of radiation biology. In the mid-1950s, the induction of a dormant virus (prophage) by irradiation of its host was an intriguing phenomenon. Soon, it was found that prophage induction results from the inactivation of the prophage repressor. Similarly, a score of induced cellular SOS functions were found to be induced when the LexA repressor is inactivated. Repressor inactivation involves the formation of a newly formed distinctive structure: a RecA-polymer wrapped around single-stranded DNA left by the arrest of replication at damaged sites. By touching this RecA nucleofilament, the LexA repressor is inactivated, triggering the sequential expression of SOS functions. The RecA nucleofilament acts as a chaperone, allowing recombinational repair to occur after nucleotide excision repair is over. The UmuD'C complex, synthesized slowly and parsimoniously, peaks at the end of recombinational repair, ready to be positioned at the tip of a RecA nucleofilament, placing the UmuD'C complex right at a lesion. At this location, UmuD'C prevents recombinational repair, and now acts as an error-prone paucimerase that fills the discontinuity opposite the damaged DNA. Finally, the elimination of lesions from the path of DNA polymerase, allows the resumption of DNA replication, and the SOS repair cycle switches to a normal cell cycle.

Expression of leading region genes on IncI1 plasmid ColIb-P9: genetic evidence for single-stranded DNA transcription

The leading region of a plasmid is the first sector to enter the recipient cell in bacterial conjugation. This sector of IncI1 plasmid ColIb-P9 includes genes that are transcribed in a transient pulse early in the conjugatively infected cell to promote establishment of the immigrant plasmid. Evidence is presented that the burst of gene expression is regulated by a process which is independent of a repressor but dependent on the orientation of the genes on the unique plasmid strand transferred in conjugation. The nucleotide sequence of 11.7 kb of the leading region was determined and found to contain 10 ORFs; all are orientated such that the template strand for transcription corresponds to the transferred strand. The leading region contains three dispersed repeats of a sequence homologous to a novel promoter in ssDNA described by H. Masai & K. Arai (1997, Cell 89, 897-907). It is proposed that the repeats are promoters that form in the transferring strand of ColIb to support transient transcription of genes transferred early in conjugation.

Nucleotide sequence of the F plasmid leading region

The entire nucleotide sequence of the first DNA segment of the conjugative F plasmid to enter the recipient cell, the leading region, is described. Analysis of the sequence provides further evidence that products encoded within the 13.2-kb leading region are likely to be expressed and perform functions associated with the transferred strand in the recipient cell.

Complete DNA sequence and detailed analysis of the Yersinia pestis KIM5 plasmid encoding murine toxin and capsular antigen

Yersinia pestis, the causative agent of plague, harbors at least three plasmids necessary for full virulence of the organism, two of which are species specific. One of the Y. pestis-specific plasmids, pMT1, is thought to promote deep tissue invasion, resulting in more acute onset of symptoms and death. We determined the entire nucleotide sequence of Y. pestis KIM5 pMT1 and identified potential open reading frames (ORFs) encoded by the 100,990-bp molecule. Based on codon usage for known yersinial genes, homology with known proteins in the databases, and potential ribosome binding sites, we determined that 115 of the potential ORFs which we considered could encode polypeptides in Y. pestis. Five of these ORFs were genes previously identified as being necessary for production of the classic virulence factors, murine toxin (MT), and the fraction 1 (F1) capsule antigen. The regions of pMT1 encoding MT and F1 were surrounded by remnants of multiple transposition events and bacteriophage, respectively, suggesting horizontal gene transfer of these virulence factors. We identified seven new potential virulence factors that might interact with the mammalian host or flea vector. Forty-three of the remaining 115 putative ORFs did not display any significant homology with proteins in the current databases. Furthermore, DNA sequence analysis allowed the determination of the putative replication and partitioning regions of pMT1. We identified a single 2,450-bp region within pMT1 that could function as the origin of replication, including a RepA-like protein similar to RepFIB, RepHI1B, and P1 and P7 replicons. Plasmid partitioning function was located ca. 36 kb from the putative origin of replication and was most similar to the parABS bacteriophage P1 and P7 system. Y. pestis pMT1 encoded potential genes with a high degree of similarity to a wide variety of organisms, plasmids, and bacteriophage. Accordingly, our analysis of the pMT1 DNA sequence emphasized the mosaic nature of this large bacterial virulence plasmid and provided implications as to its evolution.

Inhibition of Escherichia coli RecA coprotease activities by DinI

In Escherichia coli, the SOS response is induced upon DNA damage and results in the enhanced expression of a set of genes involved in DNA repair and other functions. The initial step, self-cleavage of the LexA repressor, is promoted by the RecA protein which is activated upon binding to single-stranded DNA. In this work, induction of the SOS response by the addition of mitomycin C was found to be prevented by overexpression of the dinI gene. dinI is an SOS gene which maps at 24.6 min of the E.coli chromosome and encodes a small protein of 81 amino acids. Immunoblotting analysis with anti-LexA antibodies revealed that LexA did not undergo cleavage in dinI-overexpressed cells after UV irradiation. In addition, the RecA-dependent conversion of UmuD to UmuD' (the active form for mutagenesis) was also inhibited in dinI-overexpressed cells. Conversely, a dinI-deficient mutant showed a slightly faster and more extensive processing of UmuD and hence higher mutability than the wild-type. Finally, we demonstrated, by using an in vitro reaction with purified proteins, that DinI directly inhibits the ability of RecA to mediate self-cleavage of UmuD.

Organization of the leading region of IncN plasmid pKM101 (R46): a regulation controlled by CUP sequence elements

Analysis of the nucleotide sequence of the 13.8 kb leading region of the IncN plasmid pKM101 (a deletion derivative of R46) revealed eight copies of highly conserved repetitive elements, CUP (Conserved UPstream), and at least nine novel open reading frames (ORFs). Appropriate protein products were identified for eight ORFs and the analysis of their deduced amino acid sequences revealed similarities with some well-known proteins (KorA of RK2/RP4, RecX and PsiB) that may play a role in the adaptation of promiscuous plasmids to the new host. Comparison of CUP elements revealed that the CUP core is 417 nucleotides long and consists of two portions that markedly differ in GC content. The larger portion (307 nucleotides) of the core is about 74% GC and contains at least one NotI site, while the other (110 nucleotides) is only about 40% GC. The remarkable features of CUP elements is that five of them are oriented in the same direction and fused in a similar mode to the open reading frames (ORFs) that are able to encode unrelated proteins. The spacings between the right boundary of the CUP core and the potential ATG start codons of these ORFs are slightly different in length (16 to 18 bp), highly divergent in sequence but in all cases contain the conserved hexamer 5'-AGGAGT-3' at the position that is typical for the ribosome binding site of Escherichia coli. The A+T-rich portion of the CUP sequences contains the strong negatively regulated promoter and appears to function as a genetic switch that coordinately controls the expression of CUP-fused genes during the conjugal transfer. These findings suggest that seven plasmid genes fused to the CUP elements including repA and two ard genes encoding positively acting replication protein and antirestriction proteins, respectively, may be members of one regulatory network based on the CUP elements and two plasmid-encoded regulatory proteins ArdK and ArdR. At least, the ArdK protein may act as a typical repressor by binding to the promoter region of the CUP sequence. Most of the structural and functional features of organization of the CUP-controlled regulatory network are associated with the idea that the CUP elements may be involved in the natural genetic engineering process of organizing various functionally related genes in one regulon.

Analysis of the sequence and gene products of the transfer region of the F sex factor

Bacterial conjugation results in the transfer of DNA of either plasmid or chromosomal origin between microorganisms. Transfer begins at a defined point in the DNA sequence, usually called the origin of transfer (oriT). The capacity of conjugative DNA transfer is a property of self-transmissible plasmids and conjugative transposons, which will mobilize other plasmids and DNA sequences that include a compatible oriT locus. This review will concentrate on the genes required for bacterial conjugation that are encoded within the transfer region (or regions) of conjugative plasmids. One of the best-defined conjugation systems is that of the F plasmid, which has been the paradigm for conjugation systems since it was discovered nearly 50 years ago. The F transfer region (over 33 kb) contains about 40 genes, arranged contiguously. These are involved in the synthesis of pili, extracellular filaments which establish contact between donor and recipient cells; mating-pair stabilization; prevention of mating between similar donor cells in a process termed surface exclusions; DNA nicking and transfer during conjugation; and the regulation of expression of these functions. This review is a compendium of the products and other features found in the F transfer region as well as a discussion of their role in conjugation. While the genetics of F transfer have been described extensively, the mechanism of conjugation has proved elusive, in large part because of the low levels of expression of the pilus and the numerous envelope components essential for F plasmid transfer. The advent of molecular genetic techniques has, however, resulted in considerable recent progress. This summary of the known properties of the F transfer region is provided in the hope that it will form a useful basis for future comparison with other conjugation systems.

PsiB, and anti-SOS protein, is transiently expressed by the F sex factor during its transmission to an Escherichia coli K-12 recipient

PsiB, an anti-SOS protein, shown previously to prevent activation of RecA protein, was purified from the crude extract of PsiB overproducing cells. PsiB is probably a tetrameric protein, whose subunit has a sequence-deduced molecular mass of 15741 daltons. Using an immuno-assay with anti-PsiB antibodies, we have monitored PsiB cell concentrations produced by F and R6-5 plasmids: the latter type produces a detectable level of PsiB protein while the former does not. The discrepancy can be assigned to a Tn10 out-going promoter located upstream of psiB. When we inserted a Tn10 promoter upstream of F psiB, the F PsiB protein concentration reached the level of R6-5 PsiB. We describe here the physiological role that PsiB protein may have in the cell and how it causes an anti-SOS function. We observed that PsiB protein was transiently expressed by a wild-type F sex factor during its transmission to an Escherichia coli K-12 recipient. In an F+ x F- cross, PsiB concentration increased at least 10-fold in F- recipient bacteria after 90 minutes and declined thereafter; the psiB gene may be repressed when F plasmid replicates vegetatively. PsiB protein may be induced zygotically so as to protect F single-stranded DNA transferred upon conjugation. PsiB protein, when overproduced, may interfere with RecA protein at chromosomal single-stranded DNA sites generated by discontinuous DNA replication, thus causing an SOS inhibitory phenotype.

Zygotic induction of plasmid ssb and psiB genes following conjugative transfer of Incl1 plasmid Collb-P9

The Incl1 conjugative plasmid Collb-P9 carries a psiB gene that prevents induction of the SOS response in host bacteria. This locus is located 2.5 kb downstream of the ssb (single-stranded DNA-binding protein) gene in the leading region. This portion of Collb is strikingly similar to part of the leading region of the otherwise distinct F plasmid. Expression of psiB and ssb is increased when the host cell is exposed to an SOS-inducing treatment or the Collb transfer system is derepressed. Moreover, expression of both genes on a derepressed plasmid is strongly enhanced in conjugatively infected recipient cells. Carriage of the psiB gene by Collb is shown to prevent a low level of SOS induction following conjugation. Plasmid ssb and psiB genes may function to promote installation of the replicon in the new cell.