Faecalis Plasmid pCF10 Research Articles

PrgE: an OB-fold protein from plasmid pCF10 with striking differences to prototypical bacterial SSBs.

A major pathway for horizontal gene transfer is the transmission of DNA from donor to recipient cells via plasmid-encoded type IV secretion systems (T4SSs). Many conjugative plasmids encode for a single-stranded DNA-binding protein (SSB) together with their T4SS. Some of these SSBs have been suggested to aid in establishing the plasmid in the recipient cell, but for many, their function remains unclear. Here, we characterize PrgE, a proposed SSB from the Enterococcus faecalis plasmid pCF10. We show that PrgE is not essential for conjugation. Structurally, it has the characteristic OB-fold of SSBs, but it has very unusual DNA-binding properties. Our DNA-bound structure shows that PrgE binds ssDNA like beads on a string supported by its N-terminal tail. In vitro studies highlight the plasticity of PrgE oligomerization and confirm the importance of the N-terminus. Unlike other SSBs, PrgE binds both double- and single-stranded DNA equally well. This shows that PrgE has a quaternary assembly and DNA-binding properties that are very different from the prototypical bacterial SSB, but also different from eukaryotic SSBs.

Dynamics of plasmid-mediated niche invasion, immunity to invasion, and pheromone-inducible conjugation in the murine gastrointestinal tract

Microbial communities provide protection to their hosts by resisting pathogenic invasion. Microbial residents of a host often exclude subsequent colonizers, but this protection is not well understood. The Enterococcus faecalis plasmid pCF10, whose conjugative transfer functions are induced by a peptide pheromone, efficiently transfers in the intestinal tract of mice. Here we show that an invading donor strain established in the gastrointestinal tract of mice harboring resident recipients, resulting in a stable, mixed population comprised of approximately 10% donors and 90% recipients. We also show that the plasmid-encoded surface protein PrgB (Aggregation Substance), enhanced donor invasion of resident recipients, and resistance of resident donors to invasion by recipients. Imaging of the gastrointestinal mucosa of mice infected with differentially labeled recipients and donors revealed pheromone induction within microcolonies harboring both strains in close proximity, suggesting that adherent microcolonies on the mucosal surface of the intestine comprise an important niche for cell-cell signaling and plasmid transfer.

Enterococcus faecalis Sex Pheromone cCF10 Enhances Conjugative Plasmid Transfer In Vivo.

ABSTRACTCell-cell communication mediated by peptide pheromones (cCF10 [CF]) is essential for high-frequency plasmid transfer in vitro in Enterococcus faecalis. To examine the role of pheromone signaling in vivo, we established either a CF-producing (CF+) recipient or a recipient producing a biologically inactive variant of CF (CF− recipient) in a germfree mouse model 3 days before donor inoculation and determined transfer frequencies of the pheromone-inducible plasmid pCF10. Plasmid transfer was detected in the upper and middle sections of the intestinal tract 5 h after donor inoculation and was highly efficient in the absence of antibiotic selection. The transconjugant/donor ratio reached a maximum level approaching 1 on day 4 in the upper intestinal tract. Plasmid transfer was significantly lower with the CF− recipient. While rescue of the CF− mating defect by coculture with CF+ recipients is easily accomplished in vitro, no extracellular complementation occurred in vivo. This suggests that most pheromone signaling in the gut occurs between recipient and donor cells in very close proximity. Plasmid-bearing cells (donors plus transconjugants) steadily increased in the population from 0.1% after donor inoculation to about 10% at the conclusion of the experiments. This suggests a selective advantage of pCF10 carriage distinct from antibiotic resistance or bacteriocin production. Our results demonstrate that pheromone signaling is required for efficient pCF10 transfer in vivo. In the absence of CF+ recipients, a low level of transfer to CF− recipients occurred in the gut. This may result from low-level host-mediated induction of the donors in the gastrointestinal (GI) tract, similar to that previously observed in serum.

Mechanisms of peptide sex pheromone regulation of conjugation in Enterococcus faecalis.

In many gram positive bacteria, horizontal transfer and virulence are regulated by peptide‐mediated cell‐cell signaling. The heptapeptide cCF10 (C) activates conjugative transfer of the Enterococcus faecalis plasmid pCF10, whereas the iCF10 (I) peptide inhibits transfer. Both peptides bind to the same domain of the master transcription regulator PrgX, a repressor of transcription of the prgQ operon encoding conjugation genes. We show that repression of prgQ by PrgX tetramers requires formation of a pCF10 DNA loop where each of two PrgX DNA‐binding sites is occupied by a dimer. I binding to PrgX enhances prgQ repression, while C binding has the opposite effect. Previous models suggested that differential effects of these two peptides on the PrgX oligomerization state accounted for their distinct functions. Our new results demonstrate that both peptides have similar, high‐binding affinity for PrgX, and that both peptides actually promote formation of PrgX tetramers with higher DNA‐binding affinity than Apo‐PrgX. We propose that differences in repression ability of PrgX/peptide complexes result from subtle differences in the structures of DNA‐bound PrgX/peptide complexes. Changes in the induction state of a donor cell likely results from replacement of one type of DNA‐bound peptide/PrgX tetramer with the other.

Structural analysis of the Anti-Q–Qs interaction: RNA-mediated regulation of E. faecalis plasmid pCF10 conjugation

Conjugation of the E. faecalis plasmid pCF10 is triggered in response to peptide sex pheromone cCF10 produced by potential recipients. Regulation of this response is complex and multi-layered and includes a small regulatory RNA, Anti-Q that participates in a termination/antitermination decision controlling transcription of the conjugation structural genes. In this study, the secondary structure of the Anti-Q transcript and its sites of interaction with its target, Qs, were determined. The primary site of interaction occurred at a centrally-located loop whose sequence showed high variability in analogous molecules on other pheromone-responsive plasmids. This loop, designated the specificity loop, was demonstrated to be important but not sufficient for distinguishing between Qs molecules from pCF10 and another pheromone-responsive plasmid pAD1. A loop 5′ from the specificity loop which carries a U-turn motif played no demonstrable role in Anti-Q–Qs interaction or regulation of the termination/antitermination decision. These results provide direct evidence for a critical role of Anti-Q–Qs interactions in posttranscriptional regulation of pCF10 transfer functions.

Sex Pheromone Response, Clumping, and Slime Production in Enterococcal Strains Isolated from Occluded Biliary Stents

Bile-resistant bacteria, particularly gram-positive Enterococcus faecalis and Enterococcus faecium, play an important role in biliary stent occlusion, because their sessile mode of growth protects them against host defenses and antimicrobial agents. Twelve E. faecalis and seven E. faecium strains isolated from occluded biliary stents have been investigated for slime production, presence of aggregation substance genes, and ability to adhere to Caco-2 cells. Ten isolates were strong producers of slime, and seven isolates produced clumps when exposed to pheromones of E. faecalis JH2-2 and/or OG1RF. The small E. faecium clumps differed from the large clumps of E. faecalis and were similar to those of E. faecium LS10(pBRG1) carrying a pheromone response plasmid. After induction with pheromones, the adhesion to Caco-2 cells of clumping-positive strains was found to increase from two- to fourfold. Amplicons of the expected size were detected in three clumping-positive and three clumping-negative E. faecalis isolates by using primers (agg) internal to a highly conserved region of the E. faecalis pheromone response plasmids pAD1, pPD1, and pCF10 and primers internal to prgB of the E. faecalis plasmid pCF10. The agg/prgB-positive E. faecalis strains were also positive in Southern hybridization experiments with a prgB-specific probe. No PCR products were obtained with the same primers from four clumping-positive isolates (one E. faecalis and three E. faecium strains), which were also Southern hybridization negative. Our results demonstrate that slime production and pheromone response are both present in isolated enterococci, suggesting that clinical strains with these features might have a selective advantage in colonizing biliary stents.

CcfA, the genetic determinant for the cCF10 peptide pheromone in Enterococcus faecalis OG1RF.

The nosocomial pathogen Enterococcus faecalis has a unique pheromone-inducible conjugative mating system. Conjugative transfer of the E. faecalis plasmid pCF10 is specifically induced by the cCF10 peptide pheromone (LVTLVFV). Genomic sequence information has recently allowed the identification of putative structural genes coding for the various enterococcal pheromones (D. B. Clewell et al., Mol. Microbiol. 35:246-247, 2000). The cCF10 pheromone sequence LVTLVFV was found within an open reading frame designated ccfA, encoding a putative lipoprotein precursor. Several other pheromone sequences were found in similar locations within other predicted lipoproteins. CcfA shows significant sequence relatedness to the Escherichia coli protein YidC, an inner membrane protein translocase, as well as to a large number of homologs identified in gram-positive and in gram-negative bacteria. Analysis of the deduced CcfA amino acid sequence suggested that mature cCF10 peptide could be formed from the proteolytic degradation of its signal peptide. Expression of the cloned ccfA gene with an inducible expression vector dramatically increased cCF10 production by E. faecalis and also resulted in cCF10 production by Lactococcus lactis, a non-pheromone producer. Site-directed mutagenesis of the ccfA sequence encoding the cCF10 peptide confirmed that ccfA was a functional genetic determinant for cCF10.

Two targets in pCF10 DNA for PrgX binding: their role in production of Qa and prgX mRNA and in regulation of pheromone-inducible conjugation

PrgX is the primary cytoplasmic protein involved in negative control of pheromone-inducible conjugation functions of the Enterococcus faecalis plasmid pCF10. PrgX is believed to act in concert with an antisense RNA called Qa to inhibit readthrough of transcription from the prgQ promoter into the pCF10 genes mediating conjugation functions; PrgX also positively regulates its own expression, as well as that of Qa. We found two DNA target sites for PrgX binding in the intergenic region between the prgX and prgQ genes of pCF10. The primary binding site near prgX includes an 11 bp palindromic sequence and showed relatively high affinity for His-tagged PrgX (His-PrgX). The secondary binding site is between the −35 and −10 regions of the prgQ promoter, and contains only a half of the palindromic sequence; this binding site showed weaker affinity. A region of pCF10 including the prgQ promoter and the secondary binding site reduced Qa RNA levels greatly and this reduction was overcome by the presence of the primary binding site and PrgX. In constructs where the binding sites were mutated individually or in combination, the intracellular levels of PrgX protein and Qa RNA were reduced significantly. On the basis of these results, we propose that both DNA binding sites are required for the autoregulation of PrgX expression and for positive regulation of Qa RNA.

Effect of pheromone induction on transfer of the Enterococcus faecalis plasmid pCF10 in intestinal mucus ex vivo.

The effect of synthetic sex pheromone on pheromone-inducible conjugation between the isogenic Enterococcus faecalis strains OG1RF and OG1SS was investigated in (i) Todd-Hewitt broth medium and (ii) intestinal mucus isolated from germ-free rats. In broth, the presence of synthetic pheromone cCF10 had no detectable effect on the transfer kinetics observed for the tetracycline resistance encoding plasmid pCF10. In mucus, presence of the same pheromone significantly increased the transfer efficiency observed during the first 2 h of conjugation, while the effect was less pronounced later in the experiment. We suggest that due to differences in diffusion rates and medium-binding of the pheromones, the effect of the synthetic cCF10 was immediately dominated by the effect of pheromones produced by the recipient E. faecalis strain in broth, while this happened later in mucus.

Effect of pheromone induction on transfer of the Enterococcus faecalis plasmid pCF10 in intestinal mucus ex vivo

The effect of synthetic sex pheromone on pheromone-inducible conjugation between the isogenic Enterococcus faecalis strains OG1RF and OG1SS was investigated in (i) Todd-Hewitt broth medium and (ii) intestinal mucus isolated from germ-free rats. In broth, the presence of synthetic pheromone cCF10 had no detectable effect on the transfer kinetics observed for the tetracycline resistance encoding plasmid pCF10. In mucus, presence of the same pheromone significantly increased the transfer efficiency observed during the first 2 h of conjugation, while the effect was less pronounced later in the experiment. We suggest that due to differences in diffusion rates and medium-binding of the pheromones, the effect of the synthetic cCF10 was immediately dominated by the effect of pheromones produced by the recipient E. faecalis strain in broth, while this happened later in mucus.

Sequence and analysis of the replication region of the Staphylococcus xylosus plasmid pSX267

The region of the 29.5-kb plasmid pSX267 from Staphylococcus xylosus DSM 20267 that is required for autonomous replication in staphylococci was isolated on a 1.8-kb DNA fragment. The sequence analysis of the fragment yielded two open reading frames, repA and orf2, encoding proteins of 37.2 and 13.2 kDa, respectively. The deduced amino acid sequence of repA showed similarity to the replication initiator protein of plasmid pAD1 from Enterococcus faecalis, to two proteins of unknown function encoded by the E. faecalis plasmid pCF10 and the Lactobacillus helveticus plasmid pLJ1, and surprisingly to the mouse interferon-response element binding factor 1. The repA gene of pSX267 is indispensable for replication suggesting that it encodes the replication initiator protein of pSX267. Introduction of a frameshift mutation into repA or deletion of 26 codons at its 3'-end resulted in nonreplicative plasmids. Removal of sequences required for repA expression also abolished replication. Complementation experiments with repA in trans identified the ori of pSX267 within the repA coding region. The second orf, which is located downstream of repA, could be deleted without affecting plasmid replication. It seems to be a nonfunctional remainder of a rolling circle plasmid of the pC194/pUB110 family, since its deduced amino acid sequence resembles the pC194/pUB110-type replication proteins. The minimal replicon of pSX267 defined so far comprises 1255 bp. Besides repA, no other plasmid-encoded gene is required to mediate replication in staphylococci.

Transcriptional analysis of a region of the Enterococcus faecalis plasmid pCF10 involved in positive regulation of conjugative transfer functions.

The prgB gene encodes aggregation substance (Asc10) which is essential for transfer of the pheromone-inducible conjugative plasmid pCF10 in Enterococcus faecalis. The prgQ and prgS regions, located 4 kb upstream of prgB, are required for the expression of prgB. Complementation studies indicated that the prgQ region functions in cis and in an orientation-dependent manner relative to the prgB gene (J. W. Chung and G. M. Dunny, Proc. Natl. Acad. Sci. USA 89:9020-9024, 1992). Analysis of transcriptional fusions in this study, using a promoterless lacZ gene in several locations between prgQ and prgB, confirmed that the prgQ region does not carry a promoter for the expression of prgB and that prgB does not comprise an operon with prgA (which encodes the surface exclusion protein Sec10), the gene immediately upstream from prgB. Northern (RNA) blot analysis demonstrated that two distinct transcripts (Qs RNA and QL RNA), much larger than the prgQ gene, were expressed in the prgQ region. QS RNA was produced constitutively, whereas QL RNA was produced inducibly by pheromone. The lack of any other open reading frame in QL RNA and significant sequence complementarity between the 3' end of QL RNA and the promoter region of prgB suggested that the functional products of the prgQ region might be RNA molecules rather than proteins. A mutation in prgS completely abolished the production of QL RNA. A model for transcriptional activation of prgB is presented.

Genetic analysis of a region of the Enterococcus faecalis plasmid pCF10 involved in positive regulation of conjugative transfer functions

The prgB gene encodes the surface protein Asc10, which mediates cell aggregation resulting in high-frequency conjugative transfer of the pheromone-inducible tetracycline resistance plasmid pCF10 in Enterococcus faecalis. Previous Tn5 insertional mutagenesis and sequencing analysis of a 12-kb fragment of pCF10 indicated that a region containing prgX, -Q, -R, -S, and -T, located 3 to 6 kb upstream of prgB, is required to activate the expression of prgB. Complementation studies showed that the positive regulatory region functions in cis in an orientation-dependent manner (J. W. Chung and G. M. Dunny, Proc. Natl. Acad. Sci. USA 89:9020-9024, 1992). In order to determine the involvement of each gene in the activation of prgB, Tn5 insertional mutagenesis and exonuclease III deletion analyses of the regulatory region were carried out. The results indicate that prgQ and -S are required for the expression of prgB, while prgX, -R, and -T are not required. Western blot (immunoblot) analysis of these mutants shows that prgQ is also essential for the expression of prgA (encoding the surface exclusion protein Sec10), which is located between prgB and the positive-control region. Complementation analysis demonstrates that a cis-acting regulatory element is located in the prgQ region and that pCF10 sequences in an untranslated region 3' from prgQ are an essential component of the positive-control system. Analyses of various Tn5 insertions in pCF10 genes suggest that transcription reading into this transposon is terminated in E. faecalis but that outward-reading transcripts may initiate from within the ends of Tn5 or from the junction sequences.

Conjugative mobilization of a vancomycin resistance plasmid by a putative Enterococcus faecium sex pheromone response plasmid.

Recent epidemiological evidence suggests that horizontal gene transfer may be an important mechanism for dissemination of vancomycin resistance. A filter mating survey of 21 VanA Enterococcus faecium isolates from The New York Hospital showed that 14 of these isolates transferred vancomycin resistance (Vmr) to the plasmid-free reference strain Enterococcus faecalis JH2-2. One isolate, E. faecium R7, was selected for further study based on its ability to transfer Vmr to strain JH2-2 in liquid culture. Analysis of the plasmid content of transconjugants revealed three general classes. The predominant class (28 of 47 transconjugants) contained two separate plasmids: pHKK702 and pHKK703. pHKK702 is a 41-kb plasmid that contains an element indistinguishable from the Vmr transposon Tn1546 and an element that hybridizes with an ermB probe from the Staphylococcus aureus erythromycin resistance transposon Tn551. pHKK703 is a 55-kb plasmid that hybridizes with probes for the sex pheromone response genes prgA, prgB, and prgX derived from the E. faecalis plasmid pCF10. The second group of transconjugants (18 of 47) contained various recombinant forms of pHKK702 and pHKK703, whereas a third transconjugant class contained only pHKK702 (1 of 47). Transconjugants that contained both pHKK702 and pHKK703 were able to efficiently transfer Vmr to recipient strains in broth or on filters. However, no transfer of Vmr was detected using the donor containing only pHKK702. The transfer of Vmr from the recombination-deficient derivative of E. faecalis JH2-2 [strain UV202(pHKK702, pHKK703)] was reduced 600-fold compared to that of JH2-2(pHKK702, pHKK703). We propose that pHKK703 functions as an E. faecium sex pheromone response plasmid that conjugatively mobilizes pHKK702, and that a major pathway for this mobilization may require donor-mediated recombination proficiency. This report provides the first example in which a plasmid containing a Tn1546-related element is conjugatively mobilized.

Characterization of the traC determinant of the Enterococcus faecalis hemolysin-bacteriocin plasmid pAD1: binding of sex pheromone

pAD1, a conjugative, 60-kb, hemolysin-bacteriocin plasmid in Enterococcus faecalis, encodes a mating response to a small peptide sex pheromone, cAD1, secreted by potential recipient bacteria. A gene, traC, encoding a 60.7-kDa protein with a typical amino terminal signal peptide, was identified within a region that appears to encode a product that binds to exogenous pheromone. A cloned segment of DNA containing traC resulted in specific binding of cells to synthetic cAD1. The putative traC product has strong similarity to a product of the E. faecalis plasmid pCF10 as well as oligopeptide binding proteins of Escherichia coli, Salmonella typhimurium, and Bacillus subtilis.

Role of the pheromone-inducible surface protein Asc10 in mating aggregate formation and conjugal transfer of the Enterococcus faecalis plasmid pCF10

The high transfer frequency of pheromone-inducible conjugative plasmids of Enterococcus faecalis in liquid culture is due in part to the formation of mating aggregates. These aggregates result from the interaction of two surface components, aggregation substance (AS), which is plasmid encoded, and the chromosomally encoded binding substance (BS). In the accompanying paper (S.-M. Kao, S. B. Olmsted, A. S. Viksnins, J.C. Gallo, G. M. Dunny, J. Bacteriol, 173:7650-7664, 1991), the sequence of the prgB gene encoding the AS molecule (Asc10) produced by pheromone-induced cells carrying plasmid pCF10 is presented. Here we report the results of genetic and immunological experiments which define the role of Asc10 in aggregation and plasmid transfer. These data indicate expression of AS on the surface of an E. faecalis cell and its binding to BS expressed on a second cell are required for the formation of a mating pair and the efficient transfer of pCF10 in liquid matings. However, the orientation of the receptors was not critical for transfer; ie., AS expressed on recipient cells could facilitate plasmid transfer via binding to BS on the donor. Our results suggest that additional (as yet unidentified) products are involved in forming the channel that ultimately serves to transfer the DNA, with AS-BS binding serving primarily to generate the initial attachment between cells. The putative prgC gene product, identified by DNA sequencing (data presented in the accompanying paper), could be involved in transfer events occurring subsequent to aggregation.

Two conjugation systems associated with Streptococcus faecalis plasmid pCF10: identification of a conjugative transposon that transfers between S. faecalis and Bacillus subtilis

The tetracycline resistance plasmid pCF10 (58 kilobases [kb]) of Streptococcus faecalis possesses two separate conjugation systems. A 25-kb region of the plasmid (designated TRA) was shown previously to determine pheromone response and conjugation functions required for transfer of pCF10 between S. faecalis cells (P. J. Christie and G. M. Dunny, Plasmid 15:230-241, 1986). When S. faecalis cells were mixed with Bacillus subtilis in broth, tetracycline resistance was transferred from S. faecalis. The tetracycline-resistant B. subtilis cells contained a 16-kb region of pCF10 (distinct from TRA) that carried the tetracycline resistance determinant (Tetr). This Tetr element was found to transfer between S. faecalis and B. subtilis strains in the absence of plasmids. Genetic and molecular techniques were used to establish locations of the element at several different sites on the B. subtilis chromosome. The Tetr element could be transferred in filter matings from B. subtilis to S. faecalis strains and between recombination-proficient and -deficient S. faecalis strains in the absence of any plasmid DNA. The transfer required direct cell-to-cell contact and was not inhibited by DNase. The Tetr element was shown to transpose from the S. faecalis chromosome to various locations within the hemolysin plasmid pAD1. Together, the data indicate that the Tetr element, termed transposon Tn925, is very similar to the conjugative transposon Tn916 in both structure and function. A derivative of Tn925, containing transposon Tn917 inserted into a site approximately 3 kb from one end, exhibited elevated transfer frequencies and may provide a useful means for delivering Tn917 by conjugation into various gram-positive species.

Identification of regions of the Streptococcus faecalis plasmid pCF-10 that encode antibiotic resistance and pheromone response functions

The conjugative plasmid pCF-10 (58 kb) of Streptococcus faecalis has been mapped with restriction enzymes. By restriction mapping and Southern hybridization analysis, a 16-kb segment of the plasmid was shown to resemble closely the conjugative tetracycline resistance transposon, Tn916. Mutagenesis of the plasmid with the erythromycin resistance transposon Tn917 was used to localize a tetracycline resistance determinant and several regions involved in conjugal transfer. Fifty Tn917 insertions (outside the region of the plasmid homologous to Tn916) affecting mating behavior and the ability of donor cells to respond to the sex pheromone cCF-10 were mapped to nine distinct segments, or tra regions. Insertions into tra regions 1–3 and 7–9 led to an enhanced transfer ability of mutant plasmids relative to the transfer frequency obtained for the wild-type plasmid. Cells carrying these mutant plasmids differed in colony morphology or growth in broth culture from cells carrying pCF-10. Insertions into tra regions 4–6 resulted in reduced plasmid transfer, or completely eliminated the mating potential of donor cells. Insertions generating transfer-defective plasmids could be grouped further according to the ability of strains harboring the mutant plasmids to respond to cCF-10. HindIII fragments of pCF-10 coding for transfer functions have been cloned into Escherichia coli.

Induction of surface exclusion (entry exclusion) by Streptococcus faecalis sex pheromones: use of monoclonal antibodies to identify an inducible surface antigen involved in the exclusion process.

The Streptococcus faecalis plasmid pCF-10 is representative of a class of plasmids that enables its host cells to respond to sex pheromones produced by other S. faecalis cells. The pheromone response has been previously shown to result in increased conjugal plasmid transfer, cell clumping, and multiple cell-surface antigenic changes. To test for other effects of pheromone induction, cells carrying pCF-10 were used as recipients in matings with an isogenic donor strain carrying a derivative of pCF-10, tagged with a transposon to provide an additional selective marker. Pheromone induction of the "male recipients" decreased their recipient ability by a factor of 10-300 in comparison to uninduced cells or plasmid-free recipients. These results indicate that an entry exclusion (surface exclusion) function, similar to that described in studies of plasmids in Gram-negative bacteria, is induced during the S. faecalis pheromone response process. The exclusion operates only against homologous plasmids. Immunological, biochemical, and genetic experiments using monoclonal antibodies reactive with C130, the predominant protein antigen associated with the pheromone response of cells carrying pCF-10, indicate that this antigen is involved in surface exclusion. The data also support the notion that synthesis of C130 involves a posttranslational modification of a precursor of C130 to a final product of higher molecular weight form.