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

Surface Proteins on Gram-Positive Bacteria

Surface proteins are critical for the survival of gram-positive bacteria both in the environment and to establish an infection. Depending on the organism, their surface proteins are evolutionarily tailored to interact with specific ligands on their target surface, be it inanimate or animate. Most surface molecules on these organisms are covalently anchored to the peptidoglycan through an LPxTG motif found at the C-terminus. These surface molecules are generally modular with multiple binding or enzymatic domains designed for a specific survival function. For example, some molecules will bind serum proteins like fibronectin or fibrinogen in one domain and have a separate function in another domain. In addition, enzymes such as those responsible for the production of ATP may be generally found on some bacterial surfaces, but when or how they are used in the life of these bacteria is currently unknown. While surface proteins are required for pathogenicity but not viability, targeting the expression of these molecules on the bacterial surface would prevent infection but not death of the organism. Given that the number of different surface proteins could be in the range of two to three dozen, each with two or three separate functional domains (with hundreds to thousands of each protein on a given organism), exemplifies the complexity that exists on the bacterial surface. Because of their number, we could not adequately describe the characteristics of all surface proteins in this chapter. However, since the streptococcal M protein was one of the first gram-positive surface protein to be completely sequenced, and perhaps one of the best studied, we will use M protein as a model for surface proteins in general, pointing out differences with other surface molecules when necessary.

Enterococcal Genetics

The study of the genetics of enterococci has focused heavily on mobile genetic elements present in these organisms, the complex regulatory circuits used to control their mobility, and the antibiotic resistance genes they frequently carry. Recently, more focus has been placed on the regulation of genes involved in the virulence of the opportunistic pathogenic species Enterococcus faecalis and Enterococcus faecium. Little information is available concerning fundamental aspects of DNA replication, partition, and division; this article begins with a brief overview of what little is known about these issues, primarily by comparison with better-studied model organisms. A variety of transcriptional and posttranscriptional mechanisms of regulation of gene expression are then discussed, including a section on the genetics and regulation of vancomycin resistance in enterococci. The article then provides extensive coverage of the pheromone-responsive conjugation plasmids, including sections on regulation of the pheromone response, the conjugative apparatus, and replication and stable inheritance. The article then focuses on conjugative transposons, now referred to as integrated, conjugative elements, or ICEs, and concludes with several smaller sections covering emerging areas of interest concerning the enterococcal mobilome, including nonpheromone plasmids of particular interest, toxin-antitoxin systems, pathogenicity islands, bacteriophages, and genome defense.

[Pathogenicity of Enterococci]

Enterococci belong to the group of lactic acid bacteria (LAB), and inhabit the gastrointestinal tracts of a wide variety of animals from insects and to human, and the commensal organism in humans and animals. The commensal/probiotic role of enterococci has evolved through thousands of years in mutual coexistence. Enterococcus have many favorable traits that have been appreciated in food fermentation and preservation, and many serve as probiotics to promote health. While lactobacillus have been shown to confer numerous benefits on and often regarded as health bringing organisms, enterococci have become more recognized as emerging human pathogens in recent years. Mac Callum and Hastings characterized an organism, now known to be Enterococcal faecalis, which was isolated from a lethal case of endocarditis on 1899. The report was the first detailed description of its pathogenic capabilities. Over the past few decades, multi-drug resistance enterococci have become as important health-care associated pathogen, and leading causes of drug resistance infection. The modern life style including the broad use of antibiotics in medical practice and animal husbandry have selected for the convergence of potential virulence factors to the specific enterococcus species such as E. faecium and E. faecalis. The development of modern medical care of intensive and invasive medical therapies and treatments for human disease, and existence of severe compromised patients in hospitals has contributed to the increased prevalence of these opportunistic organisms. The virulence factors converged in E. faecalis and E. faecium which have been isolated in nosocomial infections, include antibiotic resistance, extracellular proteins (toxins), extrachromosome and mobile genetic elements, cell wall components, biofilm formation, adherence factors, and colonization factor such as bacteriocin, etc. In these potential virulence factors, I presented characteristics of enterococcal conjugative plasmid, cytolysin, collagen binding protein of adhesion, bacteriocins, and drug resistances. I made reference to our original reports, and review books for this review. The review books are "Enterococci: from Commensals to Leading Causes of Drug Resistant Infection, NCBI Bookshelf. A service of the National Library of Medicine, National Institute of Health. Ed. by Michael S Gilmore, Don B Clewell, Yasuyoshi Ike, and Nathan Shankar", and "The Enterococci: Pathogenesis, Molecular Biology, and Antibiotic Resistance, Gilmore M., Clewell D., Courvadin P., Dunny G., Murray B., Rice L., (ed) 2002. ASM Press".

Identification of a peptide-pheromone that enhances Listeria monocytogenes escape from host cell vacuoles

Listeria monocytogenes is a Gram-positive facultative intracellular bacterial pathogen that invades mammalian cells and escapes from membrane-bound vacuoles to replicate within the host cell cytosol. Gene products required for intracellular bacterial growth and bacterial spread to adjacent cells are regulated by a transcriptional activator known as PrfA. PrfA becomes activated following L. monocytogenes entry into host cells, however the signal that stimulates PrfA activation has not yet been defined. Here we provide evidence for L. monocytogenes secretion of a small peptide pheromone, pPplA, which enhances the escape of L. monocytogenes from host cell vacuoles and may facilitate PrfA activation. The pPplA pheromone is generated via the proteolytic processing of the PplA lipoprotein secretion signal peptide. While the PplA lipoprotein is dispensable for pathogenesis, bacteria lacking the pPplA pheromone are significantly attenuated for virulence in mice and have a reduced efficiency of bacterial escape from the vacuoles of nonprofessional phagocytic cells. Mutational activation of PrfA restores virulence and eliminates the need for pPplA-dependent signaling. Experimental evidence suggests that the pPplA peptide may help signal to L. monocytogenes its presence within the confines of the host cell vacuole, stimulating the expression of gene products that contribute to vacuole escape and facilitating PrfA activation to promote bacterial growth within the cytosol.

Virulence Plasmids of Nonsporulating Gram-Positive Pathogens

Gram-positive bacteria are leading causes of many types of human infection, including pneumonia, skin and nasopharyngeal infections, as well as urinary tract and surgical wound infections among hospitalized patients. These infections have become particularly problematic because many of the species causing them have become highly resistant to antibiotics. The role of mobile genetic elements, such as plasmids, in the dissemination of antibiotic resistance among Gram-positive bacteria has been well studied; less well understood is the role of mobile elements in the evolution and spread of virulence traits among these pathogens. While these organisms are leading agents of infection, they are also prominent members of the human commensal ecology. It appears that these bacteria are able to take advantage of the intimate association between host and commensal, via virulence traits that exacerbate infection and cause disease. However, evolution into an obligate pathogen has not occurred, presumably because it would lead to rejection of pathogenic organisms from the host ecology. Instead, in organisms that exist as both commensal and pathogen, selection has favored the development of mechanisms for variability. As a result, many virulence traits are localized on mobile genetic elements, such as virulence plasmids and pathogenicity islands. Virulence traits may occur within a minority of isolates of a given species, but these minority populations have nonetheless emerged as a leading problem in infectious disease. This chapter reviews virulence plasmids in nonsporulating Gram-positive bacteria, and examines their contribution to disease pathogenesis.

Antibiotic resistant enterococci-tales of a drug resistance gene trafficker

Enterococci have been recognized as important hospital-acquired pathogens in recent years, and isolates of E. faecalis and E. faecium are the third- to fourth-most prevalent nosocomial pathogen worldwide. Acquired resistances, especially against penicilin/ampicillin, aminoglycosides (high-level) and glycopeptides are therapeutically important and reported in increasing numbers. On the other hand, isolates of E. faecalis and E. faecium are commensals of the intestines of humans, many vertebrate and invertebrate animals and may also constitute an active part of the plant flora. Certain enterococcal isolates are used as starter cultures or supplements in food fermentation and food preservation. Due to their preferred intestinal habitat, their wide occurrence, robustness and ease of cultivation, enterococci are used as indicators for fecal pollution assessing hygiene standards for fresh- and bathing water and they serve as important key indicator bacteria for various veterinary and human resistance surveillance systems. Enterococci are widely prevalent and genetically capable of acquiring, conserving and disseminating genetic traits including resistance determinants among enterococci and related Gram-positive bacteria. In the present review we aimed at summarizing recent advances in the current understanding of the population biology of enterococci, the role mobile genetic elements including plasmids play in shaping the population structure and spreading resistance. We explain how these elements could be classified and discuss mechanisms of plasmid transfer and regulation and the role and cross-talk of enterococcal isolates from food and food animals to humans.

Tales of conjugation and sex pheromones: A plasmid and enterococcal odyssey

This review covers highlights of the author's experience becoming and working as a plasmid biologist. The account chronicles a progression from studies of ColE1 DNA in Escherichia coli to Gram-positive bacteria with an emphasis on conjugation in enterococci. It deals with gene amplification, conjugative transposons and sex pheromones in the context of bacterial antibiotic resistance.

Cloning, E. coli overexpression, purification and binding properties of TraA and TraC, two proteins involved in the pheromone-dependent conjugation process in enterococci

The bacteriocin encoding plasmid pPD1 from Enterococcus faecalis is involved in a mating response to the sex pheromone cPD1 produced by recipient bacterial cells devoid of pPD1. Previous studies showed that cPD1 is internalized into donor cells in a process in which TraC plays the role of cell surface pheromone receptor. Inside the recipient cells, the pheromone binds to the plasmid-encoded cytoplasmic protein TraA, able to recognize specific DNA sequences and to modulate the conjugation process. To avoid self-induction of the conjugation process, donor cells produce the inhibitor iPD1, which competes with cPD1. This study was designed to produce recombinant TraA and TraC in a functionally active state and to evaluate their main functional properties. We have isolated the sequences encoding TraA and TraC from the plasmid pPD1 and cloned them in suitable expression vectors. The two recombinant proteins were successfully obtained in a soluble form using Escherichia coli as expression host and a T7 inducible expression system. TraC and TraA were purified to homogeneity by three or two chromatographic steps, respectively, leading to a final yield up to 4mg/l of cell culture for TraC and up to 10mg/l of cell culture for TraA. The ability of TraA and TraC to bind the specific pheromone and inhibitor peptides has been assessed by means of ESI-mass spectrometry. Moreover, the ability of recombinant TraA to bind DNA has been demonstrated by means of electrophoretic mobility shift assay. Overall these results are consistent with the heterologously expressed TraC and TraA being functionally active.

Properties of Enterococcus faecalis plasmid pAD1, a member of a widely disseminated family of pheromone-responding, conjugative, virulence elements encoding cytolysin

The 60-kb pAD1 represents a large and widely disseminated family of conjugative, pheromone-responding, virulence plasmids commonly found in clinical isolates of Enterococcus faecalis. It encodes a hemolysin/bacteriocin (cytolysin) shown to contribute to virulence in animal models, and the related bacteriocin is active against a wide variety of Gram-positive bacteria. This review summarizes what is currently known about the molecular biology of pAD1, including aspects of its cytolytic, UV-resistance, replication, maintenance, and conjugative properties.

Characterization of the pheromone response of the Enterococcus faecalis conjugative plasmid pCF10: complete sequence and comparative analysis of the transcriptional and phenotypic responses of pCF10-containing cells to pheromone induction

The sex pheromone plasmids in Enterococcus faecalis are one of the most efficient conjugative plasmid transfer systems known in bacteria. Plasmid transfer rates can reach or exceed 10(-1) transconjugants per donor in vivo and under laboratory conditions. We report the completion of the DNA sequence of plasmid pCF10 and the analysis of the transcription profile of plasmid genes, relative to conjugative transfer ability following pheromone induction. These experiments employed a mini-microarray containing all 57 open reading frames of pCF10 and a set of selected chromosomal genes. A clear peak of transcription activity was observed 30 to 60 min after pheromone addition, with transcription subsiding 2 h after pheromone induction. The transcript activity correlated with the ability of donor cells to transfer pCF10 to recipient cells. Remarkably, aggregation substance (Asc10, encoded by the prgB gene) was present on the cell surface for a long period of time after pheromone-induced transcription of prgB and plasmid transfer ability had ceased. This observation could have relevance for the virulence of E. faecalis.

Enterococcal peptide sex pheromones: synthesis and control of biological activity

The enterococcal pheromone-inducible plasmids such as pCF10 represent a unique class of mobile genetic elements whose transfer functions are induced by peptide sex pheromones. These pheromones are excreted by potential recipient cells and detected by plasmid-containing donor cells at the cell surface, where the pheromone is imported and signals induction of the plasmid transfer system. Pheromone is processed from a chromosomally encoded lipoprotein and excreted by both the donor and recipient cells, but a carefully controlled detection system prevents a response to self-pheromone while still allowing an extremely sensitive response to exogenous pheromone.

Comparative study using type strains and clinical and food isolates to examine hemolytic activity and occurrence of the cyl operon in enterococci

The hemolytic ability, the presence of cyl genes, and the diagnostic accuracy of cytolysin molecular detection were investigated in the genus Enterococcus by using 164 strains from 20 different species (26 reference strains, 42 clinical isolates from human and veterinary origin, and 96 isolates from ewe cheese and milk). Hemolysis was assayed with sheep and horse erythrocytes and under aerobic or anaerobic conditions. Screening of cytolysin genes (cylL(L), cylL(S), cylM, cylB, and cylA) was performed with new specific primers and the anaerobic assay of beta-hemolysis was used as the "gold standard" for the evaluation of cyl gene-based PCRs. Since beta-hemolysis and cyl genes were found in 10 and 14 species, respectively, the hemolytic ability seems to be spread throughout the genus ENTEROCOCCUS: Beta-hemolysis was observed in 6 of 26 (23%) reference strains, 14 of 42 (33%) clinical isolates, and 6 of 96 (6%) food isolates. The presence of cyl genes was detected in 15 of 26 (58%) reference strains, 37 of 42 (88%) clinical isolates, and 67 of 96 (70%) food isolates. These data indicate a virulence potential in food isolates, reinforcing the need of their safety assessment. Analysis of phenotypic-genotypic congruence suggests a divergent sequence evolution of cyl genes and the effect of environmental factors in the regulation of cytolysin expression. Evaluation of the diagnostic accuracy of cytolysin molecular detection points to cylL(L)-based PCR and cylL(L)L(S)MBA-based PCR as the most reliable approaches. Nevertheless, the low sensitivity (46%) and gene variability indicated by our study strongly recommend the phenotypic assay for the assessment of hemolytic ability in enterococci, followed by the molecular screening of cyl genes in nonhemolytic strains to evaluate their virulence potential.

The genes coding for enterocin EJ97 production by Enterococcus faecalis EJ97 are located on a conjugative plasmid

Enterococcus faecalis EJ97 produces a cationic bacteriocin (enterocin EJ97) of low molecular mass (5,327.7 Da). The complete amino acid sequence of enterocin EJ97 was elucidated after automated microsequencing of oligopeptides generated by endoproteinase GluC digestion and cyanogen bromide treatment. Transfer of the 60-kb conjugative plasmid pEJ97 from the bacteriocinogenic strain E. faecalis EJ97 to E. faecalis OG1X conferred bacteriocin production and resistance on the recipient. The genetic determinants of enterocin EJ97 were located in an 11.3-kb EcoRI-BglII DNA fragment of pEJ97. This region was cloned and sequenced. It contains the ej97A structural gene plus three open reading frames (ORFs) (ej97B, ej97C, and ej97D) and three putative ORFs transcribed in the opposite direction (orfA, orfB, and orfC). The gene ej97A translated as a 44-amino-acid residue mature protein lacking a leader peptide with no homology to other bacteriocins described so far. The product of ej97B (Ej97B) shows strong homology in its C-terminal domain to the superfamily of bacterial ATP-binding cassette transporters. The products of ej97C (Ej97C) and ej97D (Ej97D) could be proteins with 71 and 64 residues, respectively, of unknown functions and with no significant similarity to known proteins. There are two additional ORFs (ORF1 and ORF6) flanking the ej97 module, which have been identified as a transposon-like structure (tnp). ORF1 shows similarities to transposase of the Lactococcus lactis element ISS1 and is up to 50% identical to IS1216. This is flanked by two 18-bp inverted repeats (IRs) that are almost identical to those of ISS1 and IS1216. ORF6 (resEJ97) shows strong homology to the resolvase of plasmid pAM373 and up to 40 to 50% homology with the recombinase of several multiresistant plasmids and transposons from Staphylococcus aureus and E. faecalis. These data suggest that EJ97 could represent a new class of bacteriocins with a novel secretion mechanism and that the whole structure could be a composite transposon. Furthermore, two additional gene clusters were found: one cluster is probably related to the region responsible for the replication of plasmid pEJ97, and the second cluster is related to the sex pheromone response. These regions showed a high homology to the corresponding regions of the conjugative plasmids pAM373, pPD1, and pAD1 of E. faecalis, suggesting that they have a common origin.

Identification of the cAD1 sex pheromone precursor in Enterococcus faecalis

The Enterococcus faecalis virulence plasmid pAD1 encodes a mating response induced by exposure to an octapeptide sex pheromone, cAD1, secreted by plasmid-free enterococci. The determinant for the pheromone in E. faecalis FA2-2, designated cad, was found to encode a 309-amino-acid lipoprotein precursor with the last 8 residues of its 22-amino acid signal sequence representing the cAD1 moiety. The lipoprotein moiety contained two 77-amino-acid repeats (70% identity) separated by 45 residues. The nonisogenic E. faecalis strain V583 determinant encodes a homologous precursor protein, but it differs at two amino acid positions, both of which are located within the pheromone peptide moiety (positions 2 and 8). Construction of a variant of strain FA2-2 containing the differences present in V583 resulted in cells that did not produce detectable cAD1. The mutant appeared normal under laboratory growth conditions, and while significantly reduced in recipient potential, when carrying pAD1 it exhibited a normal mating response. A mutant of FA2-2 with a truncated lipoprotein moiety appeared normal with respect to recipient potential and, when carrying plasmid DNA, donor potential. A gene encoding a protein designated Eep, believed to be a zinc metalloprotease, had been previously identified as required for pheromone biosynthesis and was believed to be involved in the processing of a pheromone precursor. Our new observation that the pAD1-encoded inhibitor peptide, iAD1, whose precursor is itself a signal sequence, is also dependent on Eep is consistent with the likelihood that such processing occurs at the amino terminus of the cAD1 moiety.

The intracellular function of extracellular signaling peptides

A novel class of extracellular signaling peptides has been identified in Gram-positive bacteria that are actively transported into the cell to interact with intracellular receptors. The defining members of this novel class of signaling peptides are the Phr peptides of Bacillus subtilis and the mating pheromones of Enterococcus faecalis. These peptides are small and unmodified, gene encoded, and secreted by the bacterium. Most of these peptides diffuse into the extracellular medium, and when their concentration is sufficiently high, they are then actively transported into the cell by an oligopeptide permease (Opp). Once inside the cell, these peptides interact with an array of intracellular receptors. In B. subtilis, the Phr peptides regulate development of environmentally resistant spores and genetically competent cells (i.e. the natural ability to take up exogenous DNA). In E. faecalis, the mating pheromones regulate cell-cell transfer of plasmids, many of which encode antibiotic resistance or virulence factors. At least one component of the signaling pathway for these peptides is conserved in many bacteria, Opp. Opp is a non-specific transporter that transports peptides for use as carbon and nitrogen sources. The possibility that other bacteria could possess similar intracellularly functioning signaling peptides is discussed.

Completion of the nucleotide sequence of the Enterococcus faecalis conjugative virulence plasmid pAD1 and identification of a second transfer origin

pAD1 is a 59.3-kb plasmid in Enterococcus faecalis that has been the subject of intense investigation with regard to its pheromone-inducible conjugation behavior as well as its contribution to virulence. Approximately two-thirds of the pAD1 nucleotide sequence has been previously reported. Here we report on an analysis of the final approximately 22 kb, a significant portion of which is believed to encode structural genes associated with conjugation. The conjugation-related region was also found to contain a new (second) origin of conjugative transfer (oriT). A list of open reading frames covering the entire plasmid is presented.

Bacteriocin production in vancomycin-resistant and vancomycin-susceptible Enterococcus isolates of different origins

Bacteriocin production was determined for 218 Enterococcus isolates (Enterococcus faecalis [93] and E. faecium [125]) obtained from different origins (human clinical samples [87], human fecal samples [78], sewage [28], and chicken samples [25]) and showing different vancomycin susceptibility patterns (vancomycin resistant, all of them vanA positive [56], and vancomycin susceptible [162]). All enterococcal isolates were randomly selected except for the vancomycin-resistant ones. A total of 33 isolates of eight different bacterial genera were used as indicators for bacteriocin production. Forty-seven percent of the analyzed enterococcal isolates were bacteriocin producers (80.6% of E. faecalis and 21.6% of E. faecium isolates). The percentage of bacteriocin producers was higher among human clinical isolates (63.2%, 81.8% of vancomycin-resistant isolates and 60.5% of vancomycin-susceptible ones) than among isolates from the other origins (28 to 39.3%). Only one out of the 15 vancomycin-resistant isolates from human fecal samples was a bacteriocin producer, while 44.4% of fecal vancomycin-susceptible isolates were. The bacteriocin produced by the vanA-containing E. faecium strain RC714, named bacteriocin RC714, was further characterized. This bacteriocin activity was cotransferred together with the vanA genetic determinant to E. faecalis strain JH2-2. Bacteriocin RC714 was purified to homogeneity and its primary structure was determined by amino acid sequencing, showing an identity of 88% and a similarity of 92% with the previously described bacteriocin 31 from E. faecalis YI717. The presence of five different amino acids in bacteriocin RC714 suggest that this could be a new bacteriocin. The results obtained suggest that the epidemiology of vancomycin resistance may be influenced by different factors, including bacteriocin production.

Enterococcus faecalis conjugative plasmid pAM373: complete nucleotide sequence and genetic analyses of sex pheromone response

pAM373 is a 36.7 kb conjugative plasmid in Enterococcus faecalis that encodes a response to a peptide sex pheromone, cAM373, secreted by plasmid-free (recipient) strains of enterococci. It was identified over 15 years ago as one of five plasmids in E. faecalis strain RC73 and was of interest because a related pheromone activity could be detected in culture supernatants of Staphylococcus aureus and Streptococcus gordonii. Because of increased clinical concern relating to the possibility of mobilizing vancomycin resistance determinants from enterococci, where they are becoming common, into pathogens such as S. aureus, efforts were initiated to characterize pAM373 further. The results of a complete nucleotide sequence determination of pAM373, as well as a genetic analysis of key genes related to regulation of the pheromone response, are reported here. With regard to determinants related to conjugation, the plasmid has a structural organization similar to other known pheromone-responsive plasmids such as pAD1, pCF10 and pPD1; however, there are several unique features. Although there are significant homologues relating to a pheromone-binding surface protein (TraC) and a negatively regulating protein (TraA), there is an absence of a determinant equivalent to traB of pAD1 (reduces endogenous pheromone) and a determinant for surface-exclusion protein. The precursor structure of the inhibitor peptide iAM373 was identified, and its determinant (iam373) was found to be about 500 nt upstream of an apparent transcription terminator t1. Tn917-lac insertion analyses provided interesting insights into aspects of control of the pheromone response and showed that, although the traA product is sensitive to pheromone, it appears to act differently from the traA homologue of pAD1.

Pheromone-regulated expression of sex pheromone plasmid pAD1-encoded aggregation substance depends on at least six upstream genes and a cis-acting, orientation-dependent factor

Conjugative transfer of Enterococcus faecalis-specific sex pheromone plasmids relies on an adhesin, called aggregation substance, to confer a tight cell-to-cell contact between the mating partners. To analyze the dependence of pAD1-encoded aggregation substance, Asa1, on pheromone induction, a variety of upstream fragments were fused to an alpha-amylase reporter gene, amyL, by use of a novel promoter probe vector, pAMY-em1. For pheromone-regulated alpha-amylase activity, a total of at least six genes, traB, traC, traA, traE1, orfY, and orf1, are required: TraB efficiently represses asa1 (by a mechanism unrelated to its presumptive function in pheromone shutdown, since a complete shutdown is observed exclusively in the presence of traC); only traC can relieve traB-mediated repression in a pheromone-dependent manner. In addition to traB, traA is required but not sufficient for negative control. Mutational inactivation of traE1, orfY, or orf1, respectively, results in a total loss of alpha-amylase activity for constructs normally mediating constitutive expression. Inversion of a fragment covering traA, P(0), and traE1 without disrupting any gene or control element switches off amyL or asa1 expression, indicating the involvement of a cis-acting, orientation-dependent factor (as had been shown for plasmid pCF10). Unexpectedly, pAD1 represses all pAMY-em1 derivatives in trans, while its own pheromone-dependent functions are unaffected. The discrepancy between the new data and those of former studies defining TraE1 as a trans-acting positive regulator is discussed.

Identification and characterization of a determinant (eep) on the Enterococcus faecalis chromosome that is involved in production of the peptide sex pheromone cAD1

Plasmid-free strains of Enterococcus faecalis secrete a peptide sex pheromone, cAD1, which specifically induces a mating response by donors carrying the hemolysin plasmid pAD1 or related elements. A determinant on the E. faecalis OG1X chromosome has been found to encode a 46.5-kDa protein that plays an important role in the production of the extracellular cAD1. Wild-type E. faecalis OG1X cells harboring a plasmid chimera carrying the determinant exhibited an eightfold enhanced production of cAD1, and plasmid-free cells carrying a mutated chromosomal determinant secreted undetectable or very low amounts of the pheromone. The production of other pheromones such as cPD1, cOB1, and cCF10 was also influenced, although there was no effect on the pheromone cAM373. The determinant, designated eep (for enhanced expression of pheromone), did not include the sequence of the pheromone. Its deduced product (Eep) contains apparent membrane-spanning sequences; conceivably it is involved in processing a pheromone precursor structure or in some way regulates expression or secretion.

Regulation of the Enterococcus faecalis pAD1-related sex pheromone response: analyses of traD expression and its role in controlling conjugation functions

The Enterococcus faecalis haemolysin plasmid pAD1 (60 kb) confers a conjugative mating response to an octapeptide sex pheromone (cAD1) secreted by plasmid-free strains. The response involves two plasmid-borne regulatory determinants: traE1, whose product positively regulates all or most conjugation-related structural genes; and traA, whose product negatively regulates traE1 by controlling transcriptional readthrough of an upstream termination site (TTS1/TTS2). TraA binds to the promoter region of iad, which encodes a pheromone-inhibitor peptide, iAD1; and TTS1/TTS2 tightly terminates transcription arriving from this promoter during the uninduced state. A determinant, traD, appearing to encode a small peptide (23 amino acids), is located just downstream of iad and is in the opposite orientation. Transcripts of traD were identified and found to be present at a relatively high level in cells not expressing conjugation functions; the amount of RNA was greatly reduced, however, upon induction of the pheromone response. The decrease in traD RNA was not a consequence of the induced activity of TraE1, as it also occurred in a traE1 insertion mutant. A mutation in traD that would eliminate translation but that did not affect transcription had no apparent effect on the cell phenotype, indicating that RNA was likely to be the functional product. This was consistent with our finding that synthesis of traD RNA containing the translational defect was able to complement, in trans, a temperature-sensitive traD mutation. Thus, transcription of the traD determinant is significantly involved in downregulation of the pAD1 pheromone response.

Lyme disease-causing Borrelia species encode multiple lipoproteins homologous to peptide-binding proteins of ABC-type transporters

To identify cell envelope proteins of Borrelia burgdorferi, the causative agent of Lyme disease, we constructed a library of B. burgdorferi genes fused to the Escherichia coli phoA gene, which expresses enzymatically active alkaline phosphatase. One such gene, oppA-1, encodes a predicted polypeptide with significant similarities to various peptide-binding proteins of ABC-type transporters. Immediately downstream of oppA-1 are two genes, oppA-2 and oppA-3, whose predicted polypeptide products show strong similarities in their amino acid sequences to OppA-1, including a sequence that resembles the most highly conserved region in peptide-binding proteins. By labeling with [3H]palmitate, OppA-1, OppA-2, and OppA-3 were shown to be lipoproteins. DNA hybridization analysis showed that the oppA-1 oppA-2 oppA-3 region is located on the linear chromosome of B. burgdorferi, and the genes are conserved among different Borrelia species that cause Lyme disease (B. burgdorferi, B. garinii, and B. afzelli), suggesting that all three homologous genes are important to the maintenance of Lyme disease spirochetes in one or more of their hosts.

Efficient transfer of the pheromone-independent Enterococcus faecium plasmid pMG1 (Gmr) (65.1 kilobases) to Enterococcus strains during broth mating

Plasmid pMG1 (65.1 kb) was isolated from a gentamicin-resistant Enterococcus faecium clinical isolate and was found to encode gentamicin resistance. EcoRI restriction of pMG1 produced five fragments, A through E, with molecular sizes of 50.2, 11.5, 2.0, 0.7, and 0.7 kb, respectively. The clockwise order of the fragments was ACDEB. pMG1 transferred at high frequency to Enterococcus strains in broth mating. pMG1 transferred between Enterococcus faecalis strains, between E. faecium strains, and between E. faecium and E. faecalis strains at a frequency of approximately 10(-4) per donor cell after 3 h of mating. The pMG1 transfers were not induced by the exposure of the donor cell to culture filtrates of plasmid-free E. faecalis FA2-2 or an E. faecium strain. Mating aggregates were not observed by the naked eye during broth mating. Small mating aggregates of several cells in the broth matings were observed by microscopy, while no aggregates of donor cells which had been exposed to a culture filtrate of E. faecalis FA2-2 or an E. faecium strain were observed, even by microscopy. pMG1 DNA did not show any homology in Southern hybridization with that of the pheromone-responsive plasmids and broad-host-range plasmids pAMbeta1 and pIP501. These results indicate that there is another efficient transfer system in the conjugative plasmids of Enterococcus and that this system is different from the pheromone-induced transfer system of E. faecalis plasmids.

Regulation of the pAD1 sex pheromone response of Enterococcus faecalis by direct interaction between the cAD1 peptide mating signal and the negatively regulating, DNA-binding TraA protein

The Enterococcus faecalis conjugative plasmid pAD1 (60 kb) encodes a mating response to the recipient-produced peptide sex pheromone cAD1. The response involves two key plasmid-encoded regulatory proteins: TraE1, which positively regulates all or most structural genes relating to conjugation, and TraA, which binds DNA and negatively regulates expression of traE1. In vitro studies that included development of a DNA-associated protein-tag affinity chromatography technique showed that TraA (37.9 kDa) binds directly to cAD1 near its carboxyl-terminal end and, as a consequence, loses its affinity for DNA. Analyses of genetically modified TraA proteins indicated that truncations within the carboxyl-terminal 9 residues significantly affected the specificity of peptide-directed association/dissociation of DNA. The data support earlier observations that transposon insertions near the 3' end of traA eliminated the ability of cells to respond to cAD1.

Oligopeptide permease in Borrelia burgdorferi: putative peptide-binding components encoded by both chromosomal and plasmid loci

To elucidate the importance of oligopeptide permease for Borrelia burgdorferi, the agent of Lyme disease, a chromosomal locus in B. burgdorferi that encodes homologues of all five subunits of oligopeptide permease has been identified and characterized. B. burgdorferi has multiple copies of the gene encoding the peptide-binding component, OppA; three reside at the chromosomal locus and two are on plasmids. Northern analyses indicate that each oppA gene is independently transcribed, although the three chromosomal oppA genes are also expressed as bi- and tri-cistronic messages. Induction of one of the plasmid-encoded oppA genes was observed following an increase in temperature, which appears to be an important cue for adaptive responses in vivo. The deduced amino acid sequences suggest that all five borrelial oppA homologues are lipoproteins, but the protease-resistance of at least one of them in intact bacteria is inconsistent with outer-surface localization. Insertional inactivation of a plasmid-encoded oppA gene demonstrates that it is not essential for growth in culture.

Molecular mechanism of peptide-specific pheromone signaling in Enterococcus faecalis: functions of pheromone receptor TraA and pheromone-binding protein TraC encoded by plasmid pPD1

Conjugative transfer of the Enterococcus faecalis plasmid pPD1 is activated by cPD1, one of several peptide sex pheromones secreted by plasmid-free recipient cells, and is blocked by a donor-produced peptide inhibitor, iPD1. Using a tritiated pheromone, [3H]cPD1, we investigated how pPD1-harboring donor cells receive these peptide signals. Donor cells rapidly incorporated [3H]cPD1. The cell extract but not the membrane fraction of the donor strain exhibited significant [3H]cPD1-binding activity. On the basis of these data and those of tracer studies, it was demonstrated that cPD1 was internalized, where it bound to a high-molecular-weight compound. The cell extract of a strain carrying the traA-bearing multicopy plasmid (pDLHH21) also exhibited high [3H]cPD1-binding activity. A recombinant TraA exhibited a dissociation constant of 0.49 +/- 0.08 nM against [3H]cPD1. iPD1 competitively inhibited [3H]cPD1 binding to TraA, whereas pheromones and inhibitors relating to other plasmid systems did not. These results show that TraA is a specific intracellular receptor for cPD1 and that iPD1 acts as an antagonist for TraA. A strain carrying the traC-bearing multicopy plasmid (pDLES23) exhibited significant [3H]cPD1-binding activity. A strain carrying traC-disrupted pPD1 (pAM351CM) exhibited lower [3H] cPD1-binding activity as well as lower sensitivity to cPD1 than a wild-type donor strain. Some of the other pheromones and inhibitors inhibited [3H]cPD1 binding to the traC transformant like cPD1 and iPD1 did. These results show that TraC, as an extracellular less-specific pheromone-binding protein, supports donor cells to receive cPD1.

Control of genes for conjugative transfer of plasmids and other mobile elements

Conjugative transfer is a primary means of spread of mobile genetic elements (plasmids and transposons) between bacteria.It leads to the dissemination and evolution of the genes (such as those conferring resistance to antibiotics) which are carried by the plasmid. Expression of the plasmid genes needed for conjugative transfer is tightly regulated so as to minimise the burden on the host. For plasmids such as those belonging to the IncP group this results in downregulation of the transfer genes once all bacteria have a functional conjugative apparatus. For F-like plasmids (apart from F itself which is a derepressed mutant) tight control results in very few bacteria having a conjugative apparatus. Chance encounters between the rare transfer-proficient bacteria and a potential recipient initiate a cascade of transfer which can continue until all potential recipients have acquired the plasmid. Other systems express their transfer genes in response to specific stimuli. For the pheromone-responsive plasmids of Enterococcus it is small peptide signals from potential recipients which trigger the conjugative transfer genes. For the Ti plasmids of Agrobacterium it is the presence of wounded plants which are susceptible to infection which stimulates T-DNA transfer to plants. Transfer and integration of T-DNA induces production of opines which the plasmid-positive bacteria can utilise. They multiply and when they reach an appropriate density their plasmid transfer system is switched on to allow transfer of the Ti plasmid to other bacteria. Finally some conjugative transfer systems are induced by the antibiotics to which the elements confer resistance. Understanding these control circuits may help to modify management of microbial communities where plasmid transfer is either desirable or undesirable. z 1998 Published by Elsevier Science B.V.

Cloning and genetic and sequence analyses of the bacteriocin 21 determinant encoded on the Enterococcus faecalis pheromone-responsive conjugative plasmid pPD1

The pheromone-responsive conjugative plasmid pPD1 (59 kb) of Enterococcus faecalis encodes the bacteriocin 21 (bac21) determinant. Cloning, transposon insertion mutagenesis and sequence analysis of the bac21 determinant showed that an 8.5-kb fragment lying between kb 27.1 and 35.6 of the pPD1 map is required for complete expression of the bacteriocin. The 8.5-kb fragment contained nine open reading frames (ORFs), bacA to bac1, which were oriented in the same (upstream-to-downstream) direction. Transposon insertions into the bacA to bacE ORFs, which are located in the proximal half of bac21, resulted in defective bacteriocin expression. Insertions into the bacF to bac1 ORFs, which are located in the distal half of bac21, resulted in reduced bacteriocin expression. Deletion mutant analysis of the cloned 8.5-kb fragment revealed that the deletion of segments between kb 31.6 and 35.6 of the pPD1 map, which contained the distal region of the determinant encoding bacF to bac1, resulted in reduced bacteriocin expression. The smallest fragment (4.5 kb) retaining some degree of bacteriocin expression contained the bacA to bacE sequences located in the proximal half of the determinant. The cloned fragment encoding the 4.5-kb proximal region and a Tn916 insertion mutant into pPD1 bacB trans-complemented intracellularly to give complete expression of the bacteriocin. bacA encoded a 105-residue sequence with a molecular mass of 11.1 kDa. The deduced BacA protein showed 100% homology to the broad-spectrum antibiotic peptide AS-48, which is encoded on the E. faecalis conjugative plasmid pMB2 (58 kb). bacH encoded a 195-residue sequence with a molecular mass of 21.9 kDa. The deduced amino acid sequence showed significant homology to the C-terminal region of HlyB (31.1% identical residues), a protein located in the Escherichia coli alpha-hemolysin operon that is a representative bacterial ATP-binding cassette export protein.

Cloning and genetic analysis of the UV resistance determinant (uvr) encoded on the Enterococcus faecalis pheromone-responsive conjugative plasmid pAD1

The conjugative pheromone-responsive plasmid pAD1 (59.6 kb) of Enterococcus faecalis encodes a UV resistance determinant (uvr) in addition to the hemolysin-bacteriocin determinant. pAD1 enhances the UV resistance of wild-type E. faecalis FA2-2 and E. faecalis UV202, which is a UV-sensitive derivative of E. faecalis JH2-2. A 2.972-kb fragment cloned from between 27.7 and 30.6 kb of the pAD1 map conferred UV resistance function on UV202. Sequence analysis showed that the cloned fragment contained three open reading frames designated uvrA, uvrB, and uvrC. The uvrA gene is located on the pAD1 map between 28.1 and 29.4 kb. uvrB is located between 30.1 and 30.3 kb, and uvrC is located between 30.4 and 30.6 kb on the pAD1 map. The uvrA, uvrB, and uvrC genes encode sequences of 442, 60, and 74 amino acids, respectively. The deduced amino acid sequence of the uvrA-encoded protein showed 20% homology of the identical residues with the E. coli UmuC protein. Tn917 insertion mutagenesis and deletion mutant analysis of the cloned fragment showed that uvrA conferred UV resistance. A palindromic sequence, 5'-GAACNGTTC-3', which is identical to the consensus sequence found within the putative promoter region of the Bacillus subtilis DNA damage-inducible genes, was located within the promoter region of uvrA. Two uvrA transcripts of different lengths (i.e., 1.54 and 2.14 kb) which terminate at different points downstream of uvrA were detected in UV202 carrying the deletion mutant containing uvrA. The longer transcript, 2.14 kb, was not detected in UV202 carrying the deletion mutant containing both uvrA and uvrB, which suggests that uvrB encodes a terminator for the uvrA transcript. The uvrA transcript was not detected in any significant quantity in UV202 carrying the cloned fragment containing uvrA, uvrB, and uvrC; on the other hand, the 1.54-kb uvrA transcript was detected in the strain exposed to mitomycin C, which suggests that the UvrC protein functions as a regulator of uvrA.

Cell-cell communication in gram-positive bacteria

In gram-positive bacteria, many important processes are controlled by cell-to-cell communication, which is mediated by extracellular signal molecules produced by the bacteria. Most of these signaling molecules are peptides or modified peptides. Signal processing, in most cases, involves either transduction across the cytoplasmic membrane or import of the signal and subsequent interaction with intracellular effectors. Concentrations of signal in the nanomolar range or below are frequently sufficient for biological activity. The microbial processes controlled by extracellular signaling include the expression of virulence factors, the expression of gene transfer functions, and the production of antibiotics.

Enterococcus faecalis antigens in human infections

Genomic libraries of two Enterococcus faecalis strains, OG1RF and TX52 (an isolate from an endocarditis patient), were constructed in cosmid vectors pBeloBAC11 and pLAFRx, and screened with a serum from a rabbit immunized with surface proteins of an E. faecalis endocarditis isolate and sera from four patients with enterococcal endocarditis. Seventy-five cosmid clones reacted with at least two of the sera. Thirty-eight of the 75 immunopositive clones were considered to contain distinct inserts based on their DNA restriction patterns and were chosen for further subcloning into a pBluescript vector. Each sublibrary was screened with one of the five sera, and the DNA sequence of the immunopositive subclones was determined. Analysis of these sequences revealed similarities to a range of proteins, including bacterial virulence factors, transporters, two-component regulators, metabolic enzymes, and membrane or cell surface proteins. Fourteen subclones did not show significant similarity to any sequence in the databases and may contain novel genes. Thirteen of the immunopositive cosmid clones did not yield immunopositive subclones, and one such cosmid clone produced a nonprotein antigen in Escherichia coli.

Regulation of transfer of the Enterococcus faecalis pheromone-responding plasmid pAD1: temperature-sensitive transfer mutants and identification of a new regulatory determinant, traD

The enterococcal, conjugative, cytolysin plasmid pAD1 confers a mating response to the peptide sex pheromone cAD1 secreted by plasmid-free strains of Enterococcus faecalis. Cells carrying pAM714, a pAD1::Tn917 derivative with wild-type conjugation properties, were mutagenized with ethyl methanesulfonate to obtain variants that were induced (in the absence of pheromone) to transfer plasmid DNA upon shifting from 32 to 42 degrees C. Of 31 such mutants generated, the results of analyses of 7 are presented in detail. All seven strains were thermosensitive in the E. faecalis host FA2-2; colony morphology, clumping, and DNA transfer correlated well with each other at the two temperatures. In the nonisogenic host E. faecalis OG1X, however, only one derivative (pAM2725) exhibited correlation of all three traits at both temperatures. Three (pAM2700, pAM2703, and pAM2717) clumped and had colonies characteristic of pheromone-induced cells at 32 degrees C but transferred plasmid DNA at a higher frequency only at the elevated temperature. The other three (pAM2708, pAM2709, and pAM2712) were derepressed at both temperatures for all three characteristics. Four of the mutations, including that of pAM2725, mapped within the traA determinant, whereas two mapped identically in a previously unnoted open reading frame (designated traD) putatively encoding a short (23-amino-acid) peptide downstream of the inhibitor peptide determinant iad and in the opposite orientation. One mutant could not be located in the regions sequenced. Studies showed that the traA and traD mutations could be complemented in trans with a DNA fragment carrying the corresponding regions.

High-level plasmid-mediated gentamicin resistance and pheromone response of plasmids present in clinical isolates of Enterococcus faecalis

Eleven pheromone-responding plasmids encoding erythromycin or gentamicin resistance were isolated from multiresistant clinical Enterococcus faecalis isolates. The plasmids were classified into six types with respect to their pheromone responses. The three erythromycin resistance plasmids responded to different pheromones. Of the eight gentamicin resistance plasmids, four plasmids responded to same pheromone. Southern hybridization studies showed that the genes involved in regulation of the pheromone response were conserved in the drug resistance plasmids.

Directed insertion of a selectable marker into a circular plasmid of Borrelia burgdorferi

Studies of the biology of Borrelia burgdorferi and the pathogenesis of Lyme disease are severely limited by the current lack of genetic tools. As an initial step toward facile genetic manipulation of this pathogenic spirochete, we have investigated gene inactivation by allelic exchange using a mutated borrelial gyrB gene that confers resistance to the antibiotic coumermycin A1 as a selectable marker. We have transformed B. burgdorferi by electroporation with a linear fragment of DNA in which this selectable marker was flanked by sequences from a native borrelial 26-kb circular plasmid. We have identified coumermycin A1-resistant transformants in which gyrB had interrupted the targeted site on the 26-kb plasmid via homologous recombination with the flanking sequences. Antibiotic resistance conferred by the mutated gyrB gene on the plasmid is dominant, and transformed spirochetes carrying this plasmid do not contain any unaltered copies of the plasmid. Coumermycin A1 resistance can be transferred to naive B. burgdorferi by transformation with borrelial plasmid DNA from the initial transformants. This work represents the first example of a directed mutation in B. burgdorferi whereby a large segment of heterologous DNA (gyrB) has been inserted via homologous recombination with flanking sequences, thus demonstrating the feasibility of specific gene inactivation by allelic exchange.

Mechanisms of adhesion by oral bacteria

Adherence to a surface is a key element for colonization of the human oral cavity by the more than 500 bacterial taxa recorded from oral samples. Three surfaces are available: teeth, epithelial mucosa, and the nascent surface created as each new bacterial cell binds to existing dental plaque. Oral bacteria exhibit specificity for their respective colonization sites. Such specificity is directed by adhesin-receptor cognate pairs on genetically distinct cells. Colonization is successful when adherent cells grow and metabolically participate in the oral bacterial community. The potential roles of adherence-relevant molecules are discussed in the context of the dynamic nature of the oral econiche.

Cloning and genetic organization of the bacteriocin 31 determinant encoded on the Enterococcus faecalis pheromone-responsive conjugative plasmid pYI17

The conjugative plasmid pYI17 (57.5 kb) isolated from Enterococcus faecalis YI717 confers a pheromone response on the host and encodes the bacteriocin 31 gene. Bacteriocin 31 is active against E. hirae 9790, E. faecium, and Listeria monocytogenes. pYI17 was mapped physically by restriction enzyme analysis and the relational clone method. Deletion mutant and sequence analyses of the EcoRI fragment B cloned from pYl17 revealed that a 1.0-kb fragment contained the bacteriocin gene (bacA) and an immunity gene (bacB). This fragment induced bacteriocin activity in E. faecalis OG1X and E. hirae 9790. The bacA gene is located on the pYI17 physical map between 3.37 and 3.57 kb, and bacB is located between 3.59 kb and 3.87 kb, bacA encodes 67 amino acids, and bacB encodes 94 amino acids. The deduced amino acid sequence of the bacA protein contained a series of hydrophobic residues typical of a signal sequence at its amino terminus. The predicted mature bacA protein (43 amino acids) showed sequence homology with the membrane-active class II bacteriocins of lactic acid bacteria. Analysis of Tn5 insertion mutants and the resulting transcripts indicated that these genes are transcribed as an operon composed of bacA, bacB, and an open reading frame located downstream of bacB designated ORF3.

Enterococcus faecalis pheromone binding protein, PrgZ, recruits a chromosomal oligopeptide permease system to import sex pheromone cCF10 for induction of conjugation

Conjugative transfer of the plasmid pCF10 by Enterococcus faecalis donor cells occurs in response to a peptide sex pheromone, cCF10, secreted by recipients. The plasmid-encoded cCF10 binding protein, PrgZ, is similar in sequence to binding proteins (OppAs) encoded by oligopeptide permease (opp) operons. Mutation of prgZ decreased the sensitivity of donor cells to pheromone, whereas inactivation of the chromosomal E. faecalis opp operon abolished response at physiological concentrations of pheromone. Affinity chromatography experiments demonstrated the interaction of the pheromone with several putative intracellular regulatory molecules, including an RNA molecule required for positive regulation of conjugation functions. These data suggest that processing of the pheromone signal involves recruitment of a chromosomal Opp system by PrgZ and that signaling occurs by direct interaction of internalized pheromone with intracellular effectors.

A binding-lipoprotein-dependent oligopeptide transport system in Streptococcus gordonii essential for uptake of hexa- and heptapeptides

Cells of the oral bacterium Streptococcus gordonii express three cytoplasmic membrane-bound lipoproteins with apparent molecular masses of 76 to 78 kDa that are the products of three genes (designated hppA, hppG, and hppH). The lipoproteins are immunologically cross-reactive, contain 60% or more identical amino acid residues, and are highly similar to the AmiA, AliA (PlpA), and AliB substrate-binding protein components of an oligopeptide permease in Streptococcus pneumoniae. Insertional inactivation of the hppA or hppH gene resulted in loss of the ability of S. gordonii cells to utilize specific peptides of five to seven amino acid residues for growth. An insertion within the COOH-terminal coding region of hppG that caused apparent truncation of the HppG polypeptide had a similar effect; however, S. gordonii mutants in which HppG polypeptide production was abolished were still able to grow on all oligopeptides tested. Inactivation of hppA gene (but not inactivation of the hppG or hppH gene) caused reduced growth rate of cells in complex medium, slowed the rate of development of competence for transformation, reduced the efficiency of transformation, and increased the resistance of cells to aminopterin. These results suggest that the formation of a solute-binding-protein complex consisting of at least the HppA and the HppH lipopolypeptides is necessary for binding and subsequent uptake of primarily hexa- or heptapeptides by a Hpp (Hexa-heptapeptide permease) system in S. gordonii. In addition, Hpp may play a role in the control of metabolic functions associated with the growth of streptococcal cells on complex nitrogen sources and with the development of competence.

An unmodified heptadecapeptide pheromone induces competence for genetic transformation in Streptococcus pneumoniae

Competence for genetic transformation in Streptococcus pneumoniae has been known for three decades to arise in growing cultures at a critical cell density, in response to a secreted protease-sensitive signal. We show that strain CP1200 produces a 17-residue peptide that induces cells of the species to develop competence. The sequence of the peptide was found to be H-Glu-Met-Arg-Leu-Ser-Lys-Phe-Phe-Arg-Asp-Phe-Ile-Leu-Gln-Arg- Lys-Lys-OH. A synthetic peptide of the same sequence was shown to be biologically active in small quantities and to extend the range of conditions suitable for development of competence. Cognate codons in the pneumococcal chromosome indicate that the peptide is made ribosomally. As the gene encodes a prepeptide containing the Gly-Gly consensus processing site found in peptide bacteriocins, the peptide is likely to be exported by a specialized ATP-binding cassette transport protein as is characteristic of these bacteriocins. The hypothesis is presented that this transport protein is encoded by comA, previously shown to be required for elaboration of the pneumococcal competence activator.

Phase variation of Enterococcus faecalis pAD1 conjugation functions relates to changes in iteron sequence region

pAD1 (60 kb) is a conjugative, hemolysin/bacteriocin plasmid in Enterococcus faecalis. It confers a mating response to the peptide sex pheromone cAD1 produced by recipient (plasmid-free) cells, leading to highly efficient plasmid transfer in broth matings. Control of the physiological response to cAD1 can been overridden by a reversible phase variation event at frequencies on the order of 10(-4) to 10(-3) per cell per generation (L. T. Pontius and D. B. Clewell, Plasmid 26:172-185, 1991). The variant forms are designated Dryc and Dry+, which reflects the colony morphologies of cells whose conjugation functions are switched on and off, respectively. Here we show that Dryc variants exhibit a structural change in a region between repA and repB that contains two clusters of 8-bp iterons. The change involved a 31- or 32-bp increase in size of this region. In three or four independent variants examined, one of the iteron clusters increased in size from 13 to 17 iterons. When iteron DNA was placed on a multicopy plasmid and introduced into a wild-type pAD1 derivative, the Dryc phenotype was generated. Since traA, a key negative regulator of conjugation, bears several centrally located iteron-like sequences with the same orientation, we speculate that the protein(s) that normally binds iterons (possibly RepA and/or RepB) blocks traA transcription in Dryc variants.

Physical mapping of the conjugative bacteriocin plasmid pPD1 of Enterococcus faecalis and identification of the determinant related to the pheromone response

The pheromone-responding conjugative bacteriocin plasmid pPD1 (59 kb) of Enterococcus faecalis was mapped physically by using a relational clone approach, and transposon analysis with Tn917 (Emr) or Tn916 (Tcr) facilitated the location of the bacteriocin-related genes in a segment of about 6.7 kb. Tn917 insertions within a 3-kb region resulted in constitutive clumping. The nucleotide sequence of the region that included the insertions giving rise to constitutive clumping was determined. The region of pPD1 spanned about 8 kb and was found to contain a number of open reading frames, some of which were named on the basis of homologies with two other pheromone-responding plasmids, pAD1 and pCF10. The genes were arranged in the sequence repB-repA-traC-traB-traA-ipd-traE-traF- orfY-sea-1 with all but repB and traA oriented in the same (left-to-right) direction. traC and traB corresponded, respectively, to traC and traB of pAD1 and to prgY and prgZ of pCF10.

Cloning and characterization of a region of Enterococcus faecalis plasmid pPD1 encoding pheromone inhibitor (ipd), pheromone sensitivity (traC), and pheromone shutdown (traB) genes

Bacteriocin plasmid pPD1 in Enterococcus faecalis encodes a mating response to recipient-produced sex pheromone cPD1. Once a recipient acquires pPD1, transconjugants apparently shut off cPD1 activity in broth culture and no longer behave as recipients for pPD1. This event is performed by synthesis of the pheromone inhibitor iPD1 and also by repression of cPD1 production, the so-called "pheromone shutdown." A 5.4-kb EcoRV-HincII segment of pPD1, which expressed iPD1 in Escherichia coli, was sequenced and found to be organized as traC-traB-traA-ipd; each open reading frame is analogous to that found in other pheromone plasmids, pAD1 and pCF10, and thus is designated in accordance with the nomenclature in pAD1. The ipd gene encodes a peptide consisting of 21 amino acids, in which the C-terminal eight residues correspond to iPD1. The putative TraC product has a strong similarity to oligopeptide-binding proteins found in other bacterial species, as do pheromone-binding proteins of pCF10 and pAD1. A strain carrying traC-disrupted pPD1 required a concentration of cPD1 fourfold higher than that needed by the wild-type strain for induction of sexual aggregation. These results suggest that the TraC product contributes to pheromone sensitivity as a pheromone-binding protein. A strain transformed with traB-disrupted pPD1 produced a high level of cPD1 similar to that produced by plasmid-free recipients and underwent self-induction. Thus, the TraB product contributes to cPD1 shutdown.

Peptide permeases from Streptococcus pneumoniae affect adherence to eucaryotic cells

To gain access to tissues within the human host, Streptococcus pneumoniae initially colonizes the nasopharynx and then interacts with glycoconjugates on the surfaces of target cells at various sites of infection. Although pneumococcal adhesins are currently unknown, exported proteins on the bacterial surface are potential candidates. To identify bacterial elements involved in this process, mutants of S. pneumoniae with defects in exported proteins were screened for the inability to adhere to cells representative of three in vivo niches: (i) agglutination of bovine erythrocytes, which reflects adherence to cells which reside in the nasopharynx; (ii) human type II pneumocytes (lung cells [LC]), representing the alveolar site of infection; and (iii) human vascular endothelial cells (EC), representing the endovascular site. The capacity of the mutants to adhere during the course of pneumococcal disease was also assessed by using cytokine-activated LC and EC. All of the 30 mutants analyzed produced hemagglutination values comparable with those of the parent strain. Four independent mutants demonstrated a greater than 50% decrease in adherence to both LC and EC. Sequence analysis of the altered alleles from these strains showed that mutations had occurred in two previously identified loci, plpA and ami, which belong to the family of genes encoding protein-dependent peptide permeases. Mutations in the ami locus resulted in an inability to recognize the GalNAc beta 1-4Gal glycoconjugate receptor present on resting LC and EC, whereas mutations in plpA resulted in a failure to recognize a GalNAc beta 1-3Gal glycoconjugate receptor also present on resting cells. Mutations in neither allele affected recognition of GlcNAc receptors present on cytokine-activated LC and EC. These results suggest that peptide permeases modulate pneumococcal adherence to epithelial and endothelial cells either by acting directly as adhesins or by modulating the expression of adhesins on the pneumococcal surface during the initial stages of colonization of the lung or the vascular endothelium.

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.

Convergent sensing pathways mediate response to two extracellular competence factors in Bacillus subtilis

Development of genetic competence in Bacillus subtilis is regulated by extracellular signaling molecules, including the ComX pheromone, a modified 9- or 10-amino-acid peptide. Here, we present characterization of a second extracellular competence stimulating factor (CSF). CSF appears to be, at least in part, a small peptide of between 520 and 720 daltons. Production of CSF requires several genes that are needed both for initiation of sporulation and development of competence (spo0H, spo0A, spo0B, and spo0F). Although both peptide factors regulate competence, two different sensing pathways mediate the response to the ComX pheromone and CSF. Analysis of double mutants indicated that ComX pheromone is on the same genetic pathway as the membrane-bound histidine protein kinase encoded by comP and that CSF is on the same genetic pathway as the oligopeptide permease encoded by spo0K. Furthermore, the cellular response to partly purified ComX pheromone requires the ComP histidine protein kinase, whereas the response to partly purified CSF requires the Spo0K oligopeptide permease. These two sensing pathways converge to activate competence genes. Both factors and their convergent sensing pathways are required for normal development of competence and might function to integrate different physiological signals.