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

Structural Differences in Complexes between the Master Regulator PrgX, Peptide Pheromones, and Operator Binding Sites Determine the Induction State for Conjugative Transfer of pCF10

Pheromone-inducible conjugation in the Enterococcus faecalis pCF10 system is regulated by the PrgX transcription factor through binding interactions at two operator binding sites (XBS1 and XBS2) upstream of the transcription start site of the prgQ operon encoding the conjugation machinery. Repression of transcription requires the interaction of a PrgX tetramer with both XBSs via formation of a DNA loop. The ability of PrgX to regulate prgQ transcription is modulated by its interaction with two antagonistic regulatory peptides, ICF10 (I) and cCF10 (C); the former peptide inhibits prgQ transcription, while the latter peptide enhances prgQ transcription. In this report, we used electrophoretic mobility shift assays (EMSAs) and DNase footprinting to examine binding interactions between the XBS operator sites and various forms of PrgX (Apo-X, PrgX/I, and PrgX/C). Whereas a previous model based on high-resolution structures of PrgX proposed that the functional differences between PrgX/C and PrgX/I resulted from differences in PrgX oligomerization state, the current results show that specific differences in XBS2 occupancy by bound tetramers account for the differential regulatory properties of the two peptide/PrgX complexes and for the effects of XBS mutations on regulation. The results also confirmed a DNA looping model of PrgX function. IMPORTANCE Peptide pheromones regulate antibiotic resistance transfer in Enterococcus faecalis. Here, we present new data showing that pheromone-dependent regulation of transfer genes is mediated via effects on the structures of complexes between peptides, the intracellular peptide receptor, and operator sites on the target DNA.

Competent but complex communication: The phenomena of pheromone-responsive plasmids

Enterococci are robust gram-positive bacteria that are found in a variety of surroundings and that cause a significant number of healthcare-associated infections. The genus possesses a high-efficiency pheromone-responsive plasmid (PRP) transfer system for genetic exchange that allows antimicrobial-resistance determinants to spread within bacterial populations. The pCF10 plasmid system is the best characterised, and although other PRP systems are structurally similar, they lack exact functional homologues of pCF10-encoded genes. In this review, we provide an overview of the enterococcal PRP systems, incorporating functional details for the less-well-defined systems. We catalogue the virulence-associated elements of the PRPs that have been identified to date, and we argue that this reinforces the requirement for elucidation of the less studied systems.

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".

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.

Autoregulation of the synthesis of the MobM relaxase encoded by the promiscuous plasmid pMV158

The streptococcal promiscuous plasmid pMV158 (5,540 bp) replicates by the rolling-circle mechanism and can be mobilized among a wide number of Gram-positive and -negative bacteria. The plasmid region involved in its conjugative transfer includes the mobM gene, which encodes the MobM relaxase, and the cis-acting origin of transfer (oriT). MobM initiates transfer by cleavage of supercoiled pMV158 DNA at a specific dinucleotide within oriT. In the present work, we have performed a detailed transcriptional analysis to assess the role of MobM in the control of its own gene expression. By in vivo and in vitro approaches, we demonstrated that mobM transcription in Escherichia coli was mostly initiated from a promoter (Pmob2) different from the one (Pmob1) used in Lactococcus lactis. Whereas promoter Pmob1 was embedded within the oriT sequence, promoter Pmob2 was placed apart from but adjacent to oriT. Further, MobM was able to repress the expression of its own gene from both promoters. Given the promiscuity of pMV158, the organization of the mobM promoter region suggests a strategy of the plasmid to cope with different transcription machineries of the hosts it colonizes.

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.

Specificity of the TraA-DNA interaction in the regulation of the pPD1-encoded sex pheromone response in Enterococcus faecalis

The Enterococcus faecalis conjugative plasmid pPD1 encodes proteins responsible for the mating response to the sex pheromone cPD1 secreted by a recipient cell. This response involves the respectively negative and positive determinants traA and traE, the pheromone-inhibitor determinant ipd and structural genes participating in the conjugation process. TraA is capable of binding to key sites within the regulatory gene cluster. The binding of TraA to cognate sites is modulated by the pheromone (cPD1) and the pheromone-inhibitor (iPD1) peptides. Using atomic force microscopy and classic biochemical techniques, we mapped and characterized the TraA-DNA interactions within the pPD1 regulatory gene cluster and the role of TraA in the transcription regulation of the sex pheromone response. A previous report showed that TraA binds to three adjacent sites (tab1, tab2 and tab3) located upstream of the ipd promoter region. Here, we provide direct evidence for such interactions and show that TraA alone or in the presence of iPD1 inhibits ipd transcription by preferentially binding to tab1, whereas in the presence of saturating cPD1, the overall binding to the tab sites decreases, TraA preferentially binds to tab3 and the ipd repression is relieved. Moreover, TraA alone or in the presence of iPD1 binds to two non-adjacent sites within the ipd terminators T1 and T2, an interaction that is also relieved in the presence of cPD1. The binding of TraA to the termination region of ipd may play an important role in controlling traE and traF expression via a transcriptional read-through mechanism already postulated for the pAD1 plasmid. TraA may also regulate its own expression by binding to a site in the proximity of the traA promoter, which has been relocated 200 bp downstream of the ipd gene. A model for the TraA-mediated regulation of the pPD1-encoded sex pheromone response is presented.

Characterization of an active partition system for the Enterococcus faecalis pheromone-responding plasmid pAD1

Enterococcus faecalis plasmid pAD1 is a 60-kb conjugative, low-copy-number plasmid that encodes a mating response to the peptide sex pheromone cAD1 and a cytolytic exotoxin that contributes to virulence. Although aspects of conjugation have been studied extensively, relatively little is known about the control of pAD1 maintenance. Previous work on pAD1 identified a 5-kb region of DNA sufficient to support replication, copy control, and stable inheritance (K. E. Weaver, D. B. Clewell, and F. An, J. Bacteriol. 175:1900-1909, 1993), and recently, the pAD1 replication initiator (RepA) and the origin of vegetative replication (oriV) were characterized (M. V. Francia, S. Fujimoto, P. Tille, K. E. Weaver, and D. B. Clewell, J. Bacteriol. 186:5003-5016, 2004). The present study focuses on the adjacent determinants repB and repC, as well as a group of 25 8-bp direct repeats (iterons with the consensus sequence TAGTARRR) located between the divergently transcribed repA and repB. Through mutagenesis and trans-complementation experiments, RepB (a 33-kDa protein, a member of the ParA superfamily of ATPases) and RepC (a protein of 14.4 kDa) were shown to be required for maximal stabilization. Both were active in trans. The iteron region was shown to act as the pAD1 centromere-like site. Purified RepC was shown by DNA mobility shift and DNase I footprinting analyses to interact in a sequence-specific manner with the iteron repeats upstream of the repBC locus. The binding of RepC to the iteron region was shown to be modified by RepB in the presence of ATP via a possible interaction with the RepC-iteron complex. RepB did not bind to the iteron region in the absence of RepC.

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.

Analysis of the amino acid sequence specificity determinants of the enterococcal cCF10 sex pheromone in interactions with the pheromone-sensing machinery

The level of expression of conjugation genes in Enterococcus faecalis strains carrying the pheromone-responsive transferable plasmid pCF10 is determined by the ratio in the culture medium of two types of signaling peptides, a pheromone (cCF10) and an inhibitor (iCF10). Recent data have demonstrated that both peptides target the cytoplasmic receptor protein PrgX. However, the relative importance of the interaction of these peptides with the pCF10 protein PrgZ (which enhances import of cCF10) versus PrgX is not fully understood, and there is relatively little information about specific amino acid sequence determinants affecting the functional interactions of cCF10 with these proteins in vivo. To address these issues, we used a pheromone-inducible reporter gene system where various combinations of PrgX and PrgZ could be expressed in an isogenic host background to examine the biological activities of cCF10, iCF10, and variants of cCF10 isolated in a genetic screen. The results suggest that most of the amino acid sequence determinants of cCF10 pheromone activity affect interactions between the peptide and PrgX, although some sequence variants that affected peptide/PrgZ interactions were also identified. The results provide functional data to complement ongoing structural studies of PrgX and increase our understanding of the functional interactions of cCF10 and iCF10 with the pheromone-sensing machinery of pCF10.

Molecular basis for control of conjugation by bacterial pheromone and inhibitor peptides

In many bacteria expression of lateral gene transfer and of virulence factors is controlled by cell-cell signalling systems. Molecular interactions of microbial signal molecules with their cognate receptors are not well understood. For the Enterococcus faecalis conjugative plasmid pCF10, the PrgX protein serves as a molecular switch controlling expression of conjugation and virulence genes encoded by the plasmid. The induction state of a pCF10-carrying donor cell is determined by the ratio of two signalling peptides, cCF10 pheromone and iCF10 inhibitor. Recent analysis of PrgX/cCF10 interactions suggests a mechanism for conversion to the induced state. However, the means by which iCF10 peptide antagonizes cCF10 activity is unclear, and it has been suggested that inhibitor peptides block import of pheromone peptides. We now show that both of these peptides interact with the same binding pocket of PrgX, but they differentially alter the conformation of the protein and its oligomerization state, resulting in opposing biological activities.

Structure of peptide sex pheromone receptor PrgX and PrgX/pheromone complexes and regulation of conjugation in Enterococcus faecalis

Many bacterial activities, including expression of virulence factors, horizontal genetic transfer, and production of antibiotics, are controlled by intercellular signaling using small molecules. To date, understanding of the molecular mechanisms of peptide-mediated cell-cell signaling has been limited by a dearth of published information about the molecular structures of the signaling components. Here, we present the molecular structure of PrgX, a DNA- and peptide-binding protein that regulates expression of the conjugative transfer genes of the Enterococcus faecalis plasmid pCF10 in response to an intercellular peptide pheromone signal. Comparison of the structures of PrgX and the PrgX/pheromone complex suggests that pheromone binding destabilizes PrgX tetramers, opening a 70-bp pCF10 DNA loop required for conjugation repression.

Enterococcus faecalis pheromone-responsive protein PrgX: genetic separation of positive autoregulatory functions from those involved in negative regulation of conjugative plasmid transfer

The pCF10 plasmid in Enterococcus faecalis transfers from donor cells to recipients upon induction via peptide pheromone. Two plasmid-encoded negative regulators produced from the same transcript, PrgX protein and Qa RNA, repress conjugation genes in uninduced donor cells. PrgX positively autoregulates production of both itself and mature Qa RNA, and is believed to repress the prgQ promoter in a pheromone-sensitive fashion. Previous analysis of PrgX was complicated because mutations in prgX affecting regulation of conjugation also disrupted PrgX autoregulation, suggesting the two functions might be inseparable. In this study, we isolated 14 single amino acid substitutions in PrgX that reduced or eliminated repression of prgQ, without affecting autoregulation or DNA binding. PrgX was shown to bind to its cognate pheromone, cCF10, and most of the mutations lowered the affinity of PrgX for cCF10. Dimerization was affected by five of the mutations and the data indicate that it is required, but insufficient for pheromone induction. We propose a new model for the mechanism used by PrgX for regulation of the prgQ promoter, PrgX autoregulation, and Qa RNA processing.

Drug resistance ofEnterococcus faeciumclinical isolates and the conjugative transfer of gentamicin and erythromycin resistance traits

Drug resistance and the transferability of resistance were examined in 218 Enterococcus faecium clinical isolates obtained from in-patients of a Japanese university hospital between 1990 and 1999. One hundred and sixty one isolates (73.9%) were drug-resistant and 127 (58.2%) isolates were resistant to two or more drugs. Vancomycin resistant E. faecium (VRE) was not isolated. The transferability of drug-resistance to an E. faecium strain was examined by broth or filter mating. Six (12.5%) of the 48 gentamicin resistance traits, and fifty (50%) of the 101 erythromycin resistance traits were transferred by filter mating. The gentamicin resistance traits of five isolates and the erythromycin resistance traits of four isolates were transferred to the recipient strains by both broth mating and filter mating at a frequency of about 10(-6) and 10(-5) per donor cell, respectively. The five gentamicin resistant strains were shown to harbor pMG1-like plasmids on the basis of their Southern hybridization with pMG1 (65.1 kbp, Gm(r)), which transfers efficiently between enterococci by broth mating. Each of the four erythromycin resistant transconjugants obtained by broth mating harbored a large conjugative plasmid (more than 100 kbp). The plasmids showed no homology with well-characterized enterococcal conjugative plasmids such as pAD1, pPD1, pAM(beta)1, pIP501 and pMG1 by Southern hybridization. Of the erythromycin resistance traits that transferred only by filter mating, it was found that the erythromycin resistance trait was conferred by a 47-kbp transposable element that transferred from the chromosome of the donor strain to different sites within the pheromone responsive plasmid pAD1 (60 kbp) of the recipient strain, suggesting that the erythromycin resistance trait was encoded on a conjugative transposon, which was named Tn950.

Enterococcus faecalis sex pheromone plasmid pAM373: analyses of TraA and evidence for its interaction with RpoB

The Enterococcus faecalis plasmid pAM373 (36.7kb) encodes a mating response to the sex pheromone cAM373 secreted by recipient (plasmid-free) bacteria. Like certain other conjugative enterococcal plasmids, a key regulator of the pheromone response is a negatively acting protein, TraA, which is believed to interact with internalized pheromone to influence expression from a key transcriptional promoter P(0). An earlier report showed that in the case of pAM373 most, but not all, transposon-insertion mutations in traA differed from those in the case of pAD1 and pCF10 in that they did not give rise to the normally characteristic constitutive clumping. We show here that this phenomenon relates to a host effect involving an RpoB-related mutation associated with rifampin resistance. When harboring traA mutants, rifampin-sensitive hosts exhibited constitutive clumping, whereas rifampin-resistant hosts did not-despite the fact that the latter host exhibited a normal pheromone-inducible clumping response when harboring a wild-type plasmid. The data imply that TraA normally remains associated with the transcription complex after induction. In addition the promoter of traA, designated P(a), was shown to be located about 600bp upstream of the translational start site, as clones containing traA required this site to complement traA mutants in trans. Transcription from P(a) also gave rise to a short (130 nt) transcript, mD, expressed at a high level in uninduced cells. An earlier observation suggesting that TraA negatively affected transcriptional readthrough into the 3' end of traA from the t(ac) intrinsic bidirectional terminator between traA and the opposing, adjacent traC was supported by TraA complementation studies. Evidence is also presented suggesting that this regulation at t(ac) also involves an additional, possibly cis-acting, element.

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.

Replication of Enterococcus faecalis pheromone-responding plasmid pAD1: location of the minimal replicon and oriV site and RepA involvement in initiation of replication

The hemolysin-determining plasmid pAD1 is a member of a widely disseminated family of highly conjugative elements commonly present in clinical isolates of Enterococcus faecalis. The determinants repA, repB, and repC, as well as adjacent iteron sequences, are believed to play important roles in pAD1 replication and maintenance. The repA gene encodes an initiator protein, whereas repB and repC encode proteins related to stability and copy number. The present study focuses specifically on repA and identifies a replication origin (oriV) within a central region of the repA determinant. A small segment of repA carrying oriV was able to support replication in cis of a plasmid vector otherwise unable to replicate, if an intact RepA was supplied in trans. We demonstrate that under conditions in which RepA is expressed from an artificial promoter, a segment of DNA carrying only repA is sufficient for stable replication in E. faecalis. We also show that RepA binds specifically to oriV DNA at several sites containing inverted repeat sequences (i.e., IR-1) and nonspecifically to single-stranded DNA, and related genetic analyses confirm that these sequences play an important role in replication. Finally, we reveal a relationship between the internal structure of RepA and its ability to recognize oriV. An in-frame deletion within repA resulting in loss of 105 nucleotides, including at least part of oriV, did not eliminate the ability of the altered RepA protein to initiate replication using an intact origin provided in trans. The relationship of RepA to other known initiator proteins is also discussed.

Characterization of cis-acting prgQ mutants: evidence for two distinct repression mechanisms by Qa RNA and PrgX protein in pheromone-inducible enterococcal plasmid pCF10

The pCF10-encoded negative regulators PrgX and Qa (prgQ antisense) RNA inhibit pCF10 transfer by blocking prgQ transcription extension past a potential transcription terminator sequence IRS1. To identify potential target sites for negative regulation, we isolated and analysed 13 cis-acting mutations in the prgXQ region. Determination of the 3' end of Qa RNA showed that eight mutations mapped in the region encoding Qa RNA. Four mutations were in the Qa promoter region and one was in IRS1. Three mutations in Qa greatly reduced the intracellular level of this RNA but did not affect that of PrgX. However, both Qa RNA and PrgX protein were reduced in three Qa promoter region mutants and the expression of prgQ transcripts extending 3' from IRS1 became constitutive. Qa RNA could mediate its negative regulatory activity in the absence of PrgX, and this activity was not abolished by cCF10, the peptide pheromone that induces pCF10 transfer. RNA analysis showed that Qa RNA abolished transcription readthrough. Based on the experimental data as well as computer analysis of predicted secondary structures of prgQ mRNA in the presence or absence of Qa, we concluded that Qa RNA is a pheromone-insensitive effector of prgQ mRNA termination or degradation at IRS1. In cells lacking a Qa target sequence, expression of PrgX repressed transcription from the prgQ promoter, and this repression was relieved by addition of exogenous cCF10. Thus, even though the synthesis of these negative regulators is coupled, they each act independently on separate targets to regulate expression of conjugation functions.

Conjugative plasmid transfer in gram-positive bacteria

Conjugative transfer of bacterial plasmids is the most efficient way of horizontal gene spread, and it is therefore considered one of the major reasons for the increase in the number of bacteria exhibiting multiple-antibiotic resistance. Thus, conjugation and spread of antibiotic resistance represents a severe problem in antibiotic treatment, especially of immunosuppressed patients and in intensive care units. While conjugation in gram-negative bacteria has been studied in great detail over the last decades, the transfer mechanisms of antibiotic resistance plasmids in gram-positive bacteria remained obscure. In the last few years, the entire nucleotide sequences of several large conjugative plasmids from gram-positive bacteria have been determined. Sequence analyses and data bank comparisons of their putative transfer (tra) regions have revealed significant similarities to tra regions of plasmids from gram-negative bacteria with regard to the respective DNA relaxases and their targets, the origins of transfer (oriT), and putative nucleoside triphosphatases NTP-ases with homologies to type IV secretion systems. In contrast, a single gene encoding a septal DNA translocator protein is involved in plasmid transfer between micelle-forming streptomycetes. Based on these clues, we propose the existence of two fundamentally different plasmid-mediated conjugative mechanisms in gram-positive microorganisms, namely, the mechanism taking place in unicellular gram-positive bacteria, which is functionally similar to that in gram-negative bacteria, and a second type that occurs in multicellular gram-positive bacteria, which seems to be characterized by double-stranded DNA transfer.

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.

Functional analysis of TraA, the sex pheromone receptor encoded by pPD1, in a promoter region essential for the mating response in Enterococcus faecalis

Conjugative transfer of a bacteriocin plasmid, pPD1, of Enterococcus faecalis is induced in response to a peptide sex pheromone, cPD1, secreted from plasmid-free recipient cells. cPD1 is taken up by a pPD1 donor cell and binds to an intracellular receptor, TraA. Once a recipient cell acquires pPD1, it starts to produce an inhibitor of cPD1, termed iPD1, which functions as a TraA antagonist and blocks self-induction in donor cells. In this study, we discuss how TraA transduces the signal of cPD1 to the mating response. Gel mobility shift assays indicated that TraA is bound to a traA-ipd intergenic region, which is essential for cPD1 response. DNase I footprinting analysis suggested the presence of one strong (tab1) and two weak (tab2 and tab3) TraA-binding sites in the intergenic region. Primer extension analysis implied that the transcriptional initiation sites of traA and ipd were located in the intergenic region. Northern analysis showed that cPD1 upregulated and downregulated transcription of ipd and traA, respectively. The circular permutation assay showed that TraA bent a DNA fragment corresponding to the tab1 region, and its angle was changed in the presence of cPD1 or iPD1. From these data, we propose a model that TraA changes the conformation of the tab1 region in response to cPD1 and upregulates the transcription of ipd, which may lead to expression of genes required for the mating response.

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.

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.

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.

Peptide pheromone-induced transfer of plasmid pCF10 in Enterococcus faecalis: probing the genetic and molecular basis for specificity of the pheromone response

The tetracycline resistance plasmid pCF10 represents a class of unique mobile genetic elements of the bacterial genus Enterococcus, whose conjugative transfer functions are inducible by peptide sex pheromones excreted by potential recipient cells. These plasmids play a significant role in the dissemination of virulence and antibiotic resistance genes among the enterococci, which have become major nosocomial pathogens. Pheromone response by plasmid-carrying donor cells involves specific import of the peptide signal molecule, and subsequent interaction of the signal with one or more intracellular regulatory gene products. The pheromones are chromosomally encoded hydrophobic octa- or hepta-peptides, and different families of homologous plasmids encode the ability to respond to each pheromone. Among the four pheromone-responsive plasmids that have been characterized in some detail, there is considerable conservation in the genes encoding pheromone sensing and regulatory functions, and the peptides themselves show considerable similarity. In spite of this, there is extremely high specificity of response to each peptide, with virtually no "cross-induction" of transfer of non-cognate pheromone plasmids by the pheromones. This communication reviews the evidence for this specificity and discusses current molecular and genetic approaches to defining the basis for specificity.

Gelatinase biosynthesis-activating pheromone: a peptide lactone that mediates a quorum sensing in Enterococcus faecalis

Biosynthesis of gelatinase, a virulence factor of Enterococcus faecalis, was found to be regulated in a cell density-dependent fashion in which its production is active in late log to early stationary phase. Addition of early stationary phase culture filtrate to medium shifted the onset of gelatinase production to that of mid-log phase, suggesting that E. faecalis secretes a gelatinase biosynthesis-activating pheromone (GBAP). GBAP was isolated from culture supernatant of E. faecalis OG1S-P. Structural analysis suggested GBAP to be an 11-residue cyclic peptide containing a lactone structure, in which the alpha-carboxyl group of the C-terminal amino acid is linked to a hydroxyl group of the serine of the third residue. A synthetic peptide possessing the deduced structure showed GBAP activity at nanomolar concentrations as did natural GBAP. Database searches revealed that GBAP corresponds to a C-terminal part of a 242-residue FsrB protein. Northern analysis showed that GBAP slowly induces the transcription of two operons, fsrB-fsrC encoding FsrB and a putative histidine kinase FsrC and gelE-sprE encoding gelatinase GelE and serine protease SprE. Strains with an insertion mutation in either fsrC or a putative response regulator gene fsrA failed to respond to GBAP, suggesting that the GBAP signal is transduced by a two-component regulatory system.

Dominant-negative mutants of prgX: evidence for a role for PrgX dimerization in negative regulation of pheromone-inducible conjugation

PrgX negatively regulates prgQ transcriptional readthrough in the pheromone-inducible enterococcal conjugative plasmid pCF10. We isolated and characterized 13 dominant-negative prgX mutants, all of which mapped in either the N- or the C-terminus of PrgX. In all mutants, the in vivo level of Qa RNA, an antisense RNA to prgQ RNA, was greatly reduced. When oligomerization of PrgX was tested with a phage lambda cI repressor fusion system, the oligomerization domain was found to be between amino acid residues 78 and 280. When histidine-tagged PrgX (His-PrgX) was purified by nickel column chromatography from a strain also expressing PrgX, PrgX was co-purified with His-PrgX. Although PrgX was expressed at a much higher level than His-PrgX, an approximately equal amount of PrgX was co-purified. Pheromone induction greatly decreased the co-purification of PrgX. Based on these data, we propose that both the N- and the C-terminal domains of PrgX are required for PrgX positive autoregulation and for the repression of prgQ transcription readthrough. In vivo, PrgX exists as a dimer, and dimerization is mediated by the central region of PrgX.

pAM401-based shuttle vectors that enable overexpression of promoterless genes and one-step purification of tag fusion proteins directly from Enterococcus faecalis

Two novel Enterococcus faecalis-Escherichia coli shuttle vectors that utilize the promoter and ribosome binding site of bacA on the E. faecalis plasmid pPD1 were constructed. The vectors were named pMGS100 and pMGS101. pMGS100 was designed to overexpress cloned genes in E. coli and E. faecalis and encodes the bacA promoter followed by a cloning site and stop codon. pMGS101 was designed for the overexpression and purification of a cloned protein fused to a Strep-tag consisting of 9 amino acids at the carboxyl terminus. The Strep-tag provides the cloned protein with an affinity to immobilized streptavidin that facilitates protein purification. We cloned a promoterless beta-galactosidase gene from E. coli and cloned the traA gene of the E. faecalis plasmid pAD1 into the vectors to test gene expression and protein purification, respectively. beta-Galactosidase was expressed in E. coli and E. faecalis at levels of 10(3) and 10 Miller units, respectively. By cloning the pAD1 traA into pMGS101, the protein could be purified directly from a crude lysate of E. faecalis or E. coli with an immobilized streptavidin matrix by one-step affinity chromatography. The ability of TraA to bind DNA was demonstrated by the DNA-associated protein tag affinity chromatography method using lysates prepared from both E. coli and E. faecalis that overexpress TraA. The results demonstrated the usefulness of the vectors for the overexpression and cis/trans analysis of regulatory genes, purification and copurification of proteins from E. faecalis, DNA binding analysis, determination of translation initiation site, and other applications that require proteins purified from E. faecalis.

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.

Analysis of expression of prgX, a key negative regulator of the transfer of the Enterococcus faecalis pheromone-inducible plasmid pCF10

Conjugative transfer of pCF10, a plasmid found in Enterococcus faecalis, is induced by the peptide pheromone cCF10 and the donor-recipient aggregation is mediated by PrgB. Expression of prgB in pCF10 is negatively regulated by PrgX. The prgX gene is adjacent to prgQ which provides the promoter for prgB expression. The prgX and prgQ genes are transcribed in opposite directions. A deletion spanning nucleotides+259 to +398 from the prgQ transcription initiation site abolished the prgX gene products at both RNA and protein levels. An RNA, named Qa, was found to be transcribed in the antisense direction in the prgQ region. The transcription start site and the promoter were found to be almost identical with those of the traD determinant in pAD1, another pheromone-responsive plasmid. The first inverted repeat sequence in prgQ, IRS1, was required for the full activity of the Qa promoter. Although the size of the major Qa RNA detected by Northern blot analysis was too short (ca 120 nt) to be an mRNA for PrgX protein, the transcription from the Qa promoter was shown to proceed through to prgX. Transcriptional fusion analyses showed that the transcription of prgX requires one or more trans elements. Moreover, deletion of prgX greatly reduced the level of the Qa RNA and abolished transcription of prgX. Although transcription initiation from the Qa promoter was not increased by PrgX, transcriptional readthrough to prgX was increased by the protein. Based on these data, we conclude that transcription of prgX is initiated from the Qa promoter in prgQ, and PrgX autoregulates its transcription either by mediating transcriptional readthrough or increasing mRNA stability. The PrgX protein, prgX RNA, and the Qa RNA were downregulated by cCF10 induction; however, prgX gene products were still detected for at least 40 minutes after induction.

A pAD1-encoded small RNA molecule, mD, negatively regulates Enterococcus faecalis pheromone response by enhancing transcription termination

pAD1 is a 60-kb hemolysin-bacteriocin plasmid in Enterococcus faecalis that encodes a conjugative mating response to a peptide sex pheromone, cAD1, secreted by plasmid-free bacteria. The pheromone response is regulated by two proteins: TraE1, which positively regulates all or most conjugative structural genes, and TraA, which negatively regulates traE1. TraA binds to pAD1 DNA at the iad (encoding the inhibitor peptide iAD1) promoter but is released upon binding to imported pheromone. This leads to enhanced transcription through two closely spaced downstream terminators (t1 and t2) into traE1. TraE1 is believed to then upregulate itself from a site located within t2; thus, a small amount of transcription through t1-t2 could lead to overall induction. It is important therefore that the t1-t2 terminators be tightly controlled to keep the response shut down in the absence of pheromone. A small (200-nucleotide) RNA molecule designated mD is encoded just upstream of t1 by a determinant (traD) oriented in the direction opposite to that of transcripts utilizing t1. mD is expressed at high levels in the uninduced state, but it decreases significantly upon induction. Here we present results of genetic studies relating to the activity of t1-t2 and show that mD strongly enhances transcriptional termination at t1. The mD activity is shown to influence transcription well downstream and can affect the determinant for aggregation substance asa1. The phenomenon is specific in that there is no effect of mD on the unrelated pheromone-responding plasmids pPD1 and pCF10.

How bacteria talk to each other: regulation of gene expression by quorum sensing

Quorum sensing, or the control of gene expression in response to cell density, is used by both gram-negative and gram-positive bacteria to regulate a variety of physiological functions. In all cases, quorum sensing involves the production and detection of extracellular signalling molecules called autoinducers. While universal signalling themes exist, variations in the design of the extracellular signals, the signal detection apparatuses, and the biochemical mechanisms of signal relay have allowed quorum sensing systems to be exquisitely adapted for their varied uses. Recent studies show that quorum sensing modulates both intra- and inter-species cell-cell communication, and it plays a major role in enabling bacteria to architect complex community structures.

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.

Identification of pel, a Streptococcus pyogenes locus that affects both surface and secreted proteins

A Tn917 insertion mutant of an M49 serotype, opacity factor-positive Streptococcus pyogenes, was isolated. It had the following phenotypes: decreased beta-hemolysis mediated by streptolysin S, reduction in the activity of a secreted cysteine protease and streptokinase, and an altered immunoglobulin and fibrinogen-binding phenotype. The site of insertion of Tn917 into the chromosome and the surrounding sequence, the pel region (pleiotropic effect locus), was determined. Phage A25 transduction confirmed that the pleiotropic changes in phenotype could be cotransduced with Tn917. The pel region was cloned and sequenced, and the transposon was found to be inserted upstream of a single open reading frame which led to a failure to transcribe a 500-base mRNA. The loss of this transcript decreased the transcription of emm and speB genes and reduced the secretion of streptokinase. Enhanced Pel expression from a nisin-inducible plasmid resulted in increased message levels for emm in a wild-type organism. Characterization of the pel mutant provides evidence for the coordinated regulation of secreted and surface proteins and suggests the existence of a new global regulatory factor in S. pyogenes.

Enterococcus faecalis pheromone-responding plasmid pAD1 gives rise to an aggregation (clumping) response when cells are exposed to subinhibitory concentrations of chloramphenicol, erythromycin, or tetracycline

The Enterococcus faecalis conjugative cytolysin plasmid pAD1 encodes a specific aggregation (clumping) response to the peptide sex pheromone cAD1 secreted by plasmid-free strains. Here it is shown that, in the absence of cAD1, exposure of E. faecalis cells harboring pAD1 to subinhibitory concentrations of chloramphenicol, erythromycin, or tetracycline also results in an aggregation response that appears related to a stress-sensitive mechanism associated with a component of the pheromone response.

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.

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.