Regulatory mechanisms in expression of the traY-I operon of sex factor plasmid R100: involvement of traJ and traY gene products

Prokaryotic Information Games: How and When to Take up and Secrete DNA

Besides transduction via bacteriophages natural transformation and bacterial conjugation are the most important mechanisms driving bacterial evolution and horizontal gene spread. Conjugation systems have evolved in eubacteria and archaea. In Gram-positive and Gram-negative bacteria, cell-to-cell DNA transport is typically facilitated by a type IV secretion system (T4SS). T4SSs also mediate uptake of free DNA in Helicobacter pylori, while most transformable bacteria use a type II secretion/type IV pilus system. In this chapter, we focus on how and when bacteria "decide" that such a DNA transport apparatus is to be expressed and assembled in a cell that becomes competent. Development of DNA uptake competence and DNA transfer competence is driven by a variety of stimuli and often involves intricate regulatory networks leading to dramatic changes in gene expression patterns and bacterial physiology. In both cases, genetically homogeneous populations generate a distinct subpopulation that is competent for DNA uptake or DNA transfer or might uniformly switch into competent state. Phenotypic conversion from one state to the other can rely on bistable genetic networks that are activated stochastically with the integration of external signaling molecules. In addition, we discuss principles of DNA uptake processes in naturally transformable bacteria and intend to understand the exceptional use of a T4SS for DNA import in the gastric pathogen H. pylori. Realizing the events that trigger developmental transformation into competence within a bacterial population will eventually help to create novel and effective therapies against the transmission of antibiotic resistances among pathogens.

Spread and Persistence of Virulence and Antibiotic Resistance Genes: A Ride on the F Plasmid Conjugation Module

The F plasmid or F-factor is a large, 100-kbp, circular conjugative plasmid of Escherichia coli and was originally described as a vector for horizontal gene transfer and gene recombination in the late 1940s. Since then, F and related F-like plasmids have served as role models for bacterial conjugation. At present, more than 200 different F-like plasmids with highly related DNA transfer genes, including those for the assembly of a type IV secretion apparatus, are completely sequenced. They belong to the phylogenetically related MOB_F12A group. F-like plasmids are present in enterobacterial hosts isolated from clinical as well as environmental samples all over the world. As conjugative plasmids, F-like plasmids carry genetic modules enabling plasmid replication, stable maintenance, and DNA transfer. In this plasmid backbone of approximately 60 kbp, the DNA transfer genes occupy the largest and mostly conserved part. Subgroups of MOB_F12A plasmids can be defined based on the similarity of TraJ, a protein required for DNA transfer gene expression. In addition, F-like plasmids harbor accessory cargo genes, frequently embedded within transposons and/or integrons, which harness their host bacteria with antibiotic resistance and virulence genes, causing increasingly severe problems for the treatment of infectious diseases. Here, I focus on key genetic elements and their encoded proteins present on the F-factor and other typical F-like plasmids belonging to the MOB_F12A group of conjugative plasmids.

Cooperative Function of TraJ and ArcA in Regulating the F Plasmid tra Operon

The F plasmid tra operon encodes most of the proteins required for bacterial conjugation. TraJ and ArcA are known activators of the tra operon promoter P_Y, which is subject to H-NS-mediated silencing. Donor ability and promoter activity assays indicated that P_Y is inactivated by silencers and requires both TraJ and ArcA for activation to support efficient F conjugation. The observed low-level, ArcA-independent F conjugation is caused by tra expression from upstream alternative promoters. Electrophoretic mobility shift assays showed that TraJ alone weakly binds to P_Y regulatory DNA; however, TraJ binding is significantly enhanced by ArcA binding to the same DNA, indicating cooperativity of the two proteins. Analysis of binding affinities between ArcA and various DNA fragments in the P_Y regulatory region defined a 22-bp tandem repeat sequence (from -76 to -55 of P_Y) sufficient for optimal ArcA binding, which is immediately upstream of the predicted TraJ-binding site (from -54 to -34). Deletion analysis of the P_Y promoter in strains deficient in TraJ, ArcA, and/or H-NS determined that sequences upstream of -103 are required by silencers including H-NS for P_Y silencing, whereas sequences downstream of -77 are targeted by TraJ and ArcA for activation. TraJ and ArcA appear not only to counteract P_Y silencers but also to directly activate P_Y in a cooperative manner. Our data reveal the cooperativity of TraJ and ArcA during P_Y activation and provide insights into the regulatory circuit controlling F-family plasmid-mediated bacterial conjugation.IMPORTANCE Conjugation is a major mechanism for dissemination of antibiotic resistance and virulence among bacterial populations. The tra operon in the F family of conjugative plasmids encodes most of the proteins involved in bacterial conjugation. This work reveals that activation of tra operon transcription requires two proteins, TraJ and ArcA, to bind cooperatively to adjacent sites immediately upstream of the major tra promoter P_Y The interaction of TraJ and ArcA with the tra operon not only relieves P_Y from silencers but also directly activates it. These findings provide insights into the regulatory circuit of the F-family plasmid-mediated bacterial conjugation.

DNA-Binding Proteins Regulating pIP501 Transfer and Replication

pIP501 is a Gram-positive broad-host-range model plasmid intensively used for studying plasmid replication and conjugative transfer. It is a multiple antibiotic resistance plasmid frequently detected in clinical Enterococcus faecalis and Enterococcus faecium strains. Replication of pIP501 proceeds unidirectionally by a theta mechanism. The minimal replicon of pIP501 is composed of the repR gene encoding the essential rate-limiting replication initiator protein RepR and the origin of replication, oriR, located downstream of repR. RepR is similar to RepE of related streptococcal plasmid pAMβ1, which has been shown to possess RNase activity cleaving free RNA molecules in close proximity of the initiation site of DNA synthesis. Replication of pIP501 is controlled by the concerted action of a small protein, CopR, and an antisense RNA, RNAIII. CopR has a dual function: It acts as transcriptional repressor at the repR promoter and, in addition, prevents convergent transcription of RNAIII and repR mRNA (RNAII), which indirectly increases RNAIII synthesis. CopR binds asymmetrically as a dimer at two consecutive binding sites upstream of and overlapping with the repR promoter. RNAIII induces transcriptional attenuation within the leader region of the repR mRNA (RNAII). Deletion of either control component causes a 10- to 20-fold increase of plasmid copy number, while simultaneous deletions have no additional effect. Conjugative transfer of pIP501 depends on a type IV secretion system (T4SS) encoded in a single operon. Its transfer host-range is considerably broad, as it has been transferred to virtually all Gram-positive bacteria including Streptomyces and even the Gram-negative Escherichia coli. Expression of the 15 genes encoding the T4SS is tightly controlled by binding of the relaxase TraA, the transfer initiator protein, to the operon promoter overlapping with the origin of transfer (oriT). The T4SS operon encodes the DNA-binding proteins TraJ (VirD4-like coupling protein) and the VirB4-like ATPase, TraE. Both proteins are actively involved in conjugative DNA transport. Moreover, the operon encodes TraN, a small cytoplasmic protein, whose specific binding to a sequence upstream of the oriT nic-site was demonstrated. TraN seems to be an effective repressor of pIP501 transfer, as conjugative transfer rates were significantly increased in an E. faecalis pIP501ΔtraN mutant.

Towards an integrated model of bacterial conjugation

Bacterial conjugation is one of the main mechanisms for horizontal gene transfer. It constitutes a key element in the dissemination of antibiotic resistance and virulence genes to human pathogenic bacteria. DNA transfer is mediated by a membrane-associated macromolecular machinery called Type IV secretion system (T4SS). T4SSs are involved not only in bacterial conjugation but also in the transport of virulence factors by pathogenic bacteria. Thus, the search for specific inhibitors of different T4SS components opens a novel approach to restrict plasmid dissemination. This review highlights recent biochemical and structural findings that shed new light on the molecular mechanisms of DNA and protein transport by T4SS. Based on these data, a model for pilus biogenesis and substrate transfer in conjugative systems is proposed. This model provides a renewed view of the mechanism that might help to envisage new strategies to curb the threating expansion of antibiotic resistance.

Relaxosome function and conjugation regulation in F-like plasmids - a structural biology perspective

The tra operon of the prototypical F plasmid and its relatives enables transfer of a copy of the plasmid to other bacterial cells via the process of conjugation. Tra proteins assemble to form the transferosome, the transmembrane pore through which the DNA is transferred, and the relaxosome, a complex of DNA-binding proteins at the origin of DNA transfer. F-like plasmid conjugation is characterized by a high degree of plasmid specificity in the interactions of tra components, and is tightly regulated at the transcriptional, translational and post-translational levels. Over the past decade, X-ray crystallography of conjugative components has yielded insights into both specificity and regulatory mechanisms. Conjugation is repressed by FinO, an RNA chaperone which increases the lifetime of the small RNA, FinP. Recent work has resulted in a detailed model of FinO/FinP interactions and the discovery of a family of FinO-like RNA chaperones. Relaxosome components include TraI, a relaxase/helicase, and TraM, which mediates signalling between the transferosome and relaxosome for transfer initiation. The structures of TraI and TraM bound to oriT DNA reveal the basis of specific recognition of DNA for their cognate plasmid. Specificity also exists in TraI and TraM interactions with the transferosome protein TraD.

Regulation of bacterial conjugation: balancing opportunity with adversity

Conjugative plasmids are involved in the dissemination of important traits such as antibiotic resistance, virulence determinants and metabolic pathways involved in adapting to environmental niches, a process termed horizontal or lateral gene transfer. Conjugation is the process of transferring DNA from a donor to a recipient cell with the establishment of the incoming DNA and its cargo of genetic traits within the transconjugant. Conjugation is mediated by self-transmissible plasmids as well as phage-like sequences that have been integrated into the bacterial chromosome, such as integrative and conjugative elements (ICEs) that now include conjugative transposons. Both conjugative plasmids and ICEs can mediate the transfer of mobilizable elements by sharing their conjugative machinery. Conjugation can either be induced, usually by small molecules or peptides or by excision of the ICE from the host chromosome, or it can be tightly regulated by plasmid- and host-encoded factors. The transfer potential of these transfer regions depends on the integration of many signals in response to environmental and physiological cues. This review will focus on the mechanisms that influence transfer potential in these systems, particularly those of the IncF incompatibility group.

Conjugative DNA metabolism in Gram-negative bacteria

Bacterial conjugation in Gram-negative bacteria is triggered by a signal that connects the relaxosome to the coupling protein (T4CP) and transferosome, a type IV secretion system. The relaxosome, a nucleoprotein complex formed at the origin of transfer (oriT), consists of a relaxase, directed to the nic site by auxiliary DNA-binding proteins. The nic site undergoes cleavage and religation during vegetative growth, but this is converted to a cleavage and unwinding reaction when a competent mating pair has formed. Here, we review the biochemistry of relaxosomes and ponder some of the remaining questions about the nature of the signal that begins the process.

Regulation of bacterial conjugation in microaerobiosis by host-encoded functions ArcAB and sdhABCD

The virulence plasmid of Salmonella enterica (pSLT) is an F-like conjugative plasmid. High rates of pSLT transfer occur in the mammalian gut, a microaerobic environment. In this study, we describe genetic screens for host-encoded activators and repressors of the transfer operon (tra) of pSLT. We show that the transcription factor ArcA is an activator of conjugation, especially under microaerobiosis. In turn, succinate dehydrogenase (SdhABCD) is a repressor of mating in aerobiosis. ArcA binds upstream of the main tra promoter (p(traY)) and activates tra transcription, as previously described in F, R1, and R100. In the absence of ArcA, transfer of pSLT decreased 7-fold in aerobiosis and >100-fold in microaerobiosis. In aerobiosis, ArcA activates the traY promoter in an ArcB-independent manner, as described in other F-like plasmids. In microaerobiosis, however, the ArcB sensor is necessary for activation of p(traY). Lack of Sdh causes a >20-fold increase in pSLT transfer in aerobiosis, but has little effect under microaerobiosis. Sdh inhibits conjugal transfer by reducing traJ transcription, probably in an indirect manner. In turn, the sdhCDAB operon is repressed by the ArcAB system under microaerobiosis. Hence, the ArcAB two-component system of S. enterica stimulates pSLT transfer under microaerobiosis by two concerted actions: activation of the tra operon and repression of the sdhCDAB operon.

Activation of the Cpx regulon destabilizes the F plasmid transfer activator, TraJ, via the HslVU protease in Escherichia coli

The Escherichia coli CpxAR two-component signal transduction system senses and responds to extracytoplasmic stress. The cpxA101* allele was previously found to reduce F plasmid conjugation by post-transcriptional inactivation of the positive activator TraJ. Microarray analysis revealed upregulation of the protease-chaperone pair, HslVU, which was shown to degrade TraJ in an E. coli C600 cpxA101* background. Double mutants of cpxA101* and hslV or hslU restored TraJ and F conjugation to wild-type levels. The constitutive overexpression of nlpE, an outer membrane lipoprotein that induces the Cpx stress response, also led to HslVU-mediated degradation of TraJ and repression of F transfer. However, Cpx-mediated TraJ degradation appears to be growth phase-dependent, as induction of nlpE in mid-log phase cells did not appreciably alter TraJ levels. Further, His6-TraJ was sensitive to HslVU degradation in vitro only when it was purified from cells overexpressing nlpE. Thus, TraJ appears to become resistant to HslVU during normal growth, with this resistance mapping to the F transfer region. Extracytoplasmic stress prevents this modification of TraJ, leaving it susceptible to HslVU. Thus, the CpxAR stress response indirectly controls the synthesis of the F mating apparatus, a complex transenvelope type IV secretion system, by degrading TraJ.

GroEL plays a central role in stress-induced negative regulation of bacterial conjugation by promoting proteolytic degradation of the activator protein TraJ

Transcription of DNA transfer genes is a prerequisite for conjugative DNA transfer of F-like plasmids. Transfer gene expression is sensed by the donor cell and is regulated by a complex network of plasmid- and host-encoded factors. In this study we analyzed the effect of induction of the heat shock regulon on transfer gene expression and DNA transfer in Escherichia coli. Raising the growth temperature from 22 degrees C to 43 degrees C transiently reduced transfer gene expression to undetectable levels and reduced conjugative transfer by 2 to 3 orders of magnitude. In contrast, when host cells carried the temperature-sensitive groEL44 allele, heat shock-mediated repression was alleviated. These data implied that the chaperonin GroEL was involved in negative regulation after heat shock. Investigation of the role of GroEL in this regulatory process revealed that, in groEL(Ts) cells, TraJ, the plasmid-encoded master activator of type IV secretion (T4S) system genes, was less susceptible to proteolysis and had a prolonged half-life compared to isogenic wild-type E. coli cells. This result suggested a direct role for GroEL in proteolysis of TraJ, down-regulation of T4S system gene expression, and conjugation after heat shock. Strong support for this novel role for GroEL in regulation of bacterial conjugation was the finding that GroEL specifically interacted with TraJ in vivo. Our results further suggested that in wild-type cells this interaction was followed by rapid degradation of TraJ whereas in groEL(Ts) cells TraJ remained trapped in the temperature-sensitive GroEL protein and thus was not amenable to proteolysis.

Characterization of the opposing roles of H-NS and TraJ in transcriptional regulation of the F-plasmid tra operon

The transfer (tra) operon of the conjugative F plasmid of Escherichia coli is a polycistronic 33-kb operon which encodes most of the proteins necessary for F-plasmid transfer. Here, we report that transcription from PY, the tra operon promoter, is repressed by the host nucleoid-associated protein, H-NS. Electrophoretic mobility shift assays indicate that H-NS binds preferentially to the tra promoter region, while Northern blot and transcriptional fusion analyses indicate that transcription of traY, the first gene in the tra operon, is derepressed in an hns mutant throughout growth. The plasmid-encoded regulatory protein TraJ is essential for transcription of the tra operon in wild-type Escherichia coli; however, TraJ is not necessary for plasmid transfer or traY operon transcription in an hns mutant. This indicates that H-NS represses transcription from PY directly and not indirectly via its effects on TraJ levels. These results suggest that TraJ functions to disrupt H-NS silencing at PY, allowing transcription of the tra operon.

Regulation of traJ transcription in the Salmonella virulence plasmid by strand-specific DNA adenine hemimethylation

The traJ gene of the virulence plasmid of Salmonella enterica serovar Typhimurium (pSLT) encodes a transcriptional activator of the transfer operon. The leucine-responsive regulatory protein (Lrp) is an activator of traJ transcription. The upstream-activating-sequence of the pSLT traJ promoter contains two Lrp binding sites (LRP-1 and LRP-2), both necessary for transcriptional activation. The promoter-proximal site (LRP-2) contains a GATC site (GATC-II) whose methylation state affects Lrp binding: GATC-II methylation in both DNA strands decreases the affinity of Lrp for the LRP-2 site, while efficient Lrp binding occurs to a non-methylated GATC-II site. The effect of GATC-II hemimethylation on Lrp binding is strand-specific: methylation of the traJ non-coding strand permits formation of the major Lrp-DNA retardation complex, but methylation of the coding strand does not. This asymmetry supports a model in which passage of the replication fork may permit Lrp-mediated activation of conjugal transfer in one daughter plasmid molecule but not in the other. A remarkable trait of this regulatory design is that hemimethylation of a single GATC site can generate distinct epigenetic signals in otherwise identical plasmid DNA molecules.

Regulation of finP transcription by DNA adenine methylation in the virulence plasmid of Salmonella enterica

DNA adenine methylase (Dam(-)) mutants of Salmonella enterica serovar Typhimurium contain reduced levels of FinP RNA encoded on the virulence plasmid. Dam methylation appears to regulate finP transcription, rather than FinP RNA stability or turnover. The finP promoter includes canonical -10 and -35 modules and depends on the sigma(70) factor. Regulation of finP transcription by Dam methylation does not require DNA sequences upstream from the -35 module, indicating that Dam acts at the promoter itself or downstream. Unexpectedly, a GATC site overlapping with the -10 module is likewise dispensable for Dam-mediated regulation. These observations indicate that Dam methylation regulates finP transcription indirectly and suggest the involvement of a host factor(s) responsive to the Dam methylation state of the cell. We provide evidence that one such factor is the nucleoid protein H-NS, which acts as a repressor of finP transcription in a Dam(-) background. H-NS also restrains transcription of the overlapping traJ gene, albeit in a Dam-independent fashion. Hence, the decreased FinP RNA content found in Dam(-) hosts of S. enterica appears to result from H-NS-mediated repression of finP transcription.

The role of H-NS in silencing F transfer gene expression during entry into stationary phase

The conjugative ability of the F plasmid of Escherichia coli is highly growth phase dependent, with plasmid transfer efficiency dropping rapidly as donor cells progress through the growth cycle towards stationary phase. Transfer is dependent on the expression of the plasmid transfer (tra) genes, which are controlled by three plasmid-encoded regulatory proteins: TraJ, TraY and TraM. Here, we show that the nucleoid-associated host protein, H-NS, acts to repress the expression of traM and traJ as cells enter stationary phase, thereby decreasing mating ability to barely detectable levels. Sequence analysis identified regions of predicted intrinsic curvature, to which H-NS preferentially binds, at the promoters of both traM and traJ. H-NS binding at these regions was then confirmed by electrophoretic mobility shift and DNase I protection footprinting assays. Immunoblot assays displayed a significant increase in TraJ and TraM levels in an hns mutant strain. These findings were further supported by Northern and primer extension analyses which showed that whereas both genes were only expressed in early exponential phase in wild-type cells, hns mutant cells exhibited drastic derepression throughout the growth cycle. Transcriptional fusion studies of the individual promoters demonstrated that H-NS-mediated repression was observed when the promoters of both traM and traJ were present in cis to each other. This suggests that H-NS may bind to an extended region of the F plasmid, acting as a regional silencer of promoters for traJ and traM.

Analysis and characterization of the IncFV plasmid pED208 transfer region

pED208 is a transfer-derepressed mutant of the IncFV plasmid, F(0)lac, which has an IS2 element inserted in its traY gene, resulting in constitutive overexpression of its transfer (tra) region. The pED208 transfer region, which encodes proteins responsible for pilus synthesis and conjugative plasmid transfer, was sequenced and found to be very similar to the F tra region in terms of its organization although most pED208 tra proteins share only about 45% amino acid identity. All the essential genes for F transfer had homologs within the pED208 transfer region with the exception of traQ, which encodes the chaperone for stable F-pilin expression. F(0)lac appears to have a fertility inhibition system different than the FinOP system of other F-like plasmids, and its transfer efficiency was increased in the presence of F or R100, suggesting that it could be mobilized by these plasmids. The F-like transfer systems specified by F, R100, and F(0)lac were highly specific for their cognate origins of transfer (oriT) as measured by their abilities to mobilize chimeric oriT-containing plasmids.

Conjugal transfer of the virulence plasmid of Salmonella enterica is regulated by the leucine-responsive regulatory protein and DNA adenine methylation

Host-encoded functions that regulate the transfer operon (tra) in the virulence plasmid of Salmonella enterica (pSLT) were identified with a genetic screen. Mutations that decreased tra operon expression mapped in the lrp gene, which encodes the leucine-responsive regulatory protein (Lrp). Reduced tra operon expression in an Lrp- background is caused by lowered transcription of the traJ gene, which encodes a transcriptional activator of the tra operon. Gel retardation assays indicated that Lrp binds a DNA region upstream of the traJ promoter. Deletion of the Lrp binding site resulted in lowered and Lrp-independent traJ transcription. Conjugal transfer of pSLT decreased 50-fold in a Lrp- background. When a FinO- derivative of pSLT was used, conjugal transfer from an Lrp- donor decreased 1000-fold. Mutations that derepressed tra operon expression mapped in dam, the gene encoding Dam methyltransferase. Expression of the tra operon and conjugal transfer remain repressed in an Lrp- Dam- background. These observations support the model that Lrp acts as a conjugation activator by promoting traJ transcription, whereas Dam methylation acts as a conjugation repressor by activating FinP RNA synthesis. This dual control of conjugal transfer may also operate in other F-like plasmids such as F and R100.

Transcription of the transfer genes traY and traM of the antibiotic resistance plasmid R100-1 is linked

Three separate traY deletion mutants of R100-1 were prepared by allele replacement. These mutants retained the ability to transfer at a level 100 times greater than R100 and 1/50 that of the parental R100-1. The mutants were complemented to normal R100-1 transfer levels by pDSP06, a multicopy traY clone. Comparison of transcripts initiated at the traY promoter, P(Y), by primer extension experiments showed that there was no detectable P(Y) activity in R100 and that the level of P(Y) activity in the traY deletion mutants was lower than that in R100-1. Similar measurements performed on RNA from a set of previously described traM deletion mutants showed that those traM deletion mutants that produced more traM and finM (M) transcripts than the parental R100-1 also produced more traY transcripts than R100-1 and that those traM mutants that produced fewer M transcripts than R100-1 also produced fewer traY transcripts than R100-1. We conclude that in R100, TraY regulates P(Y) activity and that transcripts originating in traM affect P(Y) activity.

Comparison of proteins involved in pilus synthesis and mating pair stabilization from the related plasmids F and R100-1: insights into the mechanism of conjugation

F and R100-1 are closely related, derepressed, conjugative plasmids from the IncFI and IncFII incompatibility groups, respectively. Heteroduplex mapping and genetic analyses have revealed that the transfer regions are extremely similar between the two plasmids. Plasmid specificity can occur at the level of relaxosome formation, regulation, and surface exclusion between the two transfer systems. There are also differences in pilus serology, pilus-specific phage sensitivity, and requirements for OmpA and lipopolysaccharide components in the recipient cell. These phenotypic differences were exploited in this study to yield new information about the mechanism of pilus synthesis, mating pair stabilization, and surface and/or entry exclusion, which are collectively involved in mating pair formation (Mpf). The sequence of the remainder of the transfer region of R100-1 (trbA to traS) has been completed, and the complete sequence is compared to that of F. The differences between the two transfer regions include insertions and deletions, gene duplications, and mosaicism within genes, although the genes essential for Mpf are conserved in both plasmids. F+ cells carrying defined mutations in each of the Mpf genes were complemented with the homologous genes from R100-1. Our results indicate that the specificity in recipient cell recognition and entry exclusion are mediated by TraN and TraG, respectively, and not by the pilus.

The aerobic/anaerobic interface

Microorganisms have evolved intricate signal transduction mechanisms that respond both to dioxygen per se and to the consequences imparted by dioxygen on the metabolism of the cell. Escherichia coli provides examples of both types of signal sensing mechanisms, including FNR and the Arc system. The factors involved in these diverse sensory systems are proving to have a pervasive impact on controlling gene expression and cellular physiology. Similar signal transduction systems are prevalent in a diverse range of microorganisms.