Interactions between the surface exclusion systems of some F-like plasmids

Surface exclusion of IncC conjugative plasmids and their relatives

The phenomenon of exclusion allows conjugative plasmids to selectively impede the entry of identical or related elements into their host cell to prevent the resulting instability. Entry exclusion blocks DNA translocation into the recipient cell, whereas surface exclusion destabilizes the mating pair. IncC conjugative plasmids largely contribute to the dissemination of antibiotic-resistance genes in Gammaproteobacteria. IncC plasmids are known to exert exclusion against their relatives, including IncC and IncA plasmids, yet the entry exclusion factor eexC alone does not account for the totality of the exclusion phenotype. In this study, a transposon-directed insertion sequencing approach identified sfx as necessary and sufficient for the remaining exclusion phenotype. Sfx is an exclusion factor unrelated to the ones described to date. A cell fractionation assay localized Sfx in the outer membrane. Reverse transcription PCR and beta-galactosidase experiments showed that sfx is expressed constitutively at a higher level than eexC. A search in Gammaproteobacteria genomes identified Sfx homologs encoded by IncC, IncA and related, untyped conjugative plasmids and an uncharacterized family of integrative and mobilizable elements that likely rely on IncC plasmids for their mobility. Mating assays demonstrated that sfx is not required in the donor for exclusion, ruling out Sfx as the exclusion target. Instead, complementation assays revealed that the putative adhesin TraN in the donor mediates the specificity of surface exclusion. Mating assays with TraN homologs from related untyped plasmids from Aeromonas spp. and Photobacterium damselae identified two surface exclusion groups, with each Sfx being specific of TraN homologs from the same group. Together, these results allow us to better understand the apparent incompatibility between IncA and IncC plasmids and to propose a mechanistic model for surface exclusion mediated by Sfx in IncC plasmids and related elements, with implications for the rampant dissemination of antibiotic resistance.

Conjugation's Toolkit: the Roles of Nonstructural Proteins in Bacterial Sex

Bacterial conjugation, a form of horizontal gene transfer, relies on a type 4 secretion system (T4SS) and a set of nonstructural genes that are closely linked. These nonstructural genes aid in the mobile lifestyle of conjugative elements but are not part of the T4SS apparatus for conjugative transfer, such as the membrane pore and relaxosome, or the plasmid maintenance and replication machineries. While these nonstructural genes are not essential for conjugation, they assist in core conjugative functions and mitigate the cellular burden on the host. This review compiles and categorizes known functions of nonstructural genes by the stage of conjugation they modulate: dormancy, transfer, and new host establishment. Themes include establishing a commensalistic relationship with the host, manipulating the host for efficient T4SS assembly and function and assisting in conjugative evasion of recipient cell immune functions. These genes, taken in a broad ecological context, play important roles in ensuring proper propagation of the conjugation system in a natural environment.

Plasmid co-infection: linking biological mechanisms to ecological and evolutionary dynamics

As infectious agents of bacteria and vehicles of horizontal gene transfer, plasmids play a key role in bacterial ecology and evolution. Plasmid dynamics are shaped not only by plasmid-host interactions but also by ecological interactions between plasmid variants. These interactions are complex: plasmids can co-infect the same cell and the consequences for the co-resident plasmid can be either beneficial or detrimental. Many of the biological processes that govern plasmid co-infection-from systems that exclude infection by other plasmids to interactions in the regulation of plasmid copy number-are well characterized at a mechanistic level. Modelling plays a central role in translating such mechanistic insights into predictions about plasmid dynamics and the impact of these dynamics on bacterial evolution. Theoretical work in evolutionary epidemiology has shown that formulating models of co-infection is not trivial, as some modelling choices can introduce unintended ecological assumptions. Here, we review how the biological processes that govern co-infection can be represented in a mathematical model, discuss potential modelling pitfalls, and analyse this model to provide general insights into how co-infection impacts ecological and evolutionary outcomes. In particular, we demonstrate how beneficial and detrimental effects of co-infection give rise to frequency-dependent selection on plasmid variants. This article is part of the theme issue 'The secret lives of microbial mobile genetic elements'.

Extraintestinal Pathogenic Escherichia coli: Virulence Factors and Antibiotic Resistance

The One Health approach emphasizes the importance of antimicrobial resistance (AMR) as a major concern both in public health and in food animal production systems. As a general classification, E. coli can be distinguished based on the ability to cause infection of the gastrointestinal system (IPEC) or outside of it (ExPEC). Among the different pathogens, E. coli are becoming of great importance, and it has been suggested that ExPEC may harbor resistance genes that may be transferred to pathogenic or opportunistic bacteria. ExPEC strains are versatile bacteria that can cause urinary tract, bloodstream, prostate, and other infections at non-intestinal sites. In this context of rapidly increasing multidrug-resistance worldwide and a diminishingly effective antimicrobial arsenal to tackle resistant strains. ExPEC infections are now a serious public health threat worldwide. However, the clinical and economic impact of these infections and their optimal management are challenging, and consequently, there is an increasing awareness of the importance of ExPECs amongst healthcare professionals and the general public alike. This review aims to describe pathotype characteristics of ExPEC to increase our knowledge of these bacteria and, consequently, to increase our chances to control them and reduce the risk for AMR, following a One Health approach.

The impact of interspecific competition on lineage evolution and a rapid peak shift by interdemic genetic mixing in experimental bacterial populations

Epistatic interactions between genes in the genome constrain the accessible evolutionary paths of lineages. Two factors involving epistasis that can affect the evolutionary path and fate of lineages were investigated. The first factor concerns the impact of competition with another species lineage that has different epistatic constraints. Five enteric bacterial populations were evolved by point mutation in medium containing a single limiting resource. Single-species and two-species cultures were used to determine whether different asexual lineages have different capacities for producing variants due to epistatic constraints, and whether their survival is determined by local inter-lineage competition with different species. Local inter-lineage competition quickly resulted in one successful lineage, with another lineage becoming extinct before finding a higher peak. The second factor concerns a peak-shifting process, and whether the sexual recombination between different demes can cause peak shifts was investigated. An Escherichia coli population consisting of a male (Hfr) and female strain (F(-)) was evolved in a single limiting resource and compared to evolving populations containing the male or female strain alone. The E. coli sexual lineage was successful due to its ability to escape lower peaks and reach a higher peak, not because of a rapid approach to the nearest local peak the male or female asexual lineage could reach. The data in this study demonstrate that lineage survivability can be determined by the ability to produce beneficial mutations and checked by local competition between lineages of different species. Interspecific competition may prevent a population from evolving through crossing fitness valleys or adaptive ridges if it requires many generations to achieve peak shifts. The data also show that genomic recombination between different conspecific lineages can rapidly carry the combined lineage to a higher peak.

TrbK, a small cytoplasmic membrane lipoprotein, functions in entry exclusion of the IncP alpha plasmid RP4

TrbK is the only plasmid-encoded gene product involved in entry exclusion of the broad-host-range plasmid RP4. The corresponding gene, trbK, coding for a protein of 69 amino acid residues maps in the Tra2 region within the mating pair formation genes. TrbK carries a lipid moiety at the N-terminal cysteine of the mature 47-residue polypeptide. The mutant protein TrbKC23G cannot be modified or proteolytically processed but still acts in entry exclusion with reduced efficiency. An 8-amino-acid truncation at the C terminus of TrbK results in a complete loss of the entry exclusion activity but still allows the protein to be processed. TrbK localizes predominately to the cytoplasmic membrane. Its function depends on presence in the recipient cell but not in the donor cell. TrbK excludes plasmids of homologous systems of the P complex; it is inert towards the IncI system. The likely target for TrbK action is the mating pair formation system, because DNA or any of the components of the relaxosome were excluded as possible targets.

Redundant transfer of F' plasmids occurs between Escherichia coli cells during nonlethal selections

Surface exclusion is the mechanism by which F plasmids prevent the redundant entry of additional F plasmids into the host cell during exponential growth. This mechanism is relaxed in cells that are in stationary phase. Using genetically marked F' plasmids and host strains, we extend this finding to Escherichia coli populations during extended nonlethal selection in bacterial lawns. We show that a high level of redundant transfer occurs between these nongrowing cells during the selection. This result has implications for the mechanism of adaptive mutagenesis.

Surface exclusion specificity of the TraT lipoprotein is determined by single alterations in a five-amino-acid region of the protein

The TraT protein is a highly cell-surface-exposed lipoprotein specified by F-like plasmids that confers serum resistance and blocks the conjugative transfer of plasmids to cells bearing identical or closely related plasmids, a process known as surface exclusion. The TraT protein specified by the antibiotic-resistance plasmid R6-5 was purified to apparent homogeneity. When added to mating mixtures, TraT blocked the transfer of plasmids belonging to Surface Exclusion Group IV (Sfx IV) but had no significant effect on the transfer of plasmids belonging to other groups. Additionally, the purified protein has a protective effect on bacterial cells incubated in serum, indicating that it does not have to be located on the cell surface to mediate serum resistance. To localize regions of the protein that were responsible for surface exclusion specificity, the amino acid sequence of the TraT protein specified by CoIB2-K98 (Sfx II) was determined by cloning and sequencing of the corresponding gene. Comparison of the derived sequence with those of the F and R100-1 proteins indicated that surface exclusion specificity of TraT is determined by single alterations in a five-amino-acid region (residues 116-120). This was confirmed by segment swapping experiments in which the specificity of the R6-5 TraT protein (Sfx IV) was switched to that of the CoIB2-K98 protein (Sfx II). Our results suggest that the region defined by residues 116-120 is located on the external face of the outer membrane and interacts specifically with the donor cell in surface exclusion.

TraT lipoprotein, a plasmid-specified mediator of interactions between gram-negative bacteria and their environment

The TraT protein is a cell-surface-exposed, outer membrane lipoprotein specified by large, usually conjugative, F-like plasmids. Two biological activities have been associated with the protein: (i) prevention of self-mating of cells carrying identical or closely related conjugative plasmids, by blocking the formation of stable mating aggregates; and (ii) resistance to the bactericidal activities of serum, possibly by inhibiting the correct assembly or efficient functioning of the terminal membrane attack complex of complement. The protein therefore interacts not only with components of the outer membrane but also with specific external agents. In conjugative plasmids the traT gene lies within the region necessary for the conjugal transfer of DNA (tra), although its expression is not necessarily dependent on the expression of other tra genes. Recently, however, the gene has been discovered in isolation from other tra genes in nonconjugative virulence-associated plasmids, providing further evidence that the TraT protein may have a role in pathogenesis. The nucleotide sequences of several traT genes have been determined, and comparison of the corresponding amino acid sequences suggests that a central region of five amino acid residues flanked by hydrophobic domains determines the specificity of the protein in surface exclusion. Additionally, studies of mutants with different amino acid alterations within the hydrophobic domains have shown that insertion of charged residues disrupts normal outer membrane integrity. This review considers our current knowledge of the distribution, structure, and biological role(s) of the protein. Recent applications of the protein in studies of the unusual permeability properties of the outer membrane and for the transport of foreign antigenic determinants to the bacterial cell surface are also discussed.

The immunity (imm) gene of Escherichia coli bacteriophage T4

The immunity (imm) gene of the Escherichia coli bacteriophage T4 effects exclusion of phage superinfecting cells already infected with T4. A candidate for this gene was placed under the control of the lac regulatory elements in a pUC plasmid. DNA sequencing revealed the presence of an open reading frame encoding a very lipophilic 83-residue (or 73-residue, depending on the unknown site of translation initiation) polypeptide which most likely represents a plasma membrane protein. This gene could be identified as the imm gene because expression from the plasmid caused exclusion of T4 and because interruption of the gene in the phage genome resulted in a phage no longer effecting superinfection immunity. It was found that the fraction of phage which was excluded upon infection of cells possessing the plasmid-encoded Imm protein ejected only about one-half of their DNA. Therefore, the Imm protein inhibited, directly or indirectly, DNA ejection.

Two monoclonal antibodies specific for different epitopes within the amino-terminal region of F pilin

Two murine monoclonal antibodies (JEL 92 and 93) specific for adjacent epitopes on F pilin were purified and characterized. JEL 93 immunoglobulin G (IgG) and its Fab fragments were specific for the amino-terminal region and were completely reactive with a synthetic peptide representing the first eight amino acids of F pilin. The acetyl group was demonstrated to be an important part of the epitope, since an unacetylated version of the amino-terminal peptide was 100-fold less reactive with JEL 93 IgG. JEL 92 IgG reacted with the region of F pilin surrounding Met-9, represented by a tryptic peptide derived from the first 17 amino acids. This reactivity was completely abolished by cleavage of the peptide with cyanogen bromide. As shown by electron microscopy, both monoclonal antibodies bound to a vesiclelike structure at one end of purified free pili and did not bind to the sides of the pili, nor did they appear to bind to the tip. When sonication was used to break pili into shorter fragments, the number of binding sites for JEL 92 but not JEL 93 IgG increased as measured by a competitive enzyme-linked immunosorbent assay.

Evidence that TraT interacts with OmpA of Escherichia coli

The OmpA protein is one of the major outer membrane proteins of Escherichia coli. Among other functions the protein serves as a receptor for several phages and increases the efficiency of F-mediated conjugation when present in recipient cells. TraT is an F-factor-coded outer membrane lipoprotein involved in surface exclusion, the mechanism by which E. coli strains carrying F-factors become poor recipients in conjugation. To determine a possible interaction of TraT with OmpA, the influence of TraT on phage binding to cells was measured. Because TraT inhibits inactivation of OmpA-specific phages it is suggested that TraT interacts directly with OmpA. Sequence homology of TraT with proteins 38, the phage proteins recognizing outer membrane proteins, supports this finding. A model of protein interactions is discussed.

Coexistence of incompatible plasmids in a bacterial population living under a feast and famine regime

A model is formulated to examine the possibility of (co)existence of plasmids of the same incompatibility and surface exclusion group in a bacterial population living under a feast-and-famine regime. The condition is given under which a growth rate decreasing plasmid can invade a bacterial population. It appears that in case only one plasmid type is present, the frequency of plasmid bearers will tend to a stable equilibrium if the food supply at each growth site gets exhausted and if both plasmid-free and plasmid-bearing bacteria need an equal quantity of food per cell division. If these two conditions are not satisfied, the frequency of plasmid-bearers might oscillate. Two plasmids will sometimes be able to coexist, but only if they follow different survival strategies; one with a high conjugational transfer rate and a lower fitness of its host, and the other with a low transfer rate and a higher host fitness. Coexistence of three plasmids of the same surface exclusion group is impossible.

Location of the antigenic determinants of conjugative F-like pili

The amino terminus of the pilin protein constitutes the major epitope of F-like conjugative pili studied to date (F, ColB2, R1-19, R100-1, and pED208). Anti-pED208 pilus antibodies were passed through a CNBr-Sepharose affinity column linked to bovine serum albumin which was conjugated to a synthetic peptide, AcP(1-12), containing the major epitope at the amino terminus of pED208 pilin. This allowed the separation of two classes of antibodies; one was specific for the amino terminus and bound to the column, while the other, which recognizes a second epitope on the pilus, did not bind to the column. In addition, antibodies were raised against two amino-terminal peptide-bovine serum albumin conjugates [AcP(1-8) and AcP(1-12)] to ensure a source of pure, high-titer antibodies directed against the amino terminus. The location of these antibodies on intact pili was assayed by immunoelectron microscopy with a protein A-gold technique. The amino terminus-specific antibodies did not bind to the sides of the pili but appeared to be associated with the pilus tip. In addition, these antibodies were found to bind to the vesicle-like structure at the base of the pilus. The anti-pilus antibodies not specific for the amino terminus (unbound immunoglobulin G) were found to bind to the sides of the pilus. Anti-F and anti-ColB2 pilus antibodies bound to the sides of F, ColB2, and R1-19 pili, which have only their secondary epitope in common. The carboxyl-terminal lysine of R1-19 pilin prevents the absorption of anti-F plus antiserum but not anti-ColB2 pilus antiserum to the sides of the pilus, presumably by interfering with the recognition of this secondary epitope.

Specificities of IncF plasmid conjugation genes

The conjugation regions of IncF plasmids are closely related in that they share extensive DNA homology, and that they specify related pili. Variations between individual conjugation gene products of different IncF plasmids have, however, been noted. We have extended these observations by carrying out a systematic survey of twelve such plasmids, to examine the numbers and the groupings of the plasmid-specific alleles of several genes required for conjugation and its control.

Using vector plasmids carrying cloned origins of transfer (oriT), four different specificities were recognized, and these were correlated with the specificities of the genes with products that may act at this site (traM, traYandtraZ). ThetraYgene is the first gene of the major transfer operon, and is therefore located close to the site at which thetraJprotein acts to induce expression of the operon: correspondingly, correlation was observed between theoriT/traMYZandtraJspecificities in most of the plasmids. In turn,traJis negatively regulated by thefinOandfinPproducts acting in concert: thefinOproduct was relatively non-specific, but sixfinPalleles were identified, again with specificities correlated with those oftraJ. Our explanation for this unexpectedly large number offinPalleles derives from the concept that thefinPproduct is an RNA molecule rather than a protein. Although the conjugative pili encoded by IncF plasmids are closely related, they confer different efficiencies of plating of the various F-specific bacteriophages. We distinguished four groups on this basis, presumably resulting from differences in the primary amino-acid sequences of the pilin proteins. These groups could be related to the surface exclusion system specificities, consistent with the hypothesis that surface exclusion acts at least in part by preventing interaction between the pilus and the recipient cell surface.

From these data, information about the evolutionary relationships between the twelve IncF plasmids can be deduced.

Nucleotide sequence of the exclusion-determining locus of IncI plasmid R144

The exclusion-determining locus (exc) of IncI plasmid R144 has been proposed to contain two overlapping genes. The nucleotide sequence of this locus, as presently determined, reveals an open reading frame with a coding capacity of 220 amino acids (aa) and with a promoter located upstream of the translation-initiation region. Consistent with the proposed overlapping gene arrangement, a second putative promoter was found within this coding region. Thus, a polypeptide identical to the 147 C-terminal aa of the larger polypeptide can be expressed from this second promoter. The 3'-noncoding region contains a sequence that is representative for a Rho-independent transcription terminator.

Evolution of bacterial surface exclusion against incompatible plasmids

Many conjugative transferable plasmids exhibit surface exclusion against plasmids of the same incompatibility group. A mathematical model is developed to calculate under which conditions surface exclusion against incompatible plasmids can evolve. It appears that plasmids inducing surface exclusion can evolve and even replace non-excluding plasmids if the copy number is low and the transfer rate high provided that the cost of surface exclusion is small. They can more easily expel the non-excluding plasmids if the possession of a plasmid is not very harmful for a bacterium and if the rate at which plasmids are lost is small.

A mathematical model for the co-existence of incompatible, conjugative plasmids in individual bacteria of a bacterial population

A model is formulated to examine the possibility of coexistence of two or more plasmids of the same surface exclusion group in a bacterial chemostat culture. It appears that two plasmids are able to coexist. If two plasmids can coexist they will follow different survival strategies, one with a high conjugational transfer rate and a low fitness of its host, and other with a low transfer rate and a high host fitness. Coexistence of three plasmids of the same surface exclusion group is impossible.

Overproduction, purification and characterization of the F traT protein

A lambda transducing phage (ED lambda 110) which carries the sex factor F surface exclusion genes, traS and traT, was characterized by both genetic and physiochemical techniques. The transducing segment consists of 5.2 kilobases of F tra DNA, and carries the carboxy-terminal one-half of the upstream traG gene, as well as traS, traT, and the adjacent downstream gene traD. These tra proteins could be identified in infected UV-irradiated cells, and the major part of their synthesis was found to occur from the phage's late promoter pR' under Q control. Lysogens for ED lambda 110 were induced and found to greatly overproduce the traT gene product (TraTp), an outer membrane protein normally found in about 20,000 copies per cell, to levels which exceeded the major outer membrane proteins. This led to the development of a simple purification procedure for TraTp, the most important step of which was the construction of an appropriate ompB derivative to eliminate the major outer membrane porin proteins, which have several physical properties in common with TraTp. Purified TraTp was added to mixtures of donor and recipient cells and found to inhibit mating. The specificity of this assay was demonstrated by using an R100-1 donor, which responds to a heterologous surface exclusion system, and by using an altered TraTp containing a missense amino acid substitution. A mechanism by which TraTp mediates surface exclusion is proposed.

Exclusion in IncI-type Escherichia coli conjugations: the stage of conjugation at which exclusion operates

The stage at which exclusion operates in matings between donors belonging to the I-type incompatibility group (IncI) was investigated. Mating between Escherichia coli cells harbouring the I-type plasmid R144 and E. coli cells harbouring the R144-derived recombinant plasmid pRAH308, which causes a hundredfold exclusion, was performed on a membrane filter to test whether mating aggregate formation was disturbed. Besides, level and kinetics of the formation of mating aggregates in mixtures of R144+ donor cells and recipient cells carrying plasmid pRAH308 (exclusion-proficient) was compared with the aggregate formation in mixtures of the donor cells and exclusion-deficient recipient cells. Results from these experiments revealed that the exclusion by pRAH308 does not operate at the level of aggregate formation, but acts at the stage of DNA transfer. The exclusion phenomenon by the recombinant plasmid pRAH308 appeared to be representative for exclusion caused by plasmid R144, since essentially identical results were obtained if plasmid R144 was used as exclusion-determining factor.

Plasmid R46 provides a function that promotes recA-independent deletion, fusion and resolution of replicon

We report that plasmid R46 provides a function which promotes recA-independent deletion, replicon fusion, and resolution of the fusion. R46 belongs to the incompatibility group N and specifies resistance to ampicillin, tetracycline, streptomycin and sulfonamide. Four kinds of deletion derivatives were observed by selection for susceptability to tetracycline from ampicillin-resistant clones. A common region, will be called alpha region thereafter, was postulated to be involved in these deletions. The replicon fusion occurred by a conjugative mobilization of each derivative with plasmid R388. The fusion was suggested to contain both replicons linked at each junction by the sequence in the alpha region in direct orientation. The resolution of the replicon fusion was found between two alpha regions and a consequently generated, parental deletion derivative and an R388 derivative which gained one alpha region. It is possible that the alpha region contains one potential Insertion Sequence (IS) element. These events were also speculated to occur as a consequence of insertion of the potential IS onto the intramolecular or intermolecular target sequence, or reciprocal recombination between two potential IS elements.

Cloning of an exclusion-determining fragment of the IncI plasmid, R144

By cloning a distinct 8 MDa fragment of the IncI plasmid, R144, in the vector pACYC184, two recombinant plasmids were isolated. In these plasmids, pRAH303 and pRAH308, the inserted fragment was in opposite orientations. Both plasmids when present in a recipient strain caused a conjugation-specific exclusion in crosses with donor cells carrying the IncI plasmid R144. Some derivatives of the recombinant plasmids in which parts were deleted, or in which Tn5 transposons were inserted, appeared to be exclusion negative. Analysis in minicells of the gene products of such plasmids together with those of the original recombinant plasmids revealed that the presence of two proteins, with apparent molecular weights of 13,000 and 19,000 Da could be correlated with the exclusion phenomenon.

traJ independence in expression of traT on F

Studies of F plasmids in the same cell as a transfer-repressed IncFII R plasmid showed a 100 to 1,000-fold decrease in transfer and a 75-fold decrease in surface exclusion, but no detectable change was shown in the amount of TraTp synthesized. Moreover, a mutation in traJ on F which caused a 10(4)-fold reduction in transfer caused only a 3.6-fold decrease in TraTp. These two findings suggest that a significant amount of traT expression on F is independent of traJ. Furthermore, we showed, using immunoprecipitation of TraTp, that normal amounts of this protein could be present in the cell without producing normal levels of surface exclusion.

Promoter-distal region of the tra operon of F-like sex factor R100 in Escherichia coli K-12

The distal region of the tra (transfer) operon of F-like plasmid R100 was investigated, using small plasmids derived from R100, primarily the plasmid pSM6. The transposon Tn5 (which confers kanamycin resistance) was inserted at different positions into pSM6, and the transposition derivatives were tested for ability to complement defined tra mutants of the F sex factor. Thus, the tra genes traH, G, T, and D were localized on the plasmid R100. A restriction map of pSM6 was constructed, and the locations of the insertions were mapped, using restriction endonuclease digestion of the plasmid DNA and exploiting the fact that several restriction sites are localized in the inverted repeat regions of the transposon. The gene products of the genes traG, S, T, and D were identified by radioactive labeling of proteins synthesized in minicells carrying the various insertion plasmids followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The presence of another transfer gene, traI, was inferred from these data. Another protein, the r2-A protein, was also identified, and its gene was mapped. On the basis of the data, a best-fit physical map of this region of the tra operon of R100 was constructed. The results confirmed that the general order and size of the distal transfer genes is as in the F sex factor, but showed that differences exist with respect to all of the gene products. The significance of these differences are discussed in the light of the genetic and physical homology (Manning et al., J. Bacteriol. 150:76-88) of the transfer regions.

Outer membrane of Escherichia coli: properties of the F sex factor traT protein which is involved in surface exclusion

The traT protein (TraTp) of the F sex factor is the product of one of the two genes involved in surface exclusion. Several detergents were examined under different conditions in order to determine their ability to solubilize TraTp from membrane vesicles. These experiments showed that TraTp behaved similar to a number of peptidoglycan-associated outer membrane proteins and that it existed in multimeric aggregates within the membrane. However, unlike other major outer membrane proteins, the amount of TraTp incorporated into the membrane was not affected by lipopolysaccharide-deficient mutants, even when mutants totally lacking the neutral sugars in their lipopolysaccharide backbone were used. TraTp wqs also examined by two-dimensional gel electrophoresis, where it ran as a discrete spot with a very basic isoelectric point. By coupling cyanogen bromide-activated dextran onto whole cells and by labeling whole cells with 125I (via lactoperoxidase), it was shown that TraTp was exposed on the cell surface. TraTp in a membrane environment was also insensitive to proteolytic attack by trypsin.

Identification of a membrane protein associated with expression of the surface exclusion region of the F transfer operon

Membrane preparations from radioactively labeled male and female strains of Escherichia coli K-12 were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. An intensely labeled band corresponding to a protein of molecular weight of 24,000 was readily apparent in preparations from Hfr and F-prime strains but not in those from female strains. When preparations from a series of Hfr strains containing transfer operon deletions were examined, presence of the band was found to be associated with retention of the region of the F transfer operon between ilzA and traD. Thus, the band ("protein S") appears to be the product of an F tra operon activity corresponding to traS (the gene for surface or entry exclusion), or an unknown gene in its vicinity. As predicted, protein S was subject to Fin+ control; only a faint band was detectable if the repressed plasmid R100 was also present in the F lac strain. A 24,000-dalton protein was also found in membrane preparations from strains carrying the derepressed plasmids R100-1 and R1-19 but not in those from strains carrying the repressed plasmids R100 or R1. Thus, the appearance of protein S in the membrane may be a general phenomenon resulting from transfer operon expression of F-like plasmids.

Effect of entry exclusion on mating aggregates and transconjugants

Mating aggregates during conjugation directed by an F-like R factor in Escherichia coli were measured as the number of Lac+-Lac- sectored colonies present in a mating mixture. There is a high degree of correlation between the concentration of transconjugants produced in a mating mixture and the concentration of mating aggregates observed at several different concentrations of donor and recipient cells. The mating aggregates are sex pilus specific as demonstrated by the ability of donor-specific ribonucleic acid phage MS-2 to decrease both mating aggregates and transconjugants in a mating mixture. During entry exclusion by either a derepressed or a repressed F-like R factor, isogenic to the superinfecting R factor except for a resistance determinant, the number of transconjugants was markedly reduced, but the number of mating aggregates was not decreased. Entry exclusion by F-Gal toward the donor HfrH resembled that of the F-like R factor in that there was a reduction in the number of recombinants but no significant decrease in mating aggregates. These results suggest that entry exclusion inhibits conjugation at a stage after the formation of mating aggregates.

Genetic and physical characteristics of an enterotoxin plasmid

We are engaged in the genetic and physical characterization of an enterotoxin (Ent) plasmid, Ent P307, which contains genes for the production of a hear-labile and a heat-stable enterotoxin. We are using an Escherichia coli K-12 strain, 711 (P307), constructed by S. Falkow, which contains no other plasmids besides Ent P307. Our genetic studies have shown that the plasmid is incompatible with the sex factor F, both in the integrated (Hfr) and the autonomous (F-prime) state. Ent P307 can thus be assigned to incompatibility group FI. An R factor, R386, which belongs to the same incompatibility group, was also found to be incompatibile with Ent P307, whereas five other R factors belonging to different incompatibility groups were compatible with Ent P307. In the presence of Ent P307, conjugal transfer and sensitivity to a male-specific phage of a derepressed F-like R factor, R1drd19, were repressed. Ent P307 is, thus, finO+. Presumably, it also causes repression of its own transfer genes since conjugal transfer of Ent P307 could not be demonstrated. Unlike F, it does not restrict the growth of female-specific phage phiII. From physical studies on extracted deoxyribonucleic acid, the molecular weight of Ent P307 was determined to be 54 X 10(6). By electron microscope heteroduplex analysis, the plasmid was found to be homologous with F in four regions, encompassing about half of its length. One long region and two short ones contain genes for conjugal transfer; the other short region carries genes for replication and incompatibility.

Five control systems preventing transfer of Escherichia coli K-12 sex factor F

The transfer inhibition systems of 28 Fin+ plasmids have been characterized, using Flac mutants insensitive to inhibition by R100 or R62. All F-like plasmids (except R455) and one N group plasmid determined systems analogous to that of R100; this is designated the FinOP system. None of these plasmids could supply a FinP component of the transfer inhibitor able to replace that of F itself. In addition to the FinOP and R62 transfer inhibition systems described previously, new systems were encoded by the F-like plasmid R455, the I-like plasmid JR66a, and the group X plasmid R485. Besides inhibiting F transfer, JR66a also inhibited F pilus formation and surface exclusion, whereas R485 inhibited only pilus formation and R455 inhibited neither. All three R factors inhibited transfer of J-independent Flac elements, indicating that they act directly on one or more genes (or products) of the transfer operon, rather than directly via traJ. The tral products and transfer origin sequences of two Fin+ F-like plasmids, ColB2 and R124, appear to have similar specificities to those of F itself.