DNA sequence of the F traALE region that includes the gene for F pilin

Plasmids pick a bacterial partner before committing to conjugation

Bacterial conjugation was first described by Lederberg and Tatum in the 1940s following the discovery of the F plasmid. During conjugation a plasmid is transferred unidirectionally from one bacterium (the donor) to another (the recipient), in a contact-dependent manner. Conjugation has been regarded as a promiscuous mechanism of DNA transfer, with host range determined by the recipient downstream of plasmid transfer. However, recent data have shown that F-like plasmids, akin to tailed Caudovirales bacteriophages, can pick their host bacteria prior to transfer by expressing one of at least four structurally distinct isoforms of the outer membrane protein TraN, which has evolved to function as a highly sensitive sensor on the donor cell surface. The TraN sensor appears to pick bacterial hosts by binding compatible outer membrane proteins in the recipient. The TraN variants can be divided into specialist and generalist sensors, conferring narrow and broad plasmid host range, respectively. In this review we discuss recent advances in our understanding of the function of the TraN sensor at the donor-recipient interface, used by F-like plasmids to select bacterial hosts within polymicrobial communities prior to DNA transfer.

Cryo-EM structure of the Agrobacterium tumefaciens T4SS-associated T-pilus reveals stoichiometric protein-phospholipid assembly

Agrobacterium tumefaciens causes crown gall disease in plants by the horizontal transfer of oncogenic DNA. The conjugation is mediated by the VirB/D4 type 4 secretion system (T4SS) that assembles an extracellular filament, the T-pilus, and is involved in mating pair formation between A. tumefaciens and the recipient plant cell. Here, we present a 3 Å cryoelectron microscopy (cryo-EM) structure of the T-pilus solved by helical reconstruction. Our structure reveals that the T-pilus is a stoichiometric assembly of the VirB2 major pilin and phosphatidylglycerol (PG) phospholipid with 5-start helical symmetry. We show that PG head groups and the positively charged Arg 91 residues of VirB2 protomers form extensive electrostatic interactions in the lumen of the T-pilus. Mutagenesis of Arg 91 abolished pilus formation. While our T-pilus structure is architecturally similar to previously published conjugative pili structures, the T-pilus lumen is narrower and positively charged, raising questions of whether the T-pilus is a conduit for ssDNA transfer.

Plasmid Transfer by Conjugation in Gram-Negative Bacteria: From the Cellular to the Community Level

Bacterial conjugation, also referred to as bacterial sex, is a major horizontal gene transfer mechanism through which DNA is transferred from a donor to a recipient bacterium by direct contact. Conjugation is universally conserved among bacteria and occurs in a wide range of environments (soil, plant surfaces, water, sewage, biofilms, and host-associated bacterial communities). Within these habitats, conjugation drives the rapid evolution and adaptation of bacterial strains by mediating the propagation of various metabolic properties, including symbiotic lifestyle, virulence, biofilm formation, resistance to heavy metals, and, most importantly, resistance to antibiotics. These properties make conjugation a fundamentally important process, and it is thus the focus of extensive study. Here, we review the key steps of plasmid transfer by conjugation in Gram-negative bacteria, by following the life cycle of the F factor during its transfer from the donor to the recipient cell. We also discuss our current knowledge of the extent and impact of conjugation within an environmentally and clinically relevant bacterial habitat, bacterial biofilms.

Protein Dynamics in F-like Bacterial Conjugation

Efficient in silico development of novel antibiotics requires high-resolution, dynamic models of drug targets. As conjugation is considered the prominent contributor to the spread of antibiotic resistance genes, targeted drug design to disrupt vital components of conjugative systems has been proposed to lessen the proliferation of bacterial antibiotic resistance. Advancements in structural imaging techniques of large macromolecular complexes has accelerated the discovery of novel protein-protein interactions in bacterial type IV secretion systems (T4SS). The known structural information regarding the F-like T4SS components and complexes has been summarized in the following review, revealing a complex network of protein-protein interactions involving domains with varying degrees of disorder. Structural predictions were performed to provide insight on the dynamicity of proteins within the F plasmid conjugative system that lack structural information.

Tail-Anchored Inner Membrane Protein ElaB Increases Resistance to Stress While Reducing Persistence in Escherichia coli

Host-associated bacteria, such as Escherichia coli, often encounter various host-related stresses, such as nutritional deprivation, oxidative stress, and temperature shifts. There is growing interest in searching for small endogenous proteins that mediate stress responses. Here, we characterized the small C-tail-anchored inner membrane protein ElaB in E. coli ElaB belongs to a class of tail-anchored inner membrane proteins with a C-terminal transmembrane domain but lacking an N-terminal signal sequence for membrane targeting. Proteins from this family have been shown to play vital roles, such as in membrane trafficking and apoptosis, in eukaryotes; however, their role in prokaryotes is largely unexplored. Here, we found that the transcription of elaB is induced in the stationary phase in E. coli and stationary-phase sigma factor RpoS regulates elaB transcription by binding to the promoter of elaB Moreover, ElaB protects cells against oxidative stress and heat shock stress. However, unlike membrane peptide toxins TisB and GhoT, ElaB does not lead to cell death, and the deletion of elaB greatly increases persister cell formation. Therefore, we demonstrate that disruption of C-tail-anchored inner membrane proteins can reduce stress resistance; it can also lead to deleterious effects, such as increased persistence, in E. coliIMPORTANCEEscherichia coli synthesizes dozens of poorly understood small membrane proteins containing a predicted transmembrane domain. In this study, we characterized the function of the C-tail-anchored inner membrane protein ElaB in E. coli ElaB increases resistance to oxidative stress and heat stress, while inactivation of ElaB leads to high persister cell formation. We also demonstrated that the transcription of elaB is under the direct regulation of stationary-phase sigma factor RpoS. Thus, our study reveals that small inner membrane proteins may have important cellular roles during the stress response.

Bioluminescent imaging reveals novel patterns of colonization and invasion in systemic Escherichia coli K1 experimental infection in the neonatal rat

Key features of Escherichia coli K1-mediated neonatal sepsis and meningitis, such as a strong age dependency and development along the gut-mesentery-blood-brain course of infection, can be replicated in the newborn rat. We examined temporal and spatial aspects of E. coli K1 infection following initiation of gastrointestinal colonization in 2-day-old (P2) rats after oral administration of E. coli K1 strain A192PP and a virulent bioluminescent derivative, E. coli A192PP-lux2. A combination of bacterial enumeration in the major organs, two-dimensional bioluminescence imaging, and three-dimensional diffuse light imaging tomography with integrated micro-computed tomography indicated multiple sites of colonization within the alimentary canal; these included the tongue, esophagus, and stomach in addition to the small intestine and colon. After invasion of the blood compartment, the bacteria entered the central nervous system, with restricted colonization of the brain, and also invaded the major organs, in line with increases in the severity of symptoms of infection. Both keratinized and nonkeratinized surfaces of esophagi were colonized to a considerably greater extent in susceptible P2 neonates than in corresponding tissues from infection-resistant 9-day-old rat pups; the bacteria appeared to damage and penetrate the nonkeratinized esophageal epithelium of infection-susceptible P2 animals, suggesting the esophagus represents a portal of entry for E. coli K1 into the systemic circulation. Thus, multimodality imaging of experimental systemic infections in real time indicates complex dynamic patterns of colonization and dissemination that provide new insights into the E. coli K1 infection of the neonatal rat.

New insights into F-pilus structure, dynamics, and function

F-pili are thin, flexible filaments elaborated by F(+) cells of Escherichia coli. They belong to the class of Gram-negative pili that function in horizontal gene transfer. F-pili are initially required to establish contacts between DNA donor and recipient cells. Beyond that, F-pilus function, and that of other conjugative pili, has remained obscure and controversial. The idea that F-pili are dynamic structures was proposed 40 years ago. Initially, F-pili were thought to remain extended until another cell bound to the filament tip, whereupon the filament retracted to bring the contacted cell to the donor cell surface. Thereafter, secure surface-surface contacts would allow efficient DNA transfer. A later variant of this hypothesis was that F-pili are inherently dynamic, elongating and retracting even in the absence of exogenous signals. A very different hypothesis, also proposed first about 40 years ago, was that F-pili are conduits, presumably passive, for the transfer of DNA from donor to recipient. In this hypothesis, DNA transfer is not obligatorily coupled to F-pilus retraction. Here, we review recent data obtained by integrating long-established facts about the biology of F-pili with modern tools of fluorescence and electron microscopy. These data suggest that one function for F-pili is to search a large volume around donor cells in liquid culture for the presence of other cells. However, this may not be the only function. We show that F-pilin is also required at a second, largely undefined step occurring after cells have been brought into direct contact by F-pilus retraction.

The structure of F-pili

Exchange of DNA between bacteria involves conjugative pili. While the prevailing view has been that F-pili are completely retracted before single-stranded DNA is passed from one cell to another, it has recently been reported that the F-pilus, in addition to establishing the contact between mating cells, serves as a channel for passing DNA between spatially separated cells during conjugation. The structure and function of F-pili are poorly understood. They are built from a single subunit having only 70 residues, and the small size of the subunit has made these filaments difficult to study. Here, we have applied electron cryo-microscopy and single-particle methods to solve the long-existing ambiguity in the packing geometry of F-pilin subunits. We show that the F-pilus has an entirely different symmetry from any of the known bacterial pili as well as any of the filamentous bacteriophages, which have been suggested to be structural homologs. Two subunit packing schemes were identified: one has stacked rings of four subunits axially spaced by approximately 12.8 A, while the other has a one-start helical symmetry with an axial rise of approximately 3.5 A per subunit and a pitch of approximately 12.2 A. Both structures have a central lumen of approximately 30 A diameter that is more than large enough to allow for the passage of single-stranded DNA. Remarkably, both schemes appear to coexist within the same filaments, in contrast to filamentous phages that have been described as belonging to one of two possible symmetry classes. For the segments composed of rings, the twist between adjacent rings is quite variable, while the segments having a one-start helix are in multiple states of both twist and extension. This coexistence of two very different symmetries is similar to what has recently been reported for an archaeal Methanococcus maripaludis pili filament and an archaeal Sulfolobus shibatae flagellar filament.

Direct visualization of horizontal gene transfer

Conjugation allows bacteria to acquire genes for antibiotic resistance, novel virulence attributes, and alternative metabolic pathways. Using a fluorescent protein fusion, SeqA-YFP, we have visualized this process in real time and in single cells of Escherichia coli. We found that the F pilus mediates DNA transfer at considerable cell-to-cell distances. Integration of transferred DNA by recombination occurred in up to 96% of recipients; in the remaining cells, the transferred DNA was fully degraded by the RecBCD helicase/nuclease. The acquired integrated DNA was tracked through successive replication rounds and was found to occasionally split and segregate with different chromosomes, leading to the inheritance of different gene clusters within the cell lineage. The incidence of DNA splitting corresponds to about one crossover per cell generation.

Fluorescence assays for F-pili and their application

Conjugative pili are extracellular filaments elaborated by Gram-negative bacteria expressing certain type IV secretion systems. They are required at the earliest stages of conjugal DNA transfer to establish specific and secure cell-cell contacts. Conjugative pili also serve as adsorption organelles for both RNA and DNA bacteriophages. Beyond these facts, the structure, formation and function of these filaments are poorly understood. This paper describes a rapid, quantitative assay for F-pili encoded by the F plasmid type IV secretion system. The assay is based on the specific lateral adsorption of icosahedral RNA bacteriophage R17 by F-pili. Bacteriophage particles conjugated with a fluorescent dye, Alexa 488, and bound to F-pili defined filaments visible by immunofluorescence microscopy. F-pili attached to F+ cells and free F-pili were both visible by this method. For quantification, cell-bound bacteriophage were separated from free bacteriophage particles by sedimentation and released by suspending cell pellets in 0.1 % SDS. Fluorescence in cell-free supernatant fractions was measured by fluorometry. The authors present a characterization of this assay and its application to F-pilus formation by cells carrying mutations in the gene for the F-pilus subunit F-pilin. Each mutation introduced a cysteine, which F-pilin normally lacks, at a different position in its primary structure. Cysteine residues in the N-terminal domain I abolished filament formation as measured by fluorescent R17 binding. This was confirmed by measurements of DNA donor activity and filamentous DNA bacteriophage infection. With one exception (G53C), cysteines elsewhere in the F-pilin primary structure did not abolish filament formation, although some mutations differentially affected F-pilus functions.

Genetic and environmental factors affecting T-pilin export and T-pilus biogenesis in relation to flagellation of Agrobacterium tumefaciens

The T pilus, primarily composed of cyclic T-pilin subunits, is essential for the transmission of the Ti-plasmid T-DNA from Agrobacterium tumefaciens to plant cells. Although the virB2 gene of the 11-gene virB operon was previously demonstrated to encode the full-length propilin, and other genes of this operon have been implicated as members of a conserved transmembrane transport apparatus, the role of each virB gene in T-pilin synthesis and transport and T-pilus biogenesis remained undefined. In the present study, each virB gene was examined and was found to be unessential for T-pilin biosynthesis, except virB2, but was determined to be essential for the export of the T-pilin subunits and for T-pilus formation. We also find that the genes of the virD operon are neither involved in T-pilin export nor T-pilus formation. Critical analysis of three different virD4 mutants also showed that they are not involved in T-pilus biogenesis irrespective of the A. tumefaciens strains used. With respect to the environmental effects on T-pilus biogenesis, we find that T pili are produced both on agar and in liquid culture and are produced at one end of the A. tumefaciens rod-shaped cell in a polar manner. We also report a novel phenomenon whereby flagellum production is shut down under conditions which turn on T-pilus formation. These conditions are the usual induction with acetosyringone at pH 5.5 of Ti-plasmid vir genes. A search of the vir genes involved in controlling this biphasic reaction in induced A. tumefaciens cells revealed that virA on the Ti plasmid is involved and that neither virB nor virD genes are needed for this reaction. The biphasic reaction therefore appears to be mediated through a two-component signal transducing system likely involving an unidentified vir gene in A. tumefaciens.

Epitopes fused to F-pilin are incorporated into functional recombinant pili

In order to develop a system which allows infection by an epitope-specific phage-antibody via an F-pilus expressing that epitope, a study on the expression of foreign sequences on F-pilin was undertaken. Initially, a plasmid library was constructed with random sequences encoding one to five amino acid residues fused to the C terminus of F-pilin (traA) which was used to complement an F-plasmid with an amber mutation in traA. Functional F-pilin fusions were detected using the filamentous phage, fUSE2, which transduces tetracycline resistance, as well as immunoblots using a monoclonal antiserum specific for the acetylated N terminus of pilin. All the clones selected expressed the pilin-fusions and displayed full sensitivity towards fUSE2 infection, which was indistinguishable from the wild-type F-pilin. The sequences of fUSE2-sensitive clones when compared to randomly selected clones which were not fUSE2-sensitive, revealed no obvious pattern in the amino acid residues fused to the C terminus, except for a preference for a hydrophilic amino acid at position +1. Mutating the C-terminal Leu in wt (wild-type) pilin to Ser blocked pilus assembly and fUSE2 infection; the pilin was correctly processed but the level of acetylation at the N terminus appeared to decrease. Fusing a known epitope (myc) directly to the C terminus blocked processing of F-pilin leading to loss of F-pilus assembly and function. The introduction of random sequences between traA and this epitope yielded fully recombinant, functional F-pili but this appeared to be due to processing of the extension by an unidentified protease leading to loss of the epitope. Surface expression of another epitope (G2-10) was clearly demonstrated by immuno-electron microscopy of pili with a G2-10 monoclonal antibody. A different five amino acid residue spacer between the F-pilin C terminus and the G2-10 epitope produced a system that was transfer-proficient and fUSE2-sensitive, but the pili were barely detectable by immunoblots or by electron microscopy. While the underlying rules that govern successful epitope expression at the C terminus of F-pilin remain elusive, many types of foreign sequences can be displayed with varying degrees of success. Our results also suggest that pilin sequence determines a number of steps in the complex pathway for pilus assembly.

Processed VirB2 is the major subunit of the promiscuous pilus of Agrobacterium tumefaciens

Previous studies have implicated the obligatory requirement for the vir regulon (or "virulon") of the Ti plasmid for the transfer of oncogenes from Agrobacterium tumefaciens to plant cells. The machinery used in this horizontal gene transfer has been long thought to be a transformation or conjugative delivery system. Based on recent protein sequence comparisons, the proteins encoded by the virB operon are strikingly similar to proteins involved in the synthesis and assembly of conjugative pili such as the conjugative pilus of F plasmid in Escherichia coli. The F pilus is composed of TraA pilin subunits derived from TraA propilin. In the present study, evidence is provided showing that the counterpart of TraA is VirB2, which like TraA propilin is processed into a 7.2-kDa product that comprises the pilus subunit as demonstrated by biochemical and electron microscopic analyses. The processed VirB2 protein is present exocellularly on medium on which induced A. tumefaciens had grown and appears as thin filaments of 10 nm that react specifically to VirB2 antibody. Exocellular VirB2 is produced abundantly at 19 degreesC as compared with 28 degreesC, an observation that parallels the effect of low temperature on the production of vir gene-specific pili observed previously (K. J. Fullner, L. C. Lara, and E. W. Nester, Science 273:1107-1109, 1996). Export of the processed VirB2 requires other virB genes since mutations in these genes cause the loss of VirB2 pilus formation and result in processed VirB2 accumulation in the cell. The presence of exocellular processed VirB2 is directly correlated with the formation of pili, and it appears as the major protein in the purified pilus preparation. The evidence provides a compelling argument for VirB2 as the propilin whose 7.2-kDa processed product is the pilin subunit of the promiscuous conjugative pilus, hereafter called the "T pilus" of A. tumefaciens.

Role of the propilin leader peptide in the maturation of F pilin

F-pilin maturation and translocation result in the cleavage of a 51-amino-acid leader sequence from propilin and require LepB and TraQ but not the SecA-SecY secretion pathway. The unusual propilin leader peptide and the dependence of its cleavage on TraQ suggested that TraQ recognition may be specific for the leader peptide. An in vitro propilin cleavage assay yielded propilin (13 kDa), the pilin polypeptide (7 kDa), and a 5.5-kDa protein as the traA products. The 5.5-kDa protein comigrates with the full-length 51-amino-acid leader peptide, and [14C]proline labeling confirmed its identity since the only proline residues of propilin are found within the leader peptide. The in vitro and in vivo propilin-processing reactions proceed similarly in a single polypeptide cleavage step. Furthermore, TraQ dependence is a property of F-pilin maturation specifically rather than a property of the leader peptide. A propilin derivative with an amino-terminal signal sequence generated by deleting codons 2 to 28 required TraQ for processing in vivo. On the other hand, a chimeric protein with the propilin wild-type leader peptide fused to the mature portion of beta-lactamase was processed in a TraQ-independent manner. Thus, despite its unusual length, the propilin leader peptide seems to perform a function similar to that of the typical amino-terminal signal sequence. This work suggests that TraQ is not necessary for the proteolysis of propilin and therefore is likely to act as a chaperone-like protein that promotes the translocation of propilin.

Membrane insertion of the F-pilin subunit is Sec independent but requires leader peptidase B and the proton motive force

F pilin is the subunit required for the assembly of conjugative pili on the cell surface of Escherichia coli carrying the F plasmid. Maturation of the F-pilin precursor, propilin, involves three F plasmid transfer products: TraA, the propilin precursor; TraQ, which promotes efficient propilin processing; and TraX, which is required for acetylation of the amino terminus of the 7-kDa pilin polypeptide. The mature pilin begins at amino acid 52 of the TraA propilin sequence. We performed experiments to determine the involvement of host cell factors in propilin maturation. At the nonpermissive temperature in a LepBts (leader peptidase B) host, propilin processing was inhibited. Furthermore, under these conditions, only full-length precursor was observed, suggesting that LepB is responsible for the removal of the entire propilin leader peptide. Using propilin processing as a measure of propilin insertion into the plasma membrane, we found that inhibition or depletion of SecA and SecY does not affect propilin maturation. Addition of a general membrane perturbant such as ethanol also had no effect. However, dissipation of the proton motive force did cause a marked inhibition of propilin processing, indicating that membrane insertion requires this energy source. We propose that propilin insertion in the plasma membrane proceeds independently of the SecA-SecY secretion machinery but requires the proton motive force. These results present a model whereby propilin insertion leads to processing by leader peptidase B to generate the 7-kDa peptide, which is then acetylated in the presence of TraX.

Analysis of the traLEKBP sequence and the TraP protein from three F-like plasmids: F, R100-1 and ColB2

The sequence of a region of the F plasmid containing the traLEKBP genes involved in plasmid transfer was compared to the equivalent regions of two IncFII plasmids, R100-1 and ColB2. The traLEK gene products of all three plasmids were virtually identical, with the most changes occurring in TraE. The TraB genes were also nearly identical except for an 11-codon extension at the 3' end of the R100-1 traB gene. The TraP protein of R100-l differed from those of F and ColB2 at its N terminus, while the ColB2 TraP protein contained a change of sequence in a predicted loop which was shown to be exposed in the periplasmic space by TnphoA mutagenesis. The effect of the altered TraP sequences was determined by complementing a traP mutant with clones expressing the traKBP genes of F, R100-1, and ColB2. The traP mutation in pOX38 (pOX38-traP474), a derivative of F, was found to have little effect on pilus production, pilus retraction, and filamentous phage growth and only a moderate effect on transfer. The transfer ability of pOX38-traP474 was shown to be affected by mutations in the rfa (lipopolysaccharide) locus and in ompA in the recipient cell in a manner similar to that for the wild-type pOX38-Km plasmid itself and could be complemented with the traP analogs from R100-1 and ColB2 to give an F-like phenotype. Thus, the TraP protein appears to play a minor role in conjugation and may interact with TraB, which varies in sequence along with TraP, in order to stabilize the proposed transmembrane complex formed by the tra operon products.

Effects of F-encoded components and F-pilin domains on the synthesis and membrane insertion of TraA'-'PhoA fusion proteins

F-pilin, the 70-amino-acid F-pilus subunit, accumulates in the cell envelope of F+ strains in a process that requires interactions between its precursor (the traA gene product) and other host and F-encoded proteins. Here, we have used a set of phi(traA-phoA) genes to explore the effects of different TraA domains on the synthesis and membrane insertion of TraA-PhoA fusion proteins, particularly in relation to other F-encoded gene products. The 51-amino-acid TraA leader peptide fused directly to alkaline phosphatase was synthesized at comparable rates and incorporated rapidly and efficiently into the inner membrane in F' and F- cells. A second fusion gene encoded the TraA leader peptide and the first 51 amino acids of F-pilin itself fused to PhoA (TraA'-'PhoA-102 polypeptide). Alkaline phosphatase activities and patterns of pulse-labelled polypeptides indicated that TraA'-'PhoA-102 was synthesized at comparable rates in F' and F- cells, but in neither was the TraA'-'PhoA-102 polypeptide efficiently processed as a membrane protein. A third gene encoded the entire 121-amino-acid TraA polypeptide fused to PhoA (TraA-'PhoA-121 polypeptide). About 70% of the pulse-labelled TraA-'PhoA-121 polypeptide was rapidly processed in F' cells, where it accumulated in the cell envelope as active alkaline phosphatase, whereas in F- cells, < 5% of the pulse-labelled polypeptide was processed. Additionally, the apparent rate of TraA-'PhoA-121 polypeptide synthesis was threefold higher in F' cells. The traQ gene alone could not substitute for F in restoring TraA-'PhoA-121 (or wild-type F-pilin) accumulation.

Identification of Porphyromonas gingivalis prefimbrilin possessing a long leader peptide: possible involvement of trypsin-like protease in fimbrilin maturation

Fimbriae of Porphyromonas gingivalis have been shown to be important as one of the virulence factors for colonization on mucosal surfaces. The gene (fimA) encoding the fimbrial subunit (fimbrilin) was overexpressed in Escherichia coli by using a bacteriophage T7 promoter-polymerase expression vector system. Analysis of the resulting fimA gene product revealed that the prefimbrilin had a 46 amino acid leader peptide. This extremely long leader peptide was cleaved from the prefimbrilin by treatment with trypsin or P. gingivalis extracts containing trypsin-like protease activity, resulting in production of a mature fimbrilin. We also found that some transposon-induced trypsin-like protease deficient mutants of P. gingivalis exhibited deficiency in fimbriation and that one of the mutants accumulated a fimbrilin precursor possessing a 25 amino acid leader peptide in the cell. The presence of an extremely long leader peptide and the requirement for a leader peptidase with a substrate specificity similar to that of P. gingivalis trypsin-like protease for fimbrilin maturation indicate that P. gingivalis fimbrilin is a novel type that is different from fimbrilins of type I and IV families.

Gene 19 of plasmid R1 is required for both efficient conjugative DNA transfer and bacteriophage R17 infection

F-like plasmids require a number of genes for conjugation, including tra operon genes and genes traM and traJ, which lie outside the tra operon. We now establish that a gene in the "leading region," gene 19, provides an important function during conjugation and RNA phage infection. Mutational inactivation of gene 19 on plasmid R1-16 by introduction of two nonpolar stop codons results in a 10-fold decrease in the conjugation frequency. Furthermore, infection studies with the male-specific bacteriophage R17 revealed that the phage is not able to form clear plaques in Escherichia coli cells carrying an R1-16 plasmid with the defective copy of gene 19. The total number of cells infected by phage R17 is reduced by a factor of 10. Both the conjugation- and infection-attenuated phenotypes caused by the defective gene 19 can be complemented in trans by introducing gene 19 alleles encoding the wild-type protein. Restoration of the normal phenotypes is also possible by introduction of the pilT gene encoded by the unrelated IncI plasmid R64. Our functional studies and similarities of protein 19 to proteins encoded by other DNA transfer systems, as well as the presence of a conserved motif in all of these proteins (indicative for a putative muramidase activity) suggest that protein 19 of plasmid R1 facilitates the passage of DNA during conjugation and entry of RNA during phage infection.

Characterization of traX, the F plasmid locus required for acetylation of F-pilin subunits

Acetylation of F-pilin subunits has previously been shown to depend upon expression of the F plasmid transfer operon gene traX. To assess the requirement for pilin acetylation in conjugative transfer of F, we constructed traX::kan insertion mutations and crossed them onto the transmissible F derivative pOX38. Under standard conditions, the function of traX seemed to be dispensable. Although pilin synthesized by mutant plasmids pOX38-traX482 and pOX38-traX483 was not acetylated, F-pilus production and F-pilus-specific phage infection appeared to be normal and transfer occurred at wild-type frequency. Analysis of labeled products showed that TraX+ plasmids expressed two approximately 24- (TraX1) and 22-kDa (TraX2) polypeptides that localized in the cytoplasmic membranes of cells. No product that was similar in size to the product predicted from the traX open reading frame (27.5 kDa) was detected. Therefore, we used site-directed mutagenesis, stop codon linker insertions, and phoA fusion analysis to investigate traX expression. Both TraX1 and TraX2 appeared to be encoded by the traX open reading frame. Insertion of a stop codon linker into the traX C-terminal coding region led to synthesis of two correspondingly truncated products, and fusions to phoA indicated that only the traX reading frame was translated. Expression was also very dependent on the traX M1 start codon; when this was altered, no protein products were observed. However, pilin acetylation activity was still detectable, indicating that some other in-frame start codon(s) can also be used. All sequences that are essential for activity are contained between traX codons 29 and 225. Sequence analysis indicated that traX mRNA is capable of forming a variety of base-paired structures. We suggest that traX expression is translationally controlled and that F-pilin acetylation activity may be regulated by physiological conditions in cells.

The role of the pilus in recipient cell recognition during bacterial conjugation mediated by F-like plasmids

The effects of defined mutations in the lipopolysaccharide (LPS) and the outer membrane protein OmpA of the recipient cell on mating-pair formation in liquid media by the transfer systems of the F-like plasmids pOX38 (F), ColB2 and R100-1 were investigated. Transfer of all three plasmids was affected differently by mutations in the rfa (LPS) locus of the recipient cell, the F plasmid being most sensitive to mutations that affected rfaP gene expression which is responsible for the addition of pyrophosphorylethanolamine (PPEA) to heptose I of the inner core of the LPS. ColB2 transfer was more strongly affected by mutations in the heptose II-heptose III region of the LPS (rfaF) whereas R100-1 was not strongly affected by any of the rfa mutations tested. ompA but not rfa mutations further decreased the mating efficiency of an F plasmid carrying a mutation in the mating-pair stabilization protein TraN. An F derivative with a chloramphenicol acetyltransferase (CAT) cassette interrupting the traA pilin gene was constructed and pilin genes from F-like plasmids (F, ColB2, R100-1) were used to complement this mutation. Unexpectedly, the results suggested that the differences in the pilin sequences were not responsible for recognizing specific groups in the LPS, OmpA or the TraT surface exclusion protein. Other corroborating evidence is presented suggesting the presence of an adhesin at the F pilus tip.

Analysis of the sequence and gene products of the transfer region of the F sex factor

Bacterial conjugation results in the transfer of DNA of either plasmid or chromosomal origin between microorganisms. Transfer begins at a defined point in the DNA sequence, usually called the origin of transfer (oriT). The capacity of conjugative DNA transfer is a property of self-transmissible plasmids and conjugative transposons, which will mobilize other plasmids and DNA sequences that include a compatible oriT locus. This review will concentrate on the genes required for bacterial conjugation that are encoded within the transfer region (or regions) of conjugative plasmids. One of the best-defined conjugation systems is that of the F plasmid, which has been the paradigm for conjugation systems since it was discovered nearly 50 years ago. The F transfer region (over 33 kb) contains about 40 genes, arranged contiguously. These are involved in the synthesis of pili, extracellular filaments which establish contact between donor and recipient cells; mating-pair stabilization; prevention of mating between similar donor cells in a process termed surface exclusions; DNA nicking and transfer during conjugation; and the regulation of expression of these functions. This review is a compendium of the products and other features found in the F transfer region as well as a discussion of their role in conjugation. While the genetics of F transfer have been described extensively, the mechanism of conjugation has proved elusive, in large part because of the low levels of expression of the pilus and the numerous envelope components essential for F plasmid transfer. The advent of molecular genetic techniques has, however, resulted in considerable recent progress. This summary of the known properties of the F transfer region is provided in the hope that it will form a useful basis for future comparison with other conjugation systems.

The nature of the traK4 mutation in the F sex factor of Escherichia coli

The sequence of traK gene of the F sex factor of Escherichia coli is presented; the traK gene product is predicted to be a protein of 25,627 Da with a signal sequence of 21 amino acids to give a mature protein of 23,307 Da. The traK4 mutation is an extremely polar mutation in the F plasmid that affects F pilus synthesis and plasmid transfer. traK genes carrying the traK4 mutation and a nonpolar mutation traK105 were cloned, sequenced, and identified as an amber nonsense and a frameshift mutation, respectively. The traK4 mutation occurred within one predicted rho-dependent transcription termination element (TTE) and immediately upstream of another, while the traK105 mutation occurred after the two potential TTEs within the traK gene. S1 nuclease protection analysis and Northern (RNA) blot analysis were used to confirm that the traK4 mutation, but not the traK105 mutation, caused premature termination of transcription. Computer analysis of the F transfer region suggested the presence of TTE motifs at regular intervals throughout the 33.4-kb sequence.

Promiscuous DNA transfer system of Agrobacterium tumefaciens: role of the virB operon in sex pilus assembly and synthesis

Conjugative transfer of DNA that occurs between bacteria also operates between bacteria and higher organisms. The transfer of DNA between Gram-negative bacteria requires initial contact by a sex pilus followed by DNA traversing four membranes (donor plus recipient) using a transmembrane pore. Accumulating evidence suggests that transfer of the T-DNA from Agrobacterium tumefaciens to plants may also occur via a conjugative mechanism. The virB operon of the Ti plasmid exhibits close homologies to genes that are known to encode the pilin subunits and pilin assembly proteins. The proteins encoded by the PilW operon of IncW plasmid R388 share strong similarities (average similarity = 50.8%) with VirB proteins. Similarly, the TraA, TraL and TraC proteins of IncF plasmid F have similarities to VirB2, VirB3 and VirB4 respectively (average similarity = 45.3%). VirB2 protein (12.3 kDa) contains a signal peptidase-I cleavage sequence that generates a polypeptide of 7.2 kDa. Likewise, the 12.8 kDa propilin protein TraA of plasmid F also possesses a peptidase-I cleavage site that generates the 7.2 kDa pilin structural protein. Similar amino acid sequences of the conjugative transfer genes of F, R388 as well as plasmid RP4 and the genes of the ptI operon of Bortedella pertussis suggest the existence of a superfamily of transmembrane proteins adapted to the promiscuous transfer of DNA-protein complexes.

Membrane location of the Ti plasmid VirB proteins involved in the biosynthesis of a pilin-like conjugative structure on Agrobacterium tumefaciens

The virB operon of the Agrobacterium tumefaciens Ti plasmid encodes 11 proteins. Specific antisera to VirB2, VirB3 and VirB9 were used to locate these virulence proteins in the A. tumefaciens cell. Immunoblot analysis located VirB2 protein to the inner and outer membranes; VirB3 and VirB9 were likewise associated with both membranes, but mainly in the outer membrane. VirB2 is processed from a 12.3-kDa protein into a 7.2-kDa polypeptide. Such sized protein results from cleavage at residue Ala47, upstream of which two additional alanine residues Ala45-Ala46 are contained and bearing resemblance to a signal peptide peptidase-I cleavage sequence. VirB2 and VirB3 sequences are strikingly similar to the pilin biosynthetic proteins TraA and TraL encoded by the tra operon of F and R1-19 plasmids. Since traA encodes a propilin that is cleaved into a 7.2-kDa conjugative pilin product and since this cleavage site is present in both TraA and VirB2, we propose that virB2 encodes a pilin-like protein which together with VirB3 and VirB9 as well as other VirB proteins may be used for interkingdom T-DNA transfer between bacteria and plants.

Unipolar localization and ATPase activity of IcsA, a Shigella flexneri protein involved in intracellular movement

Shigella flexneri uses elements of the host cell cytoskeleton to move within cells and from cell to cell. IcsA, an S. flexneri protein involved in this movement, was purified and studied in vitro. IcsA bound the radiolabelled ATP analog 3'(2')-O-(4-benzoyl)benzoyl-ATP and hydrolyzed ATP. In addition, the surface localization of IcsA on both extracellular and intracellular shigellae was unipolar. Further, in HeLa cells infected with shigellae, IcsA antiserum labelled the actin tail throughout its length, thereby suggesting that IcsA interacts with elements within the tail. Localization of IcsA within the tail at a distance from the bacterium would require its secretion; we demonstrate here that in vitro IcsA is secreted into the culture supernatant in a cleaved form.

The Escherichia coli K-12 F plasmid gene traX is required for acetylation of F pilin

The Escherichia coli F plasmid gene required for amino-terminal acetylation of F-pilin subunits was identified. Using Western blots (immunoblots), we assayed the reaction of monoclonal antibodies with F-pilin polypeptides in inner membrane preparations from various F mutant strains. It was known that JEL92 recognizes an internal pilin epitope and JEL93 recognizes the acetylated amino-terminal sequence (L.S. Frost, J.S. Lee, D.G. Scraba, and W. Paranchych, J. Bacteriol. 168:192-198, 1986). As expected, neither antibody reacted with inner membranes from F- cells or Flac derivatives that do not synthesize pilin. Mutations that affected the individual activities of F tra genes traA, -B, -C, -D, -E, -F, -G, -H, -I, -J, -K, -L, -M, -N, -P, -R, -U, -V and -W or trb genes trbA, -B, -C, -D, -E, -G, -H, and -I did not prevent JEL92 or JEL93 recognition of membrane pilin. However, Hfr deletion mutants that lacked the most-distal transfer region genes did not express pilin that reacted with JEL93. Nevertheless, all strains that retained traA and traQ did express JEL92-reactive pilin polypeptides. Analysis of strains expressing cloned tra segments showed that traA and traQ suffice for synthesis of JEL92-reactive pilin, but synthesis of JEL93-reactive pilin is additionally dependent on traX. We concluded that the traX product is required for acetylation of F pilin. Interestingly, our data also showed that TraA+ TraQ+ cells synthesize two forms of pilin which migrate at approximately 7 and 8 kDa. In TraX+ cells, both become acetylated and react with JEL93. Preparations of wild-type F-pilus filaments contain both types of subunits.

Synthesis of F pilin

Transfer of the Escherichia coli fertility plasmid, F, is dependent on expression of F pili. Synthesis of F-pilin subunits is known to involve three F plasmid transfer (tra) region products: traA encodes the 13-kDa precursor protein, TraQ permits this to be processed to the 7-kDa pilin polypeptide, and TraX catalyzes acetylation of the pilin amino terminus. Using cloned tra sequences, we performed a series of pulse-chase experiments to investigate the effect of TraQ and TraX on the fate of the traA product. In TraQ- cells, the traA gene product was found to be very unstable. While traA polypeptides of various sizes were detected early in the chase period, almost all were degraded within 5 min. Rapid traA product degradation was also observed in TraX+ cells, although an increased percentage of these products persisted during the chase. In TraQ+ cells, most of the traA product was processed to the 7-kDa pilin polypeptide within the 1-min pulse period; this product [7(Q)] was not degraded but was increasingly converted to an 8-kDa form [8(Q)] as the chase continued, suggesting that host enzymes can modify the pilin polypeptide. Similar results were observed in TraQ+ TraX+ cells, but the primary 7-kDa product appeared to be N-acetylated pilin (Ac-7). An 8-kDa product (Ac-8) was also detected, but this band did not increase in intensity during the chase. We suggest a pathway in which TraQ prevents the traA product from folding to a readily degradable conformation and assists its entry into the membrane, Leader peptidase I cleaves the traA product signal sequence, and a subset of the pilin polypeptides becomes modified by host enzymes; TraX then acetylates the N terminal of both the modified and unmodified pilin polypeptides.

Characterization of the F plasmid bifunctional conjugation gene, traG

The Escherichia coli F plasmid gene, traG, is required for two stages of the conjugation process: pilus biosynthesis and mating aggregate stabilization. The nucleotide sequence of traG has been determined and the topology of its product in the cytoplasmic membrane analysed using protease accessibility experiments. Complementation analysis employing plasmid deletions revealed a correlation between an N-terminal periplasmic segment of the protein product (TraGp) and its pilus assembly activity. Production of an anti-TraGp antiserum has facilitated the detection of TraGp*, a possible internal cleavage product of TraGp. Although its function is unknown. TraGp* is located in the periplasm and has been shown to possess sequences required for aggregate stabilization. The detection of TraGp* raises the possibility that the two functions of traG are carried out by separate products.

Conjugational junctions: morphology of specific contacts in conjugating Escherichia coli bacteria

F-plasmid-mediated bacterial conjugation was studied with hfr (traDts) and tra I mutant Escherichia coli donor strains. This allowed us to observe a statistically significant number of conjugation-specific contacts by video and electron microscopy. Single mating events between E. coli were observed in real time by video-enhanced light microscopy. Conjugation in vivo takes place by initial contact formation via pili, followed by direct and transient wall-to-wall contact, during which DNA is transferred and disaggregated. Electron microscopic observations of the contact zone between donor and recipient bacteria were made by thin sectioning of mating pairs that were arranged in monolayers. We defined the conjugation-specific contact found in stabilized mating pairs as the conjugational junction. Within this junction no specific substructure such as plasma bridges by fusion could be detected during transfer of DNA.

Construction and characterization of derivatives carrying insertion mutations in F plasmid transfer region genes, trbA, artA, traQ, and trbB

We devised a method for construction of insertion mutations in F plasmid tra region genes as a means of investigating the functions associated with previously uncharacterized loci. First, we constructed mutations in vitro, by insertion of a kanamycin resistance gene into a unique restriction site within a tra region fragment carried by a small, chimeric plasmid. Second, we crossed the insertion mutations, in vivo, onto a plasmid containing the complete F tra region sequence (either F lac, or pOX38, a Tra+ F plasmid derivative). Using this method, we obtained F lac mutant derivatives carrying KmR gene insertions in traQ, and a set of pOX38 mutant derivatives carrying a KmR gene insertion in trbA, artA, traQ, or trbB. Analysis of these derivatives showed that insertion of a kan gene at the NsiI site of traQ resulted in transfer deficiency, F-pilus-specific-phage resistance and an absence of detectable F-pilin subunit synthesis. Since the traQ mutants regained a wild-type phenotype when complemented with a traQ+ plasmid clone, we concluded that traQ expression is essential to transfer and F-pilus synthesis. However, pOX38 derivatives carrying kan gene inserts in genes trbA, artA, or trbB retained F-pilus-specific phage sensitivity and transferred at normal levels. Thus, these three gene products may not be essential for F-transfer from Escherichia coli K-12 under standard mating conditions.

The ferric-pseudobactin receptor PupA of Pseudomonas putida WCS358: homology to TonB-dependent Escherichia coli receptors and specificity of the protein

The initial step in the uptake of iron via ferric pseudobactin by the plant-growth-promoting Pseudomonas putida strain WCS358 is binding to a specific outer-membrane protein. The nucleotide sequence of the pupA structural gene, which codes for a ferric pseudobactin receptor, was determined. It contains a single open reading frame which potentially encodes a polypeptide of 819 amino acids, including a putative N-terminal signal sequence of 47 amino acids. Significant homology, concentrated in four boxes, was found with the TonB-dependent receptor proteins of Escherichia coli. The pupA mutant MH100 showed a residual efficiency of 30% in the uptake of 55Fe3+ complexed to pseudobactin 358, whereas the iron uptake of four other pseudobactins was not reduced at all. Cells of strain WCS374 supplemented with the pupA gene of strain WCS358 could transport ferric pseudobactin 358 but showed no affinity for three other pseudobactins. It is concluded that PupA is a specific receptor for ferric pseudobactin 358, and that strain WCS358 produces at least one other receptor for other pseudobactins.

traY and traI are required for oriT-dependent enhanced recombination between lac-containing plasmids and lambda plac5

Recombination between F42lac and lambda plac5 is typically 20- to 50-fold more efficient than recombination between chromosomal lac and lambda plac5. This enhancement of recombination requires trans-acting factors located in the promoter-distal and promoter-proximal regions of the main traY-to-traI (traZ) operon. By testing the ability of deletion mutants of tra to support enhanced recombination, we have identified traY as the only product has been ruled out. We also report that traI is the only gene from the promoter-distal end of the traY to traI operon that is required for recombination enhancement. Of the two proposed domains of traI, we conclude that the oriT-nicking activity is essential, whereas the helicase activity is largely dispensable. The possibility of a third traI activity is also discussed.

Characterization of the F-plasmid conjugative transfer gene traU

We characterized the traU gene of the Escherichia coli K-12 conjugative plasmid F. Plasmids carrying segments of the F transfer operon were tested for their capacity to complement F lac traU526. The protein products of TraU+ clones were identified, and the nucleotide sequence of traU was determined. traU mapped between traW and trbC. It encodes a 330-amino-acid, Mr36,786 polypeptide that is processed. Ethanol caused accumulation of a precursor polypeptide; removal of ethanol permitted processing of the protein to occur. Because F lac traU526 strains appear to be resistant to F-pilus-specific phages, traU has been considered an F-pilus assembly gene. However, electron microscopic analysis indicated that the traU526 amber mutation caused only a 50% reduction in F-piliation. Since F lac traU526 strains also retain considerable transfer proficiency, new traU mutations were constructed by replacing a segment of traU with a kanamycin resistance gene. Introduction of these mutations into a transfer-proficient plasmid caused a drastic reduction in transfer proficiency, but pilus filaments remained visible at approximately 20% of the wild-type frequency. Like traU526 strains, such mutants were unable to plaque F-pilus-specific phages but exhibited a slight sensitivity on spot tests. Complementation with a TraU+ plasmid restored the wild-type transfer and phage sensitivity phenotypes. Thus, an intact traU product appears to be more essential to conjugal DNA transfer than to assembly of pilus filaments.

Structure and function of conjugative pili: monoclonal antibodies as probes for structural variants of F pili

The lac-tra operon fusion plasmid pTG801 contains the known F plasmid DNA transfer (tra) genes required by Escherichia coli to elaborate functional F pili (T. Grossman and P. M. Silverman, J. Bacteriol. 171:650-656, 1989). Here, we show that these pili are actually structural variants of normal F pili and that the F plasmid must contain additional genes that affect pilus structure and function. We confirmed a previous report that two monoclonal antibodies that recognize epitopes at and near the amino terminus of F pilin do not decorate the sides of normal F pili, as determined by immunogold electron microscopy. However, both antibodies laterally decorated pTG801 pili. The epitope for one of the antibodies has been shown to include the amino-terminal acetyl group of F pilin, which must therefore also be present on pTG801 pilin. Normal antibody staining was restored to pTG801 pili when cells contained, in addition to pTG801, the compatible plasmid pRS31, which must therefore include at least one gene affecting F-pilus structure. One candidate, traD, was excluded as the sole such gene, since traD+ derivatives of a pTG801 strain still elaborated pili that could be laterally decorated with antibody. Moreover, although traD alone restored RNA bacteriophage R17 infectivity to pTG801 cells, as expected, it did not mimic pRS31 in restoring to pTG801 pili other characteristics of normal F pili. We conclude that pRS31 contains as yet uncharacterized genes required for elaboration of structurally normal F pili. Finally, we identified vesicular material, especially abundant in cultures of pTG801 transformants, that stained heavily with the anti-F-pilin monoclonal antibodies. This material may reflect the inner membrane pool of F pilin.

Sense and antisense transcripts of traM, a conjugal transfer gene of the antibiotic resistance plasmid R100

The region of the antibiotic resistance plasmid R100 that encodes the plasmid-specific transfer gene traM has two tandemly aligned promoters separated by 145 nucleotides. The principal transcripts are 705 and 562 nucleotides long. Minor transcripts are 1550 and 1700 nucleotides long. The 705-base transcript appears to encode an 11 kD traM protein. The 562-base transcript does not encode a detectable protein. When subcloned on short fragments, the promoter for the 562-base transcript initiates efficiently but that for the 705 site does not. The 3' ends of the 705 and 562 base transcripts end inside the traJ ORF. Thus they provide additional sense RNA to compete with traJ for finP, the antisense translational regulator of traJ. A model is proposed for the participation of these sense and antisense transcripts in the control of expression of the traJ gene.

A stable core region of the tra operon mRNA of plasmid R1-19

The degradation of the polycistronic tra-mRNA of the resistance plasmid R1-19 leads to the accumulation of a well defined series of stable mRNA species. The majority of the most stable mRNAs contains the message for the traA gene only. The differently sized stable mRNAs possess a common 3'terminus within the traL gene but vary at their 5' ends. The 3'terminus probably results from protection against exoribonucleases by a secondary structural feature. We propose that the 5' ends are generated by endoribonucleolytic cleavage. The stability of this part of the tra-mRNA exceeds 30 minutes and probably increases the rate of expression of the traA gene product propilin, the precursor of the sex pilus subunit. The expression of propilin and its processing into a protein of the molecular weight of mature pilin is demonstrated with the isolated gene. The sequence of the so far unknown genes traL and traE of R1-19 is presented.

Nucleotide sequence of the F plasmid transfer gene, traH: identification of a new gene and a promoter within the transfer operon

The nucleotide sequence of the F plasmid transfer gene traH, which is involved in F-pilus assembly in Escherichia coli K-12, has been determined. From the sequence data, it would appear that traH encodes a 38,897-dalton precursor polypeptide which is processed to give a periplasmic protein. Furthermore, a new gene, trbF, has been located immediately upstream of traH and shown to be expressed by means of a translational fusion to lacZ. Using galK fusion and S1 nuclease protection studies, a weak traJ-dependent promoter, P trbF, has been mapped upstream and adjacent to trbF. Transcription of trbF and traH from P trbF may well serve to complement transcription from the major tra operon promoter PY located some 16 kb upstream of these genes.

Nucleotide sequence of traQ and adjacent loci in the Escherichia coli K-12 F-plasmid transfer operon

The F tra operon region that includes genes trbA, traQ, and trbB was analyzed. Determination of the DNA sequence showed that on the tra operon strand, the trbA gene begins 19 nucleotides (nt) distal to traF and encodes a 115-amino-acid, Mr-12,946 protein. The traQ gene begins 399 nt distal to trbA and encodes a 94-amino-acid, Mr-10,867 protein. The trbB gene, which encodes a 179-amino-acid, Mr-19,507 protein, was found to overlap slightly with traQ; its start codon begins 11 nt before the traQ stop codon. Protein analysis and subcellular fractionation of the products expressed by these genes indicated that the trbB product was processed and that the mature form of this protein accumulated in the periplasm. In contrast, the protein products of trbA and traQ appeared to be unprocessed, membrane-associated proteins. The DNA sequence also revealed the presence of a previously unsuspected locus, artA, in the region between trbA and traQ. The artA open reading frame was found to lie on the DNA strand complementary to that of the F tra operon and could encode a 104-amino-acid, 12,132-dalton polypeptide. Since this sequence would not be expressed as part of the tra operon, the activity of a potential artA promoter region was assessed in a galK fusion vector system. In vivo utilization of the artA promoter and translational start sites was also examined by testing expression of an artA-beta-galactosidase fusion protein. These results indicated that the artA gene is expressed from its own promoter.

Low-frequency infection of F- bacteria by transducing particles of filamentous bacteriophages

Filamentous particles containing single-stranded plasmid and bacteriophage DNA are able to infect F- Escherichia coli at frequencies of approximately 10(-6). This infection is dependent on an intact particle and requires the products of the tolQ, tolR, and tolA genes of the bacteria. The addition of CaCl2 can increase the frequency about 100-fold, presumably by increasing the concentration of particles at the bacterial surface.

The product of the F plasmid transfer operon gene, traF, is a periplasmic protein

The products of clones carrying the F plasmid transfer operon gene, traF, were analyzed. Proteins expressed in maxicells were labeled with [35S]methionine and examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Clones carrying the wild-type traF gene expressed two polypeptide products that were not products of clones containing the traF13 amber mutation. These migrated with apparent molecular weights (Ma) of 27,000 and 25,000. A pulse-chase experiment suggested that the larger product was a precursor of the smaller one. In the presence of ethanol, the Ma-27,000 polypeptide accumulated and the Ma-25,000 product was not expressed. These results indicated that the traF protein undergoes proteolytic processing associated with export. Cell fractionation experiments further indicated that the greatest concentration of the mature (Ma 25,000) TraF protein was located in the periplasm. The DNA sequence of traF and the position of the transition mutation in traF13 DNA were also determined. Sequence analysis suggested that traF would be expressed as a 247-amino-acid, Mr-28,006 polypeptide. The 19 amino acids at the amino terminus of this polypeptide appear to constitute a typical membrane leader peptide, while the remainder of the molecule (Mr 25,942) is predicted to be primarily hydrophilic in character.

DNA sequence analysis of point mutations in traA, the F pilin gene, reveal two domains involved in F-specific bacteriophage attachment

Six missense point mutations in traA (WPFL43,44,45,46,47 and 51), the gene encoding F pilin in the transfer region of the F plasmid, have been characterized for their effect on the transfer ability, bacteriophage (R17, QB and fl) sensitivity and levels of piliation expressed by the plasmid. The sequence analysis of the first five of these mutations revealed two domains in the F pilin subunit exposed on the surface of the F pilus which mediate phage attachment. These two domains include residues 14-17 (approximately) and the last few residues at the carboxy-terminus of the pilin protein. One of these mutants had a pleiotropic affect on pilus function and was thought to have affected pilus assembly. The sixth point mutant (WPFL51), previously thought to be in traA, was complemented by chimeric plasmids carrying the traG gene of the F transfer region, which may be involved in the acetylation of the pilin subunit. A traA nonsense mutant (JCFL1) carried an amber mutation near the amino-terminus which is well suppressed in SuI+ (supD) and SuIII+ (supF) strains. Neither the antigenicity of the pilin nor the efficiency of plating of F-specific bacteriophages were affected when this plasmid was harbored by either suppressor strain. A second amber mutant (JCFL25) which is not suppressible, carried its mutation in the codon for the single tryptophan in F pilin, suggesting that this residue is important in subunit interactions during pilus assembly. Two other point mutants (JCFL32 and 44) carried missense mutations in the leader sequence (positions 9 and 13) which affected the number of pili per cell presumably by altering the processing of propilin to pilin.

Identification and characterization of the products from the traJ and traY genes of plasmid R100

The nucleotide sequence of part of the tra region of R100 including traJ and traY was determined, and the products of several tra genes were identified. The nucleotide sequence of traJ, encoding a protein of 223 amino acids, showed poor homology with the corresponding segments of other plasmids related to R100, but the deduced amino acid sequences showed low but significant homology. The first four amino acids at the N-terminal region of the TraJ protein were not essential for positive regulation of expression of traY, the first gene of the traYZ operon. The nucleotide sequence of traY shows that this gene may use TTG as the initiation codon and that it encodes a protein of 75 amino acids. Analysis of the traY gene product, which was obtained as the fusion protein with beta-galactosidase, showed that the N-terminal region of the product has an amino acid sequence identical to that deduced from the assigned frame but lacks formylmethionine. traY of plasmid F, which encodes a larger protein than the TraY protein of R100, is thought to use ATG as an initiation codon. However, a TTG initiation codon was found in the preceding region of the previously assigned traY coding frame of F. Interestingly, when translation of traY of F was initiated from TTG, the amino acid sequence homologous to the TraY protein of R100 appeared in tandem in the TraY protein of F. This may suggest that traY of F has undergone duplication of a gene like the traY gene of R100.

The expression of Pseudomonas aeruginosa PAK pilin gene mutants in Escherichia coli

Previous work has demonstrated the expression of the cloned pilin gene of Pseudomonas aeruginosa PAK within Escherichia coli and has pinpointed this protein's localization exclusively to the cytoplasmic membrane (Finlay et al., 1986). To define regions of the pilin subunit necessary for its stability and transport within E. coli, we constructed six mutants of the pilin gene and studied their expression and localization using a T7 promoter system. Two of the mutants have either a 4- or 8-amino-acid deletion at the N-terminus and both were stably expressed and transported primarily to the cytoplasmic membrane of E. coli. The other four mutants are C-terminal truncations having between 36 and 56 amino acids of the N-terminal region of the unprocessed pilin. Studies with these truncated mutants revealed that only the first 36 residues of the unprocessed pilin subunit were required for insertion into the E. coli membrane.