Type IV prepilin peptidase gene of Neisseria gonorrhoeae MS11: presence of a related gene in other piliated and nonpiliated Neisseria strains

The structure and mechanism of the bacterial type II secretion system

The type II secretion system (T2SS) is a multi-protein complex used by many bacteria to move substrates across their cell membrane. Substrates released into the environment serve as local and long-range effectors that promote nutrient acquisition, biofilm formation, and pathogenicity. In both animals and plants, the T2SS is increasingly recognized as a key driver of virulence. The T2SS spans the bacterial cell envelope and extrudes substrates through an outer membrane secretin channel using a pseudopilus. An inner membrane assembly platform and a cytoplasmic motor controls pseudopilus assembly. This microreview focuses on the structure and mechanism of the T2SS. Advances in cryo-electron microscopy are enabling increasingly elaborate sub-complexes to be resolved. However, key questions remain regarding the mechanism of pseudopilus extension and retraction, and how this is coupled with the choreography of the substrate moving through the secretion system. The T2SS is part of an ancient type IV filament superfamily that may have been present within the last universal common ancestor (LUCA). Overall, mechanistic principles that underlie T2SS function have implication for other closely related systems such as the type IV and tight adherence pilus systems.

Organization of the electron transfer chain to oxygen in the obligate human pathogen Neisseria gonorrhoeae: roles for cytochromes c4 and c5, but not cytochrome c2, in oxygen reduction

Although Neisseria gonorrhoeae is a prolific source of eight c-type cytochromes, little is known about how its electron transfer pathways to oxygen are organized. In this study, the roles in the respiratory chain to oxygen of cytochromes c(2), c(4), and c(5), encoded by the genes cccA, cycA, and cycB, respectively, have been investigated. Single mutations in genes for either cytochrome c(4) or c(5) resulted in an increased sensitivity to growth inhibition by excess oxygen and small decreases in the respiratory capacity of the parent, which were complemented by the chromosomal integration of an ectopic, isopropyl-beta-d-thiogalactopyranoside (IPTG)-inducible copy of the cycA or cycB gene. In contrast, a cccA mutant reduced oxygen slightly more rapidly than the parent, suggesting that cccA is expressed but cytochrome c(2) is not involved in electron transfer to cytochrome oxidase. The deletion of cccA increased the sensitivity of the cycB mutant to excess oxygen but decreased the sensitivity of the cycA mutant. Despite many attempts, a double mutant defective in both cytochromes c(4) and c(5) could not be isolated. However, a strain with the ectopically encoded, IPTG-inducible cycB gene with deletions in both cycA and cycB was constructed: the growth and survival of this strain were dependent upon the addition of IPTG, so gonococcal survival is dependent upon the synthesis of either cytochrome c(4) or c(5). These results define the gonococcal electron transfer chain to oxygen in which cytochromes c(4) and c(5), but not cytochrome c(2), provide alternative pathways for electron transfer from the cytochrome bc(1) complex to the terminal oxidase cytochrome cbb(3).

The tad locus: postcards from the widespread colonization island

The Tad (tight adherence) macromolecular transport system, which is present in many bacterial and archaeal species, represents an ancient and major new subtype of type II secretion. The tad genes are present on a genomic island named the widespread colonization island (WCI), and encode the machinery that is required for the assembly of adhesive Flp (fimbrial low-molecular-weight protein) pili. The tad genes are essential for biofilm formation, colonization and pathogenesis in the genera Aggregatibacter (Actinobacillus), Haemophilus, Pasteurella, Pseudomonas, Yersinia, Caulobacter and perhaps others. Here we review the structure, function and evolution of the Tad secretion system.

The TadV protein of Actinobacillus actinomycetemcomitans is a novel aspartic acid prepilin peptidase required for maturation of the Flp1 pilin and TadE and TadF pseudopilins

The tad locus of Actinobacillus actinomycetemcomitans encodes genes for the biogenesis of Flp pili, which allow the bacterium to adhere tenaciously to surfaces and form strong biofilms. Although tad (tight adherence) loci are widespread among bacterial and archaeal species, very little is known about the functions of the individual components of the Tad secretion apparatus. Here we characterize the mechanism by which the pre-Flp1 prepilin is processed to the mature pilus subunit. We demonstrate that the tadV gene encodes a prepilin peptidase that is both necessary and sufficient for proteolytic maturation of Flp1. TadV was also found to be required for maturation of the TadE and TadF pilin-like proteins, which we term pseudopilins. Using site-directed mutagenesis, we show that processing of pre-Flp1, pre-TadE, and pre-TadF is required for biofilm formation. Mutation of a highly conserved glutamic acid residue at position +5 of Flp1, relative to the cleavage site, resulted in a processed pilin that was blocked in assembly. In contrast, identical mutations in TadE or TadF had no effect on biofilm formation, indicating that the mechanisms by which Flp1 pilin and the pseudopilins function are distinct. We also determined that two conserved aspartic acid residues in TadV are critical for function of the prepilin peptidase. Together, our results indicate that the A. actinomycetemcomitans TadV protein is a member of a novel subclass of nonmethylating aspartic acid prepilin peptidases.

Neisserial pilin genes display extensive interspecies diversity

All Neisseria live in association with host cells, however, little is known about the genetic potential of nonpathogenic Neisseria species to express attachment factors such as pili. In this study, we demonstrate that type IV pilin-encoding genes are present in a wide range of Neisseria species. N. sicca, N. subflava, and N. elongata each contain two putative pilE genes arranged in tandem, while single genes were identified in N. polysaccharea, N. mucosa, and N. denitrificans. Neisserial pilE genes are highly diverse and display features consistent with a history of horizontal gene transfer.

Low-level pilin expression allows for substantial DNA transformation competence in Neisseria gonorrhoeae

The gonococcal pilus is a major virulence factor that has well-established roles in mediating epithelial cell adherence and DNA transformation. Gonococci expressing four gonococcal pilin variants with distinct piliation properties under control of the lac regulatory system were grown in different levels of the inducer isopropyl-beta-D-thiogalactopyranoside (IPTG). These pilin variants expressed various levels of pilin message and pilin protein in response to the level of IPTG in the growth medium. Moreover, posttranslational modifications of the variant pilin proteins were detected, including S-pilin production and glycosylation. The ratio of the modified and unmodified pilin forms did not substantially change with different levels of pilin expression, showing that these modifications are not linked to pilin expression levels. DNA transformation competence was also influenced by IPTG levels in the growth medium. Substantial increases in transformation competence over an isogenic, nonpiliated mutant were observed when limited amounts of three of the pilin variants were expressed. Immunoelectron microscopy showed that when limited amounts of pilin are expressed, pili are rare and do not explain the pilin-dependent transformation competence. This pilin-dependent transformation competence required prepilin processing, the outer membrane secretin PilQ, and the twitching-motility-regulating protein PilT. These requirements show that a fully functional pilus assembly apparatus is required for DNA uptake when limited pilin is produced. We conclude that the pilus assembly apparatus functions to import DNA into the bacterial cell in a pilin-dependent manner but that extended pili are not required for transformation competence.

Purification of post-translationally modified proteins from bacteria: homologous expression and purification of histidine-tagged pilin from Neisseria meningitidis

Until recently, glycosylation of proteins in prokaryotes was regarded as uncommon and thought to be limited to special cases such as S-layer proteins and some archeal outer membrane proteins. Now, there are an increasing number of reports of bacterial proteins that are glycosylated. Pilin of pathogenic Neisseria is one of the best characterised post-translationally modified bacterial proteins, with four different types of modifications reported, including a novel glycosylation. Pilin monomers assemble to form pilus fibres, which are long protein filaments that protrude from the surface of bacterial cells and are key virulence factors. To aid in the investigation of these modifications, pure pilin is required. A number of pilin purification methods have been published, but none are appropriate for the routine purification of pilin from many different isolates. This study describes a novel, rapid, and simple method of pilin purification from Neisseria meningitidis C311#3, which facilitates the production of consistent quantities of pure, native pilin. A 6x histidine tag was fused to the C-terminus of the pilin subunit structural gene, pilE, via homologous recombination placing the 6x histidine-tagged allele in the chromosome of N. meningitidis C311#3. Pilin was purified under non-denaturing conditions via a two-step process using immobilised metal affinity chromatography (IMAC), followed by dye affinity chromatography. Analysis of the purified pilin confirmed that it retained both of the post-translational modifications examined. This novel approach may prove to be a generally applicable method for purification and analysis of post-translationally modified proteins in bacteria.

The archaeal flagellum: a different kind of prokaryotic motility structure

The archaeal flagellum is a unique motility apparatus distinct in composition and likely in assembly from the bacterial flagellum. Gene families comprised of multiple flagellin genes co-transcribed with a number of conserved, archaeal-specific accessory genes have been identified in several archaea. However, no homologues of any bacterial genes involved in flagella structure have yet been identified in any archaeon, including those archaea in which the complete genome sequence has been published. Archaeal flagellins possess a highly conserved hydrophobic N-terminal sequence that is similar to that of type IV pilins and clearly unlike that of bacterial flagellins. Also unlike bacterial flagellins but similar to type IV pilins, archaeal flagellins are initially synthesized with a short leader peptide that is cleaved by a membrane-located peptidase. With recent advances in genetic transfer systems in archaea, knockouts have been reported in several genes involved in flagellation in different archaea. In addition, techniques to isolate flagella with attached hook and anchoring structures have been developed. Analysis of these preparations is under way to identify minor structural components of archaeal flagella. This and the continued isolation and characterization of flagella mutants should lead to significant advances in our knowledge of the composition and assembly of archaeal flagella.

PpdD type IV pilin of Escherichia coli K-12 can Be assembled into pili in Pseudomonas aeruginosa

Escherichia coli K-12 possesses at least 16 chromosomal genes related to genes involved in the formation of type IV pili in other gram-negative bacteria. However, E. coli K-12 does not produce type IV pili when grown under standard laboratory conditions. The results of reverse transcription-PCR, operon fusion analysis, and immunoblotting demonstrated that several of the putative E. coli piliation genes are expressed at very low levels. Increasing the level of expression of the major pilin gene (ppdD) and the linked assembly genes hofB and hofC (homologues of the Pseudomonas aeruginosa type IV pilus assembly genes pilB and pilC) did not lead to pilus production. However, expression of the ppdD gene in P. aeruginosa led to assembly of PpdD into pili that were recognized by antibodies directed against the PpdD protein. Assembly of PpdD into pili in P. aeruginosa was dependent on the expression of the pilB and pilC genes and independent of expression of the P. aeruginosa pilin structural gene pilA.

The gene encoding the prepilin peptidase involved in biosynthesis of pilus colonization factor antigen III (CFA/III) of human enterotoxigenic Escherichia coli

The assembly of pilus colonization factor antigen III (CFA/III) of human enterotoxigenic Escherichia coli requires the processing of CFA/III major pilin (CofA) by a peptidase, likely another type IV pilus formation system. Western blot analysis of CofA reveals that CofA is produced initially as a 26.5-kDa preform pilin (prepilin) and then processed to 20.5-kDa mature pilin by a prepilin peptidase. This processing is essential for exportation of the CofA from the cytoplasm to the periplasm. In this experiment, the structural gene, cofP, encoding CFA/III prepilin peptidase which cleavages at the Gly-30-Met-31 junction of CofA was identified, and the nucleotide sequence of the gene was determined. CofP consists of 819 bp encoding a 273-amino acid protein with a relative molecular mass of 30,533 Da. CofP is predicted to be localized in the inner membrane based on its hydropathy index. The amino acid sequence of CofP shows a high degree of homology with other prepilin peptidases which play a role in the assembly of type IV pili in several gram-negative bacteria.

A second prepilin peptidase gene in Escherichia coli K-12

Escherichia coli K-12 strains grown at 37 degrees C or 42 degrees C, but not at 30 degrees C, process the precursors of the Neisseria gonorrhoeae type IV pilin PilE and the Klebsiella oxytoca type IV pseudopilin PulG in a manner reminiscent of the prepilin peptidase-dependent processing of these proteins that occurs in these bacteria. Processing of prePulG in Escherichia coli requires a glycine at position -1, as does processing by the cognate prepilin peptidase (PulO), and is unaffected by mutations that inactivate several non-specific proteases. These data suggested that E. coli K-12 has a functional prepilin peptidase, despite the fact that it does not itself appear to express either type IV pilin or pseudopilin genes under the conditions that allow prePilE and prePulG processing. The E. coli K-12 genome contains two genes encoding proteins with significant sequence similarity to prepilin peptidases: gspO at minute 74.5 and pppA (f310c) at minute 67 on the genetic map. We have previously obtained evidence that gspO encodes an active enzyme but is not transcribed. pppA was cloned and shown to code for a functional prepilin peptidase capable of processing typical prepilin peptidase substrates. Inactivation of pppA eliminated the endogenous, thermoinducible prepilin peptidase activity. PppA was able to replace PulO prepilin peptidase in a pullulanase secretion system reconstituted in E. coli when expressed from high-copy-number plasmids but not when present in a single chromosomal copy. The analysis of pppA-lacZ fusions indicated that pppA expression was very low and regulated by the growth temperature at the level of translation, in agreement with the observed temperature dependence of PppA activity. Polymerase chain reaction and Southern hybridization analyses revealed the presence of the pppA gene in 12 out of 15 E. coli isolates.

Complementation analysis of the Dichelobacter nodosus fimN, fimO, and fimP genes in Pseudomonas aeruginosa and transcriptional analysis of the fimNOP gene region

The causative agent of ovine footrot, the gram-negative anaerobe Dichelobacter nodosus, produces polar type IV fimbriae, which are the major protective antigens. The D. nodosus genes fimN, fimO, and fimP are homologs of the Pseudomonas aeruginosa fimbrial assembly genes, pilB, pilC, and pilD, respectively. Both the pilD and fimP genes encode prepilin peptidases that are responsible for cleavage of the leader sequence from the immature fimbrial subunit. To investigate the functional similarity of the fimbrial biogenesis systems from these organisms, the D. nodosus genes were introduced into P. aeruginosa strains carrying mutations in the homologous genes. Analysis of the resultant derivatives showed that the fimP gene complemented a pilD mutant of P. aeruginosa for both fimbrial assembly and protein secretion. However, the fimN and fimO genes did not complement pilB or pilC mutants, respectively. These results suggest that although the PilD prepilin peptidase can be functionally replaced by the heterologous FimP protein, the function of the PilB and PilC proteins may require binding or catalytic domains specific for the P. aeruginosa fimbrial assembly system. The transcriptional organization and regulation of the fimNOP gene region were also examined. The results of reverse transcriptase PCR and primer extension analysis suggested that these genes form an operon transcribed from two sigma70-type promoters located upstream of ORFM, an open reading frame proximal to fimN. Transcription of the D. nodosus fimbrial subunit was found to increase in cells grown on solid media, and it was postulated that this regulatory effect may be of significance in the infected footrot lesion.

General secretion pathway (eps) genes required for toxin secretion and outer membrane biogenesis in Vibrio cholerae

The general secretion pathway (GSP) of Vibrio cholerae is required for secretion of proteins including chitinase, enterotoxin, and protease through the outer membrane. In this study, we report the cloning and sequencing of a DNA fragment from V. cholerae, containing 12 open reading frames, epsC to -N, which are similar to GSP genes of Aeromonas, Erwinia, Klebsiella, Pseudomonas, and Xanthomonas spp. In addition to the two previously described genes, epsE and epsM (M. Sandkvist, V. Morales, and M. Bagdasarian, Gene 123: 81-86, 1993; L. J. Overbye, M. Sandkvist, and M. Bagdasarian, Gene 132:101-106, 1993), it is shown here that epsC, epsF, epsG, and epsL also encode proteins essential for GSP function. Mutations in the eps genes result in aberrant outer membrane protein profiles, which indicates that the GSP, or at least some of its components, is required not only for secretion of soluble proteins but also for proper outer membrane assembly. Several of the Eps proteins have been identified by use of the T7 polymerase-promoter system in Escherichia coli. One of them, a pilin-like protein, EpsG, was analyzed also in V. cholerae and found to migrate as two bands on polyacrylamide gels, suggesting that in this organism it might be processed or otherwise modified by a prepilin peptidase. We believe that TcpJ prepilin peptidase, which processes the subunit of the toxin-coregulated pilus, TcpA, is not involved in this event. This is supported by the observations that apparent processing of EpsG occurs in a tcpJ mutant of V. cholerae and that, when coexpressed in E. coli, TcpJ cannot process EpsG although the PilD peptidase from Neisseria gonorrhoeae can.

The conserved tetracysteine motif in the general secretory pathway component PulE is required for efficient pullulanase secretion

The PulE component of the pullulanase secretion pathway, a typical main terminal branch of the general secretory pathway, has a tetracysteine motif (4Cys) that is also present in almost all of the many PulE homologues, including those involved in type-IV piliation and conjugal DNA transfer. The 4Cys resembles a zinc-binding motif found in other proteins such as adenylate kinases, which may be pertinent in view of the fact that PulE has a consensus ATP-binding motif and since at least one PulE homologue has been reported to have kinase activity. In PulE, the Cys residues of this motif form scrambled intra- and intermolecular disulfide bonds when cells are disrupted. Replacement of one or more Cys of this motif by Ser reduces PulE function, but at least two adjacent Cys must be replaced to prevent intramolecular disulfide bond formation.

Structure-function relationship of type-IV prepilin peptidase of Pseudomonas aeruginosa--a review

The bifunctional enzyme prepilin peptidase (PilD) from Pseudomonas aeruginosa is a key determinant in both type-IV pilus biogenesis and extracellular protein secretion, in its roles as a leader peptidase and MTase. It is responsible for endopeptidic cleavage of the unique leader peptides that characterize type-IV pilin precursors, as well as proteins with homologous leader sequences that are essential components of the general secretion pathway found in a variety of Gram-negative pathogens. Following removal of the leader peptides, the same enzyme is responsible for the second posttranslational modification that characterizes the type-IV pilins and their homologues, namely N-methylation of the newly exposed N-terminal amino acid residue. This review discusses some of the work begun in order to answer questions regarding the structure-function relationships of the active sites of this unique enzyme.

Multimers of the precursor of a type IV pilin-like component of the general secretory pathway are unrelated to pili

Both the mature and precursor forms of PulG, a type IV pilin-like component of the general secretory pathway of Klebsiella oxytoca, can be chemically cross-linked into multimers similar to those obtained by cross-linking the components of type IV pili. To explore the possibility that the PulG precursor could form a pilus-like structure, the PulG sequence was altered in a variety of ways, including (i) replacement of the characteristic hydrophobic region, which is required for the assembly of type IV pilins by the MalE signal peptide, or (ii) fusion of beta-lactamase (beta laM) to the C-terminus. Neither of these changes affected multimerization. PulG precursor could be post-translationally processed by prepilin peptidase (PulO), indicating that the N-terminus of prePulG remains on the cytoplasmic side of the cytoplasmic membrane where it is accessible to the catalytic site of this enzyme. Finally, precursor and mature forms of PulG could be efficiently cross-linked in a mixed dimer, indicating that at least a subpopulation of the two forms of the protein are probably located in clusters in the cytoplasmic membrane. These results provide further evidence that the cross-linked multimers of the precursor form of PulG are unrelated to type IV pilus-like structures. It is still unclear whether a subpopulation of processed PulG can be assembled into a rudimentary pilus-like structure.

The cryptic general secretory pathway (gsp) operon of Escherichia coli K-12 encodes functional proteins

Systematic sequencing of the Escherichia coli K-12 chromosome (GenBank entry U18997) has revealed the presence of an apparently complete operon of genes (the gspC-0 operon) similar to genes coding for components of the main terminal branch of the general secretory pathway (e.g., the Klebsiella oxytoca pulC-0 pullulanase secretion operon) and to related genes required for type IV pilus biogenesis. For example, the last gene in the gsp operon, gspO (formerly hopD), encodes a protein which is similar to several type IV prepilin peptidases. Expression of gspO from lacZp promotes cleavage of two known prepilin peptidase substrates in E. coli K-12: Neisseria gonorrhoeae type IV prepilin and K. oxytoca prePulG protein. gspO also complements a mutation in the corresponding gene (pulO) of the pullulanase secretion operon when it is expressed from lacZp. Another gene in the gsp operon, gspG (formerly hopG), encodes a protein similar to prePulG, a component of the pullulanase secretion pathway. Expression of gspG from lacZp leads to production of a protein which (i) is recognized by PulG-specific antiserum (and by antiserum against the Pseudomonas aeruginosa PulG homolog XcpG [formerly XcpT]), (ii) is processed in cells expressing gspO, and (iii) restores secretion in cells carrying a pulG mutation. The chromosomal copies of gspG and gspO are apparently not expressed, probably because of very weak transcription from the upstream region, as measured by using a chromosomal gspC-lacZ operon fusion. Thus, the gsp operon of E. coli K-12 includes at least two functional genes which, together with the rest of the operon, are probably not expressed under laboratory conditions.

Structure of the fibre-forming protein pilin at 2.6 A resolution

The crystallographic structure of Neisseria gonorrhoeae pilin, which assembles into the multifunctional pilus adhesion and virulence factor, reveals an alpha-beta roll fold with a striking 85 A alpha-helical spine and an O-linked disaccharide. Key residues stabilize interactions that allow sequence hypervariability, responsible for pilin's celebrated antigenic variation, within disulphide region beta-strands and connections. Pilin surface shape, hydrophobicity and sequence variation constrain pilus assembly to the packing of flat subunit faces against alpha 1 helices. Helical fibre assembly is postulated to form a core of coiled alpha 1 helices banded by beta-sheet, leaving carbohydrate and hypervariable sequence regions exposed to solvent.

Characterization of the pilF-pilD pilus-assembly locus of Neisseria gonorrhoeae

Expression of Type IV pili by the bacterial pathogen Neisseria gonorrhoeae appears to be essential for colonization of the human host. Several N. gonorrhoeae gene products have been recently identified which bear homology to proteins involved in pilus assembly and protein export in other bacterial systems. We report here the isolation and characterization of transposon insertion mutants in N. gonorrhoeae whose phenotypes indicate that the N. gonorrhoeae pilF and pilD gene products are required for gonoccocal pilus biogenesis. Mutants lacking the pilD gene product, a pre-pilin peptidase, were unable to process the pre-pilin subunit into pilin and thus were non-piliated. pilF mutants processed pilin but did not assemble the mature subunit. Both classes of mutants released S-pilin, a soluble, truncated form of the pilin subunit previously correlated with defects in pilus assembly. In addition, mutants containing transposon insertions in pilD or in a downstream gene, orfX, exhibited a severely restricted growth phenotype. Deletion analysis of pilD indicated that the poor growth phenotype observed for the pilD transposon mutants was a result of polar effects of the insertions on orfX expression. orfX encodes a predicted polypeptide of 23 kDa which contains a consensus nucleotide-binding domain and has apparent homologues in Pseudomonas aeruginosa, Pseudomonas putida, Thermus thermophilus, and the eukaryote Caenorhabditis elegans. Although expression of orfX and pilD appears to be transcriptionally coupled, mutants containing transposon insertions in orfX expressed pili. Unlike either pilF or pilD mutants, orfX mutants were also competent for DNA transformation.

Identification and characterization of pilG, a highly conserved pilus-assembly gene in pathogenic Neisseria

Expression of type IV pili appears to be a requisite determinant of infectivity for the strict human pathogens Neisseria gonorrhoeae and Neisseria meningitidis. The assembly of these colonization factors is a complex process. This report describes a new pilus-assembly gene, pilG, that immediately precedes the gonococcal (Gc) pilD gene encoding the pre-pilin leader peptidase. The nucleotide sequence of this region revealed a single complete open reading frame whose derived polypeptide displayed significant identities to the pilus-assembly protein PilC of Pseudomonas aeruginosa and other polytopic integral cytoplasmic membrane constituents involved in protein export and competence. A unique polypeptide of M(r) 38 kDa corresponding to the gene product was identified. A highly related gene and flanking sequences were cloned from a group B polysaccharide-producing strain of N. meningitidis (Mc). The results indicate that the pilG genes and genetic organization at these loci in Gc and Mc are extremely conserved. Hybridization studies strongly suggest that pilG-related genes exist in commensal Neisseria species and other species known to express type IV pili. Defined genetic lesions were created by using insertional and transposon mutagenesis and moved into the Gc and Mc chromosomes by allelic replacement. Chromosomal pilG insertion mutants were devoid of pili and displayed dramatically reduced competence for transformation. These findings could not be ascribed to pilin-gene alterations or to polarity exerted on pilD expression. The results indicated that PilG exerts its own independent role in neisserial pilus biogenesis.

ComC is required for the processing and translocation of comGC, a pilin-like competence protein of Bacillus subtilis

ComGC is a cell surface-localized protein required for DNA binding during transformation in Bacillus subtilis. It resembles type IV prepilins in its N-terminal domain, particularly in the amino acid sequence surrounding the processing cleavage sites of these proteins. ComC is another protein required for DNA binding, which resembles the processing proteases that cleave type IV prepilins. We show here that ComGC is processed in competent cells and that this processing requires ComC. We also demonstrate that the PilD protein of Neisseria gonorrhoeae, a ComC homologue, can process ComGC in Escherichia coli, and that the ComC protein itself is the only B. subtilis protein needed to accomplish cleavage of ComGC in the latter organism. Based on NaOH-solubility studies, we have shown that in the absence of ComC, but in the presence of all other competence proteins, B. subtilis is incapable of correctly translocating ComGC to the outer face of the cell membrane. Finally, we show that ComGC can be cross-linked to yield a form with higher molecular mass, possibly a dimer, and present evidence suggesting that formation of the higher mass complex takes place in the membrane, prior to translocation. Formation of this complex does not require ComC or any of the comG products, other than ComGC itself.

Escherichia coli contains a set of genes homologous to those involved in protein secretion, DNA uptake and the assembly of type-4 fimbriae in other bacteria

A specialised system involved in a diverse array of functions, including the biogenesis of fimbriae, protein secretion and DNA uptake, has recently been found to be widespread in the eubacteria. These systems have in common several sets of related genes, including those encoding proteins containing leader sequences homologous to that of the type-4 fimbrial subunit (prepilin), a prepilin-type leader peptidase, a cytoplasmic nucleotide-binding protein, and other proteins located in the inner and outer membranes [Hobbs, M. and Mattick, J.S., Mol Microbiol. 10 (1993) 233-243]. Here, we show that Escherichia coli contains at least nine homologs of this system, and present complete sequence data for five of the genes involved (ppdD. hopB, hopC, hopD and pshM), as well as for an adjacent gene (nadC), which encodes quinolic acid phosphoribosyltransferase. Insertional mutagenesis of hopB and hopD failed to reveal any obvious effects on cell viability, morphogenesis of M13 phage, conjugative transfer of the F plasmid, or protein secretion.

A plasmid-encoded prepilin peptidase gene from enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli, a leading agent of infantile diarrhea worldwide, adheres to tissue culture cells in a pattern called "localized adherence." Localized adherence is associated with bundle-forming pili encoded by the plasmid bfpA gene, the product of which is homologous with the major structural subunit proteins of type IV fimbriae in other bacteria. Several of these proteins have been shown to be processed from a precursor by a specific prepilin peptidase. We cloned restriction fragments downstream of the bfpA gene into an E. coli-Pseudomonas aeruginosa shuttle vector and mobilized them into a P. aeruginosa prepilin peptidase (pilD) mutant. A plasmid containing a 1.3-kb PstI-BamHI fragment was able to complement the pilD mutation, as demonstrated by restoration of sensitivity to the pilus-specific bacteriophage PO4. The DNA sequence of this fragment revealed an open reading frame, designated bfpP, the predicted product of which is homologous to other prepilin peptidases, including TcpJ of Vibrio cholerae (30% identical amino acids), PulO of Klebsiella oxytoca (29%), and PilD of P. aeruginosa (28%). A bfpA::TnphoA mutant complemented with a bfpA-containing DNA fragment only partially processes the BfpA protein. When complemented with a larger fragment containing bfpP as well as bfpA, the mutant expresses the fully processed BfpA protein. P. aeruginosa PAK, but not a pilD mutant of PAK, expresses mature BfpA protein when the bfpA gene is mobilized into this strain. Thus, as in other type IV fimbria systems, enteropathogenic E. coli utilizes a specific prepilin peptidase to process the major subunit of the bundle-forming pilus. This prepilin petidase contains sequence and reciprocal functional homologies with the PilD protein of P. aeruginosa.