Characterization of a five-gene cluster required for the biogenesis of type 4 fimbriae in Pseudomonas aeruginosa

Pulling Together with Type IV Pili

Type IV pili are an efficient and versatile device for bacterial surface motility. They are widespread among the beta-, gamma-, and delta-proteobacteria and the cyanobacteria. Within that diversity, there is a core of conserved proteins that includes the pilin (PilA), the motors PilB and PilT, and various components of pilus biogenesis and assembly, PilC, PilD, PilM, PilN, PilO, PilP, and PilQ. Progress has been made in understanding the motor and the secretory functions. PilT is a motor protein that catalyzes pilus retraction; PilB may play a similar role in pilus extension. Type IV pili are multifunctional complexes that can act as bacterial virulence factors because pilus-based motility is used to spread pathogens over the surface of a tissue, or to build multicellular structures such as biofilms and fruiting bodies.

Marrow stem cells shift gene expression and engraftment phenotype with cell cycle transit

We studied the genetic and engraftment phenotype of highly purified murine hematopoietic stem cells (lineage negative, rhodamine-low, Hoechst-low) through cytokine-stimulated cell cycle. Cells were cultured in interleukin (IL)-3, IL-6, IL-11, and steel factor for 0 to 48 h and tested for engraftment capacity in a lethally irradiated murine competitive transplant model. Engraftment showed major fluctuations with nadirs at 36 and 48 h of culture and recovery during the next G1. Gene expression of quiescent (0 h) or cycling (48 h) stem cells was compared with lineage positive cells by 3' end PCR differential display analysis. Individual PCR bands were quantified using a 0 to 9 scale and results were visually compared using color-coded matrices. We defined a set of 637 transcripts expressed in stem cells and not expressed in lineage positive cells. Gene expression analyzed at 0 and 48 h showed a major shift from "stem cell genes" being highly expressed at 0 h and turned off at 48 h, while "cell division" genes were turned on at 48 h. These observations suggest stem cell gene expression shifts through cell cycle in relation to cell cycle related alterations of stem cell phenotype. The engraftment defect is related to a major phenotypic change of the stem cell.

Type IV pili and twitching motility

Twitching motility is a flagella-independent form of bacterial translocation over moist surfaces. It occurs by the extension, tethering, and then retraction of polar type IV pili, which operate in a manner similar to a grappling hook. Twitching motility is equivalent to social gliding motility in Myxococcus xanthus and is important in host colonization by a wide range of plant and animal pathogens, as well as in the formation of biofilms and fruiting bodies. The biogenesis and function of type IV pili is controlled by a large number of genes, almost 40 of which have been identified in Pseudomonas aeruginosa. A number of genes required for pili assembly are homologous to genes involved in type II protein secretion and competence for DNA uptake, suggesting that these systems share a common architecture. Twitching motility is also controlled by a range of signal transduction systems, including two-component sensor-regulators and a complex chemosensory system.

A component of innate immunity prevents bacterial biofilm development

Antimicrobial factors form one arm of the innate immune system, which protects mucosal surfaces from bacterial infection. These factors can rapidly kill bacteria deposited on mucosal surfaces and prevent acute invasive infections. In many chronic infections, however, bacteria live in biofilms, which are distinct, matrix-encased communities specialized for surface persistence. The transition from a free-living, independent existence to a biofilm lifestyle can be devastating, because biofilms notoriously resist killing by host defence mechanisms and antibiotics. We hypothesized that the innate immune system possesses specific activity to protect against biofilm infections. Here we show that lactoferrin, a ubiquitous and abundant constituent of human external secretions, blocks biofilm development by the opportunistic pathogen Pseudomonas aeruginosa. This occurs at lactoferrin concentrations below those that kill or prevent growth. By chelating iron, lactoferrin stimulates twitching, a specialized form of surface motility, causing the bacteria to wander across the surface instead of forming cell clusters and biofilms. These findings reveal a specific anti-biofilm defence mechanism acting at a critical juncture in biofilm development, the time bacteria stop roaming as individuals and aggregate into durable communities.

Molecular analyses of the natural transformation machinery and identification of pilus structures in the extremely thermophilic bacterium Thermus thermophilus strain HB27

Thermus thermophilus HB27, an extremely thermophilic bacterium, exhibits high competence for natural transformation. To identify genes of the natural transformation machinery of T. thermophilus HB27, we performed homology searches in the partially completed T. thermophilus genomic sequence for conserved competence genes. These analyses resulted in the detection of 28 open reading frames (ORFs) exhibiting significant similarities to known competence proteins of gram-negative and gram-positive bacteria. Disruption of 15 selected potential competence genes led to the identification of 8 noncompetent mutants and one transformation-deficient mutant with a 100-fold reduced transformation frequency. One competence protein is similar to DprA of Haemophilus influenzae, seven are similar to type IV pilus proteins of Pseudomonas aeruginosa or Neisseria gonorrhoeae (PilM, PilN, PilO, PilQ, PilF, PilC, PilD), and another deduced protein (PilW) is similar to a protein of unknown function in Deinococcus radiodurans R1. Analysis of the piliation phenotype of T. thermophilus HB27 revealed the presence of single pilus structures on the surface of the wild-type cells, whereas the noncompetent pil mutants of Thermus, with the exception of the pilF mutant, were devoid of pilus structures. These results suggest that pili and natural transformation in T. thermophilus HB27 are functionally linked.

DNA as a nutrient: novel role for bacterial competence gene homologs

The uptake and stable maintenance of extracellular DNA, genetic transformation, is universally recognized as a major force in microbial evolution. We show here that extracellular DNA, both homospecific and heterospecific, can also serve as the sole source of carbon and energy supporting microbial growth. Mutants unable to consume DNA suffer a significant loss of fitness during stationary-phase competition. In Escherichia coli, the use of DNA as a nutrient depends on homologs of proteins involved in natural genetic competence and transformation in Haemophilus influenzae and Neisseria gonorrhoeae. Homologs of these E. coli genes are present in many members of the gamma subclass of Proteobacteria, suggesting that the mechanisms for consumption of DNA may have been widely conserved during evolution.

Structure-function analysis of BfpB, a secretin-like protein encoded by the bundle-forming-pilus operon of enteropathogenic Escherichia coli

Production of type IV bundle-forming pili by enteropathogenic Escherichia coli (EPEC) requires BfpB, an outer-membrane lipoprotein and member of the secretin protein superfamily. BfpB was found to compose a ring-shaped, high-molecular-weight outer-membrane complex that is stable in 4% sodium dodecyl sulfate at temperatures of < or = 65 degrees C. Chemical cross-linking and immunoprecipitation experiments disclosed that the BfpB multimeric complex interacts with BfpG, and mutational studies showed that BfpG is required for the formation and/or stability of the multimer but not for the outer-membrane localization of BfpB. Formation of the BfpB multimer also does not require BfpA, the repeating subunit of the pilus filament. Functional studies of the BfpB-BfpG complex revealed that its presence confers vancomycin sensitivity, indicating that it may form an incompletely gated channel through the outer membrane. BfpB expression is also associated with accumulation of EPEC proteins in growth medium, suggesting that it may support both pilus biogenesis and protein secretion.

Mutational analysis of genes involved in pilus structure, motility and transformation competency in the unicellular motile cyanobacterium Synechocystis sp. PCC 6803

The relevance of pilus-related genes to motility, pilus structure on the cell surface and competency of natural transformation was studied by gene disruption analysis in the unicellular motile cyanobacterium Synechocystis: sp. PCC 6803. The genes disrupted in this study were chosen as related to the pil genes for biogenesis of the type IV pili in a Gram-negative bacterium Pseudomonas aeruginosa. It was found that motility of Synechocystis cells was lost in the mutants of slr0063, slr1274, slr1275, slr1276, slr1277 and sll1694 together with a simultaneous loss of the thick pili on the cell surface. Competency of the natural transformation was lost in the mutants listed above and slr0197-disruptant. The gene slr0197 was previously predicted as a competence gene by a search with sequence-independent DNA-binding structure [Yura et al. (1999) DNA Res. 6: 75]. It was suggested that both DNA uptake for natural transformation and motility are mediated by a specific type IV-like pilus structure, while a putative DNA-binding protein encoded by slr0197 is additionally required for the DNA uptake. Based on the homology with the pil genes in P: aeruginosa, slr0063, slr1274, slr1275, slr1276, slr1277 and sll1694 were designated pilB1, pilM, pilN, pilO, pilQ and pilA1, respectively. The gene slr0197 was designated comA.

The Myxococcus xanthus pilQ (sglA) gene encodes a secretin homolog required for type IV pilus biogenesis, social motility, and development

The Myxococcus xanthus sglA1 spontaneous mutation was originally isolated because it allowed dispersed cell growth in liquid yet retained the ability to form fruiting bodies. Consequently, most of today's laboratory strains either contain the sglA1 mutation or were derived from strains that carry it. Subsequent work showed that sglA was a gene for social gliding motility, a process which is mediated by type IV pili. Here sglA is shown to map to the major pil cluster and to encode a 901-amino-acid open reading frame (ORF) that is homologous to the secretin superfamily of proteins. Secretins form a channel in the outer membrane for the transport of macromolecules. The closest homologs found were PilQ proteins from Pseudomonas aeruginosa and Neisseria gonorrhoeae, which are required for type IV pili biogenesis and twitching motility. To signify these molecular and functional similarities, we have changed the name of sglA to pilQ. The hypomorphic pilQ1 (sglA1) allele was sequenced and found to contain two missense mutations at residues 741 (G-->S) and 762 (N-->G). In addition, 19 independent social (S)-motility mutations are shown to map to the pilQ locus. In-frame deletions of pilQ and its downstream gene, orfL, were constructed. pilQ is shown to be essential for pilus biogenesis, S-motility, rippling, and fruiting body formation, while orfL is dispensable for these processes. The pilQ1 allele, but not the DeltapilQ allele, was found to render cells hypersensitive to vancomycin, suggesting that PilQ1 alters the permeability properties of the outer membrane. Many differences between pilQ1 and pilQ+ strains have been noted in the literature. We discuss some of these observations and how they may be rationalized in the context of our molecular and functional findings.

Characterization of type IV pilus genes in Pseudomonas syringae pv. tomato DC3000

Many strains of Pseudomonas syringae produce retractile pili that act as receptors for lytic bacteriophage phi 6. As these are also characteristics of type IV pili, it was postulated that P. syringae may possess genes for type IV pilus biogenesis. A cosmid clone bank of P. syringae pv. tomato DC3000 genomic DNA was used to complement a mutant of Pseudomonas aeruginosa defective in the PilD (XcpA) prepilin peptidase gene by selection for restoration of extracellular protein secretion, a function also known to require PilD. A cosmid able to complement this mutant was also able to complement mutations in the pilB and pilC genes, suggesting that, if the organization of these genes is similar to that of P. aeruginosa, the cosmid may contain the P. syringae pilA. This was confirmed by sequencing a region from this plasmid that was shown to hybridize at low stringency to the P. aeruginosa pilA gene. The deduced P. syringae PilA polypeptide possesses the characteristic properties of the type IV pilins. Heterologous expression of the P. syringae pilA in P. aeruginosa was also shown, conferring not only phi 6 phage sensitivity to P. aeruginosa pilA mutants but also sensitivity to PO4, a lytic bacteriophage specific for the pilus of P. aeruginosa. This suggests that additional components might be present in the mature pilus of P. aeruginosa that are the true receptors for this phage. Chromosomal mutations in P. syringae pv. tomato DC3000 pilA and pilD genes were shown to abolish its sensitivity to bacteriophage phi 6. To determine the importance of P. syringae pilus in plant leaf interactions, these mutations were tested under laboratory and field conditions. Although little effect was seen on pathogenicity, culturable leaf-associated population sizes of the pilA mutant were significantly different from those of the wild-type parent. In addition, the expression of the DC3000 pilA gene appears to contribute to the UV tolerance of P. syringae and may play a role in survival on the plant leaf surface.

The pilH gene encodes an ABC transporter homologue required for type IV pilus biogenesis and social gliding motility in Myxococcus xanthus

Type IV pilus genes have been shown to be required for social gliding motility in Myxococcus xanthus. We report the discovery of four additional pil genes: pilD, a homologue of type IV prepilin leader peptidases; and pilG, pilH and pilI, which have no known homologues in other type IV pilus systems. pilH encodes an ATP-binding cassette (ABC) transporter homologue, the first such homologue to be required for the biogenesis of any bacterial pilus type. pilG and pilI are co-transcribed with pilH and appear to be functionally related to pilH. Null mutants of pilG, pilH and pilI all lack social motility, are deficient in pilus production, have elevated sporulation efficiencies and display similar developmental abnormalities. In addition, all three mutations reduced the amount of PilA found in the supernatant after cells were sedimented from liquid culture. We suggest that the products of these three genes form a single ABC exporter complex, in which pilI is an integral membrane protein with membrane-spanning domains, and pilG is an accessory factor. The complex may participate in pilus assembly and/or the export of PilA pilin.

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.

Role of BfpF, a member of the PilT family of putative nucleotide-binding proteins, in type IV pilus biogenesis and in interactions between enteropathogenic Escherichia coli and host cells

Adherence of enteropathogenic Escherichia coli (EPEC) to epithelial cells is dependent on a type IV fimbria, termed the bundle-forming pilus (BFP). A cluster of 14 genes is required for expression of BFP. The eighth gene in the cluster, bfpF, encodes a putative nucleotide-binding protein which resembles the PilT protein of Pseudomonas aeruginosa. It has been proposed that PilT is required for the retraction of the P. aeruginosa pilus, which results in twitching motility. To test the role of BfpF in BFP function and EPEC pathogenesis, two different mutations were constructed in the bfpF gene, one in the cloned gene cluster in a laboratory E. coli strain and one in wild-type EPEC. Neither mutation affected prepilin synthesis, leader sequence processing, or pilus biogenesis. However, both mutations resulted in increased localized adherence. In addition, the EPEC bfpF mutant displayed increased aggregation. The EPEC bfpF mutant was not deficient in attaching and effacing activity or invasion capacity. These results suggest that BfpF decreases aggregation and adherence by EPEC but that subsequent steps in EPEC pathogenesis do not require this protein.

Sequence of the ponA gene and characterization of the penicillin-binding protein 1A of Pseudomonas aeruginosa PAO1

The nucleotide sequence of the ponA gene encoding the high molecular-mass penicillin-binding protein 1A (PBP1A) of Pseudomonas aeruginosa (Pa) PAO1 was determined and characterized. The predicted PBP1A protein of 822 amino acids (aa) has a calculated molecular mass of 91.2 kDa corresponding to the size of the protein expressed in vitro and in vivo. A penicillin-binding (PB) assay showed that the Pa ponA gene product covalently binds penicillin. The deduced PBP1A aa sequence has features typical of class-A high-molecular-mass PBPs: a highly hydrophobic N-terminus portion containing a potential transmembrane segment which might anchor the protein to the cytoplasmic membrane; an N-terminal module with the conserved boxes 1 (E86D(DN)F(AN)H(Y)G), 2 (G117GS(T)I(TM)Q), 3 (R139K(IN)E(ILL)AL) and 4 (R221R(NW)IL); a PB module with the conserved boxes 5 (S461SFK), (S520RN) and (K695TG); an internal extension at aa 297-407 between the N-terminal and PB modules; and a C-extension at the end of the PB module at aa 742 to 822. The highest percentage of similarity (62.8%) was found with the class A high-molecular-mass PBP1A of Escherichia coli (Ec) and Haemophilus influenzae. The observed extensive homology in the modular design of the Pa PBP1A with the bifunctional Ec PBP1A suggests structural and functional relationships between these proteins and refutes the proposed correspondence between Pa PBP1A and Ec PBP1B.

The pilus colonization factor of pathogenic neisserial species: organelle biogenesis and structure/function relationships--a review

Type-IV pilus expression plays a critical role in the interactions between Neisseria gonorrhoeae, Neisseria meningitidis and their human host. We have focused on experiments designed to elucidate the mechanisms of organelle biogenesis as one means of understanding the complexities of pilus biology in these species. Employing a variety of approaches, genes and gene products essential to pilus biogenesis have been identified and characterized. The findings indicate that the neisserial type-IV pilus biogenesis machinery is most closely related to that operating in Pseudomonas aeruginosa and other pseudomonad species. This interrelatedness is documented at the levels of gene organization, DNA homologies and identities between the primary structures of the components. Despite these similarities, the biological correlates of pilus expression in the pathogenic Neisseria are quite unique. The current status of our embryonic understanding of the factors influencing organelle biogenesis is presented. In the context of this workshop, emphasis has been placed on specific contributions made through studies of gonococci and meningococci to the field as a whole..

Genes involved in the biogenesis and function of type-4 fimbriae in Pseudomonas aeruginosa

Type-4 fimbriae are filamentous polar organelles which are found in a wide variety of pathogenic bacteria. Their biogenesis and function is proving to be extremely complex, involving the expression and coordinate regulation of a large number of genes. Type-4 fimbriae mediate attachment to host epithelial tissues and a form of surface translocation called twitching motility. In Pseudomonas aeruginosa they also appear to function as receptors for fimbrial-dependent bacteriophages. Analysis of mutants defective in fimbrial function has allowed the identification of many of the genes involved in the biogenesis of these organelles. Thus far over 30 genes have been characterized, which fall into two broad categories: those encoding regulatory networks that control the production and function of these fimbriae (and other virulence determinants such as alginate) in response to alterations in environmental conditions; and those encoding proteins involved in export and assembly of these organelles, many of which are similar to proteins involved in protein secretion and DNA uptake. These systems all appear to be closely related and to function in the assembly of surface-associated protein complexes that have been adapted to different biological functions.

Molecular genetic analysis of type-4 pilus biogenesis and twitching motility using Pseudomonas aeruginosa as a model system--a review

Genetic analysis of Pseudomonas aeruginosa pilus biogenesis and twitching motility has revealed the requirement for several pil loci which have been localized to different regions of the chromosome. One pil locus, designated pilE, resides at approx. 71 min on the PAO genetic map, a region of the chromosome previously shown to harbor a number of genes required for pilus assembly (i.e., pilA, -B, -C, -D, -R and -S). The PilE protein shows significant sequence identity to the N-terminal domain of PilA as well as to the pilin precursors from a variety of type-4 pilus producers. Included within this homologous region is a short, positively charged leader sequence followed by a prepilin peptidase cleavage site and a largely hydrophobic region. Additionally, an unlinked set of pil genes, designated pilG, -H, -I, -J and -K, has been localized to the SpeI fragment H which corresponds to approx. 20 min on the PAO genetic map. This gene cluster encodes proteins that demonstrate remarkable similarity to the chemotaxis proteins of enterics and the gliding bacterium Myxococcus xanthus and are thought to be part of a signal transduction system that controls P. aeruginosa pilus biosynthesis and twitching motility.

Cloning and characterization of the ponA gene encoding penicillin-binding protein 1 from Neisseria gonorrhoeae and Neisseria meningitidis

The ponA gene encoding penicillin-binding protein 1 (PBP 1) from Neisseria gonorrhoeae was cloned by a reverse genetic approach. PBP 1 was purified from solubilized membranes of penicillin-susceptible strain FA19 by covalent ampicillin affinity chromatography and used to obtain an NH2-terminal amino acid sequence. A degenerate oligonucleotide based on this protein sequence and a highly degenerate oligonucleotide based on a conserved amino acid motif found in all class A high-molecular-mass PBPs were used to isolate the PBP 1 gene (ponA). The ponA gene encodes a protein containing all of the conserved sequence motifs found in class A PBPs, and expression of the gene in Escherichia coli resulted in the appearance of a new PBP that comigrated with PBP 1 purified from N. gonorrhoeae. A comparison of the gonococcal ponA gene to its homolog isolated from Neisseria meningitidis revealed a high degree of identity between the two gene products, with the greatest variability found at the carboxy terminus of the two deduced PBP 1 protein sequences.

Identification of a gene, pilF, required for type 4 fimbrial biogenesis and twitching motility in Pseudomonas aeruginosa

Many bacterial pathogens produce a class of surface structures called type 4 fimbriae. In Pseudomonas aeruginosa these fimbriae are responsible for adhesion and translocation across host epithelial surfaces. We have identified a novel gene involved in the complex process of type 4 fimbrial biogenesis. This gene, termed pilF, is located on SpeI fragment S at 30 min on the P. aeruginosa genomic map, which is the sixth region on the chromosome shown to contain a fimbrial-associated gene. The PilF protein has a predicted M(r) of 22402, and together with a highly homologous upstream ORF shares a chromosomal arrangement similar to that found in Haemophilus influenzae. A pilF mutant is blocked in the export/assembly of the fimbrial subunit PilA, and accumulates this protein in the membrane fraction. Complementation studies indicate that the cloned pilF gene is able to restore the expression of surface fimbriae, twitching motility and susceptibility to fimbrial-specific bacteriophage.

The molecular genetics of type-4 fimbriae in Pseudomonas aeruginosa--a review

Type-4 fimbriae (or pili) are filaments found at the poles of a wide range of bacterial pathogens, including Neisseria gonorrhoeae, Moraxella bovis, Dichelobacter nodosus and Pseudomonas aeruginosa. They are composed of a small subunit which is highly conserved among different species and appear to mediate adhesion and translocation across epithelial surfaces via a phenomenon termed "twitching motility'. These fimbriae are key host colonisation factors and important protective antigens. We have analysed the genetics and biosynthesis of type-4 fimbriae in P. aeruginosa, which is an opportunistic pathogen of compromised individuals, including those suffering cystic fibrosis, AIDS or burns. A library of P. aeruginosa transposon mutants was constructed which exhibited loss of twitching motility, as determined by altered colony morphology. Analysis of these mutants, and of similar collections by other groups, have revealed that there are at least 22 genes involved in type-4 fimbrial assembly and function. A large number (pilA, B, C, D, E, M, N, O, P, Q, T, U, V and Z) appear to be involved in the biogenesis of the fimbriae and to represent a subset of a supersystem involved in the assembly of surface-associated protein complexes. Homologs of at least some of these genes have subsequently been identified in other type-4 fimbriate bacteria. In P. aeruginosa, the system is also regulated via two signal transduction pathways-a classic sensor-regulator system (encoded by pilS, pilR and rpoN) which controls transcription of the fimbrial subunit, presumably in response to host cues, and a chemotactic system (encoded by pilG, H, I, J, K and L) which may be involved in the directional or rate control of twitching motility in response to local environmental variables.

Fimbrial biogenesis genes of Pseudomonas aeruginosa: pilW and pilX increase the similarity of type 4 fimbriae to the GSP protein-secretion systems and pilY1 encodes a gonococcal PilC homologue

Type 4 fimbriae of Pseudomonas aeruginosa are surface filaments involved in host colonization. They mediate both attachment to host epithelial cells and flagelia-independent twitching motility. Four additional genes, pilW, pilX, pilY1 and pilY2, are located on Spel fragment E in the 5 kb intergenic region between the previously characterized genes pilV and pilE, which encode prepilin-like proteins involved in type 4 fimbrial biogenesis. The phenotypes of a transposon insertion and other mutations constructed by allelic exchange show that these genes are involved in the assembly of type 4 fimbriae. The PilW and PilX proteins are membrane located, possess the hydrophobic N-terminus characteristic of prepilin-like proteins, and appear to belong to the GspJ and GspK group of proteins that are required for protein secretion in a wide range of Gram-negative bacteria. These findings increase the similarities between the fimbrial biogenesis and the Gsp-based protein-secretion supersystems. PilY1 is a large protein with C-terminal homology to the PilC2 protein of Neisseria gonorrhoeae, thought to be a fimbrial tip-associated adhesin, and which, like PilY1, is involved in fimbrial assembly. PilY1 appears to be located in both the membrane and the external fimbrial fractions. PilY2 is a small protein that appears to play a subtle role in fimbrial biogenesis and represents a new class of protein.

Identification of two genes with prepilin-like leader sequences involved in type 4 fimbrial biogenesis in Pseudomonas aeruginosa

Type 4 fimbriae are surface filaments produced by a range of bacterial pathogens for colonization of host epithelial surfaces. In Pseudomonas aeruginosa, they are involved in adhesion as well as in a form of surface translocation called twitching motility, and sensitivity to infection by fimbria-specific bacteriophage. Analysis of the 2.5-kb intergenic region between the previously defined pilR and pilV genes on P. aeruginosa genomic SpeI fragment E has identified three new genes, fimT, fimU, and dadA*. The predicted 18.5-kDa products of the fimT and fimU genes contain prepilin-like leader sequences, whereas the third gene, dadA*, encodes a protein similar to the D-amino acid dehydrogenase of Escherichia coli. Isogenic mutants constructed by allelic exchange demonstrated that the fimU gene was required for fimbrial biogenesis and twitching motility, whereas the fimT and dada* mutants retained wild-type phenotypes. However, overexpression of the fimT gene was found to be able to functionally replace the lack of a fimU gene product, suggesting a subtle role in fimbrial biogenesis. The identification of these proteins increases the similarity between type 4 fimbrial biogenesis and the supersystems involved in macromolecular traffic, such as extracellular protein secretion and DNA uptake, all of which now possess multiple protein species that possess prepilin-like leader sequences.

Physical linkage of the Vibrio cholerae mannose-sensitive hemagglutinin secretory and structural subunit gene loci: identification of the mshG coding sequence

Vibrio cholerae O1 expresses a variety of cell surface factors which mediate bacterial adherence and colonization at the intestinal epithelium. The mannose-sensitive hemagglutinin (MSHA), a type IV pilus, is a potential attachment factor of the V. cholerae El Tor biotype. We describe a TnphoA mutant that is defective in its ability to hemagglutinate mouse erythrocytes. The TnphoA insertion maps to a recently identified genetic locus that encodes products that are predicted to be essential for assembly and export of the MSHA pilus. Insertional disruption at this locus in a mshA-phoA reporter strain provides evidence for a role of this locus in the latter stages of pilus assembly and/or export. These constructs have provided physical markers by which we have established close physical linkage of this secretion locus to a set of genes that includes the mshA structural gene. Sequence analysis of the intervening region between these two loci has revealed the presence of an open reading frame with homology to pilus biogenesis genes of several gram-negative bacteria. This genetic organization suggests an entire operon encoding the MSHA pilus and the components necessary for its assembly and secretion to the bacterial cell surface. The nomenclature of the MSHA structural and secretory locus has been redefined accordingly.

Characterization of genes required for pilus expression in Pseudomonas syringae pathovar phaseolicola

Nonpiliated, phage phi 6-resistant mutants of Pseudomonas syringae pv. phaseolicola were generated by Tn5 transposon mutagenesis. A P. syringae pv. phaseolicola LR700 cosmid library was screened with Tn5-containing EcoRI fragments cloned from nonpiliated mutants. The cosmid clone pVK253 complemented the nonpiliated mutant strain HB2.5. A 3.8-kb sequenced region spanning the Tn5 insertion site contained four open reading frames. The transposon-inactivated gene, designated pilP, is 525 bp long, potentially encoding a 19.1-kDa protein precursor that contains a typical membrane lipoprotein leader sequence. Generation of single mutations in each of the three remaining complete open reading frames by marker exchange also resulted in a nonpiliated phenotype. Expression of this gene region by the T7 expression system in Escherichia coli resulted in four polypeptides of approximately 39, 26, 23, and 18 kDa, in agreement with the sizes of the open reading frames. The three genes upstream of pilP were designated pilM (39 kDa), pilN (23 kDa), and pilO (26 kDa). The processing of the PilP precursor into its mature form was shown to be inhibited by globomycin, a specific inhibitor of signal peptidase II. The gene region identified shows a high degree of homology to a gene region reported to be required for Pseudomonas aeruginosa type IV pilus production.

Identification of a gene, pilV, required for type 4 fimbrial biogenesis in Pseudomonas aeruginosa, whose product possesses a pre-pilin-like leader sequence

Type 4 fimbriae are important colonization factors in Pseudomonas aeruginosa and other pathogens that mediate attachment to epithelial cells of the host. They are also responsible for a form of translocation termed 'twitching motility' and are implicated in the susceptibility to fimbrial-specific bacteriophage. Analysis of a transposon mutant which lacks functional fimbriae has identified a new gene which is required for fimbrial biogenesis. This gene, termed pilV, is located on chromosomal SpeI fragment E, 2 kb downstream of the previously characterized pilSR genes involved in transcriptional activation of the fimbrial subunit gene. The pilV gene encodes a 20 kDa membrane-located protein with considerable amino-terminal homology to the type 4 consensus pre-pilin leader sequence, suggesting that it is processed by a leader peptidase. Site-directed mutagenesis has shown that PilV requires such cleavage to be functional. PilV also exhibits close similarity to a group of proteins involved in extracellular protein secretion from a number of Gram-negative bacteria, suggesting that the biogenesis of type 4 fimbriae may have a similar basis.