Transposon mutagenesis was used to identify genes necessary for the expression of Pseudomonas aeruginosa type 4 fimbriae. In a library of 12,700 mutants, 147 were observed to have lost the spreading colony morphology associated with the presence of functional fimbriae. Of these, 28 had also acquired resistance to the fimbrial-specific bacteriophage PO4. The mutations conferring this phage resistance were found to have occurred at at least six different loci, including the three that had been previously shown to be required for fimbrial biosynthesis or function: the structural subunit (pilA) and adjacent genes (pilB,C,D), the twitching motility gene (pilT), and the sigma 54 RNA polymerase initiation factor gene (rpoN). One novel group of phage-resistant mutants was identified in which the transposon had inserted near sequences that cross-hybridized to an oligonucleotide probe designed against conserved domains in regulators of RpoN-dependent promoters. These mutants had no detectable transcription of pilA and did not produce fimbriae. A probe derived from inverse polymerase chain reaction was used to isolate the corresponding wild-type sequences from a P. aeruginosa PAO cosmid reference library, and two adjacent genes affected by transposon insertions, pilS and pilR, were located and sequenced. These genes were shown to be capable of complementing the corresponding mutants, both at the level of restoring the phenotypes associated with functional fimbriae and by the restoration of pilA transcription. The pilSR operon was physically mapped to Spel fragment 5 (corresponding to about 72-75/0 min on the genetic map), and shown to be located approximately 25 kb from pilA-D. PilS and PilR clearly belong to the family of two-component transcriptional regulatory systems which have been described in many bacterial species. PilS is predicted to be a sensor protein which when stimulated by the appropriate environmental signals activates PilR through kinase activity. PilR then activates transcription of pilA, probably by interacting with RNA polymerase containing RpoN. The identification of pilS and pilR makes possible a more thorough examination of the signal transduction systems controlling expression of virulence factors in P. aeruginosa.
Type 4 fimbriae are produced by a variety of pathogens, in which they appear to function in adhesion to epithelial cells, and in a form of surface translocation called twitching motility. Using transposon mutagenesis of Pseudomonas aeruginosa, we have identified a new locus required for fimbrial assembly. This locus contains the gene pilQ which encodes a 77 kDa protein with an N-terminal hydrophobic signal sequence characteristic of secretory proteins. pilQ mutants lack the spreading colony morphology characteristic of twitching motility, are devoid of fimbriae, and are resistant to the fimbrial-specific bacteriophage PO4. The pilQ gene was mapped to Spel fragment 2, which is located at 0-5 minutes on the P. aeruginosa PAO1 chromosome, and thus it is not closely linked to the previously characterized pilA-D, pilS,R or pilT genes. The pilQ region also contains ponA, aroK and aroB-like genes in an organization very similar to that of corresponding genes in Escherichia coli and Haemophilus influenzae. The predicted amino acid sequence of PilQ shows homology to the PulD protein of Klebsiella oxytoca and related outer membrane proteins which have been found in association with diverse functions in other species including protein secretion, DNA uptake and assembly of filamentous phage. PilQ had the highest overall homology to an outer membrane antigen from Neisseria gonorrhoeae, encoded by omc, that may fulfil the same role in type 4 fimbrial assembly in this species.
The opportunistic pathogen Pseudomonas aeruginosa produces type 4 fimbriae which promote adhesion to epithelial cells and are associated with a form of surface translocation called twitching motility. We have used transposon mutagenesis to identify loci required for fimbrial assembly or function by screening for mutants that lack the spreading colony morphology characteristic of twitching motility. A subset of these mutants is resistant to fimbria-specific phage. One of these mutants (R270) was found to contain a transposon insertion in a new gene, termed pilZ, which is located on chromosomal SpeI fragment I at about 40 min on the P. aeruginosa map, a position remote from other loci involved in fimbrial biogenesis. pilZ appears to be linked to and possibly forms an operon with a gene, holB*, which is homologous to the gene encoding the ␦ subunit of Escherichia coli DNA polymerase III. The product of the pilZ gene is a protein of 118 amino acids (predicted molecular weight, 12,895) which probably has a cytoplasmic location. PilZ appears to be a new class of protein which has not hitherto been represented in the sequence databases, and its function is unknown. Complementation studies indicate that pilZ is able to restore the expression of fimbriae on the surface of P. aeruginosa, as well as twitching motility and sensitivity to fimbria-specific phage when provided in trans to the R270 mutant.Type 4 fimbriae (or common pili) are flexible, filamentous surface appendages which are found in a number of important pathogenic bacteria, including Neisseria gonorrhoeae, Neisseria meningitidis, Moraxella bovis, Dichelobacter nodosus, and Pseudomonas aeruginosa, and which mediate adherence to host epithelial tissue during colonization (33,40,42). Type 4 fimbriae were originally defined by their polar arrangement on the cell and their ability to promote a form of motion called twitching motility (14, 32). Type 4 fimbriae are also characterized by conserved features of the structural subunit proteins which make up the fimbrial strand. These include a short, positively charged leader sequence of six to eight amino acids which is removed during fimbrial assembly, a modified amino acid, Nmethylphenylalanine, as the first residue in the mature protein, and a highly conserved hydrophobic amino-terminal domain (7,42).The biogenesis of type 4 fimbriae is a complex process involving a large network of genes that have thus far been localized to three separate regions of the Pseudomonas chromosome. The largest locus is located on SpeI fragment E at about 70 min on the physical map and contains the fimbrial subunit gene (pilA), two ancillary genes (pilB and pilC), the specific leader peptidase gene (pilD), and the sensor/regulator genes (pilS and pilR) responsible for the transcriptional activation of the fimbrial subunit gene (16,21,41). Recently, two further genes (pilE and pilV) have been localized to this region of the genome, both of which encode proteins possessing prepilinlike leader sequences that appear to be cleaved by the PilD...
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