Bacterial fimbriae (common pili) are hairlike surface appendages which are though: to be important colonisation factors. Type 4 fimbriae have a polar location on the cell, impart the phenotype of twitching motility, and are composed of a protein subunit whose amino acid sequence has certain characteristic features. Important pathogenic bacteria with type 4 fimbriae include Bacteroides nodosus and Pseudomonas aeruginosa. The aims of this work were firstly, to identify and characterise genes

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... ved in the processing and assembly of type 4 fimbrial subunits into mature organelles; and secondly, to investigate the transcriptional organisation of the fimbrial gene region. The approach adopted was to examine sequences in the immediate vicinity of the fimbrial subunit gene (fimA) of B. nodosus and P. aeruginosa, reasoning that like the fimbrial systems of Escherichia coli genes involved in fimbrial biosynthesis might be clustered together, and to map the RNA transcripts of these regions. The fimbrial genes of B. nodosus may be divided into two distinct classes, based on sequence of the major subunit gene (fimA) and downstream regional organisation The class II-specific region was sequenced and three genes were identified. One of these (fimZ) represents a duplicate, possibly redundant, fimbrial subunit gene, closely related to the progenitor of fimA in class II strains, whose sequence suggests that the entire class II-specific region of B. nodosus was originally acquired from another spec.es by recombination. fimZ is closely preceded by two unassigned open reading frames (fimC and fimD), neither of which is homologous to fimB, the single class I-specific gene. fimB, fimC and fimD are expressed only at low levels, and possibly encode fimbrial biosynthetic proteins. Downstream of the fimbrial gene region is a gene (clpB) which appears to encode the regulatory component of an ATP-dependent protease. This protein is a member of a new family of highly conserved proteins which occur widely, perhaps ubiqqitously, in both prokaryotes and eukaryotes. Characterisation of fimA mRNA from B. nodosus and P. aeruginosa indicated that this gene is transcribed from a class of promoters subject to transcriptional regulation. Further evidence that the cloned P. aeruginosa fimA gene is subject to transcriptional activation involving flanking sequences is also described. This transcriptional control suggests that type 4 fimbrial genes are co-ordinately expressed with a range of other virulence determinants. vi * The designation "type 1" is derived from a now defunct nomenclature introduced by Brinton (1965).