STRUCTURE AND FUNCTION OF REPETITIVE SEQUENCE ELEMENTS ASSOCIATED WITH A HIGHLY POLYMORPHIC DOMAIN OF THE NEISSERIA-MENINGITIDIS PILQ PROTEIN

Secretins are a large family of proteins associated with membrane tran slocation of macromolecular complexes, and a subset of this family, te rmed PilQ proteins, is required for type IV pilus biogenesis. We analy sed the status of PilQ expression in Neisseria meningitidis (Mc) and f ound that PilQ(-) mutants were non-piliated and deficient in the expre ssion of pilus-associated phenotypes. Sequence analysis of the 5' port ion of the pilQ ORF of the serogroup B Me strain 44/76 showed the pres ence of seven copies of a repetitive sequence element, in contrast to the situation in N, gonorrhoeae (Gc) strains, which carry either two o r three copies of the repeat. The derived amino acid sequence of the c onsensus nucleotide repeat was an octapeptide PAKQQAAA, designated as the small basic repeat (SBR), This gene segment was studied in more de tail in a collection of 52 Me strains of diverse origin by screening f or variability in the size of the PCR-generated DNA fragments spanning the SBRs. These strains were found to harbour from four to seven copi es of the repetitive element. No association between the number of cop ies and the serogroup, geographic origin or multilocus genotype of the strains was evident. The presence of polymorphic repeat elements in M e PilQ is unprecedented within the secretin family. To address the pot ential function of the repeat containing domain, Me strains were const ructed so as to express chimeric PilQ molecules in which the number of SBR repeats was increased or in which the repeat containing domain wa s replaced in toto by the corresponding region of the Pseudomonas aeru ginosa (Pa) PilQ protein. Although the strain expressing PilQ with an increased number of SBRs was identical to the parent strain in pilus p henotype, a strain expressing PilQ with the equivalent Pa domain had a n eightfold reduction in pilus expression level. The findings suggest that the repeat containing domain of PilQ influences Me pilus expressi on quantitatively but not qualitatively.