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 translocation of macromolecular complexes, and a subset of this family, termed PilQ proteins, is required for type IV pilus biogenesis. We analysed the status of PilQ expression in Neisseria meningitidis (Mc) and found that PilQ(-) mutants were non-piliated and deficient in the expression of pilus-associated phenotypes. Sequence analysis of the 5' portion of the pilQ ORF of the serogroup B Me strain 44/76 showed the presence of seven copies of a repetitive sequence element, in contrast to the situation in N, gonorrhoeae (Gc) strains, which carry either two or three copies of the repeat. The derived amino acid sequence of the consensus nucleotide repeat was an octapeptide PAKQQAAA, designated as the small basic repeat (SBR), This gene segment was studied in more detail in a collection of 52 Me strains of diverse origin by screening for variability in the size of the PCR-generated DNA fragments spanning the SBRs. These strains were found to harbour from four to seven copies of the repetitive element. No association between the number of copies and the serogroup, geographic origin or multilocus genotype of the strains was evident. The presence of polymorphic repeat elements in Me PilQ is unprecedented within the secretin family. To address the potential function of the repeat containing domain, Me strains were constructed so as to express chimeric PilQ molecules in which the number of SBR repeats was increased or in which the repeat containing domain was replaced in toto by the corresponding region of the Pseudomonas aeruginosa (Pa) PilQ protein. Although the strain expressing PilQ with an increased number of SBRs was identical to the parent strain in pilus phenotype, a strain expressing PilQ with the equivalent Pa domain had an eightfold reduction in pilus expression level. The findings suggest that the repeat containing domain of PilQ influences Me pilus expression quantitatively but not qualitatively.