Genetic organization of fimbriae's secretion/glycosylation machinery in Streptococcus salivarius

Background: Fimbriae are filamentous appendages found on the surface of numerous bacterial species. These structures are involved in the adherence to surfaces and in coaggregation among bacterial cells. Streptococcus salivarius, a pioneer species of the human oral mucosa, expresses long flexible peritrichous fimbriae involved in the coaggregation with the periodontopathogen Prevotella intermedia. The major structural subunit, FimS, of S. salivarius ATCC 25975 belongs to the serine-rich proteins (SRP) family whose members are highly glycosylated, surface-localized proteins of adhesive functions. In other streptococcal species, SRPs were shown to be clustered with genes encoding the dedicated secretion/glycosylation (SG) machinery. Objective: To identify and analyze the genes involved in the S/G of S. salivarius fimbriae. Methods: The chromosomic region located immediately upstream of fimS was isolated by chromosome walking, direct or inverse PCR, while the gtfA/B genes were amplified using primers derived from sequences conserved among orthologues. DNA sequence analysis was performed using NCBI software tools and the GCG package. Maximum likelihood phylogenetic analyses were conducted using PHYML. Results: The sequence analysis of the 15-kb region located upstream of fimS revealed the presence of genes encoding for the secretion machinery (secY2, SecA2, asp1/2/3/4/5) and enzymes involved in glycosylation. The gtfA/B genes that are encoding two other SRP-glycosylating enzymes, are not contiguous to the other FimS S/G genes. The secretion- and glycosyltransferase-encoding genes have orthologues in other streptococcal genomes including: S. pneumoniae, S. sanguinis, S. gordonii and S. agalactiae. Conclusion: The E values, the genetic organization and phylogenetic comparisons suggest that the fimbrial S/G machinery of S. salivarius is more closely evolutionary related to components of the pneumococcal S/G system. Fund: CIHR operating grant MOP 177248.