Regulatory Characteristics of Vibrio vulnificus gbpA Gene Encoding a Mucin-binding Protein Essential for Pathogenesis

Binding to mucin is the initial step for enteropathogens to establish pathogenesis. An open reading frame, gbpA, of Vibrio vulnificus was identified and characterized in this study. Compared with wild type, the gbpA mutant was impaired in binding to mucin-agar and the mucin-secreting HT29-methotrexate cells, and the impaired mucin binding was restored by the purified GbpA provided exogenously. The gbpA mutant had attenuated virulence and ability of intestinal colonization in a mouse model, indicating that GbpA is a mucin-binding protein and essential for pathogenesis of V. vulnificus. The gbpA transcription was growth phase-dependent, reaching a maximum during the exponential phase. The Fe-S cluster regulator (IscR) and the cyclic AMP receptor protein (CRP) coactivated, whereas SmcR, a LuxR homologue, repressed gbpA. The cellular levels of IscR, CRP, and SmcR were not significantly affected by one another, indicating that the regulator proteins function cooperatively to regulate gbpA rather than sequentially in a regulatory cascade. The regulatory proteins directly bind upstream of the gbpA promoter PgbpA. DNase I protection assays, together with the deletion analyses of PgbpA, demonstrated that IscR binds to two specific sequences centered at -164.5 and -106, and CRP and SmcR bind specifically to the sequences centered at -68 and -45, respectively. Furthermore, gbpA was induced by exposure to H2O2, and the induction appeared to be mediated by elevated intracellular levels of IscR. Consequently, the combined results indicated that IscR, CRP, and SmcR cooperate for precise regulation of gbpA during the V. vulnificus pathogenesis.