Coordinate Regulation of Vibrio vulnificus Heme Receptor HupA Expression by Cyclic AMP-receptor Protein and Ferric Uptake Regulator

ABSTRACT

Vibrio vulnificus causes rapid progressing fulminant infections in susceptible individuals, especially those with elevated serum iron levels. This ferrophilic bacterium can directly acquire iron from heme-containing proteins, such as, hemoglobin, via its heme receptor protein HupA. This study was undertaken to determine the roles of cyclic AMP-receptor protein (Crp) as an activator and of ferric uptake regulator (Fur) as a repressor in regulating hupA expression at various iron and glucose concentrations. Under severely iron-deficient conditions, hupA expression in the absence of Crp was induced albeit at low levels and repressed by the addition of iron. In contrast, hupA expression in the presence of Crp was increased by the addition of iron. Under moderately iron-deficient and iron-sufficient conditions, iron addition repressed hupA expression in the presence of Fur, but not in the absence of Fur. Glucose addition repressed hupA expression in the presence of Fur but not in the absence of Fur. Furthermore, a mutation in cyaA encoding adenylate cyclase required for cAMP synthesis hupA expression, and this repression was prevented by the exogenous addition of cAMP. These results indicate that hupA expression is under the coordinate control of cAMP or Crp, which responds to glucose availability, and of Fur, which responds to iron availability, and that Crp is not essential for the constitutional expression of hupA, but is required for the optimal expression of hupA, whereas Fur is essential for the prevention of hupA over-expression.