An iron-regulated gene required for utilization of aerobactin as an exogenous siderophore in Vibrio parahaemolyticus

A previous investigation using the Fur titration assay system showed that Vibrio parahaemolyticus possesses a gene encoding a protein homologous to lutA, the outer-membrane receptor for ferric aerobactin in Escherichia coli. In this study, a 5.6 kb DNA region from the V. parahaemolyticus WP1 genome was cloned and two entire genes, iutA and alcD homologues, were identified which are absent from Vibrio cholerae genomic sequences. The V parahaemolyticus lutA and AlcD proteins share 43% identity with the Escherichia coli lutA protein and 24% identity with the Bordetella bronchiseptica AlcD protein of unknown function, respectively. Primer extension analysis revealed that the iutA gene is transcribed in response to low-iron availability from a putative promoter overlapped with a sequence resembling a consensus E coli Fur-binding sequence. In agreement with the above finding, V parahaemolyticus effectively utilized exogenously supplied aerobactin for growth under iron-limiting conditions. Moreover, insertional inactivation of iutA impaired growth in the presence of aerobactin and incapacitated the outer-membrane fraction from iron-deficient cells for bindin g 55 Fe-labelled aerobactin. These results indicate that the V parahaemolyticus iutA homologue encodes an outer-membrane protein which functions as the receptor for ferric aerobactin. Southern blot analysis revealed that the iutA homologues are widely distributed in clinical and environmental isolates of V parahaemolyticus. However, additional genes required for ferric aerobactin transport across the inner membrane remain to be clarified.