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 IutA, 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 IutA and AlcD proteins share 43 % identity with the Escherichia coli IutA 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 binding ⁵⁵Fe-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.