VIBRIO-CHOLERAE IRON TRANSPORT - HEME TRANSPORT GENES ARE LINKED TO ONE OF 2 SETS OF TONB, EXBB, EXBD GENES

Vibrio cholerae was found to have two sets of genes encoding TonB, Exb B and ExbD proteins. The first set (tonB1, exbB1, exbD1) was obtained by complementation of a V. cholerae tonB mutant. In the mutant, a plas mid containing these genes permitted transport via the known V. choler ae high-affinity iron transport systems, including uptake of haem, vib riobactin and ferrichrome, When chromosomal mutations in exbB1 or exbD 1 were introduced into a wild-type V. cholerae background, no defect i n iron transport was noted, indicating the existence of additional gen es that can complement the defect in the wild-type background. Another region of the V,cholerae chromosome was cloned that encoded a second functional TonB/Exb system (tonS2, exbB2, exbD2), A chromosomal mutati on in exbB2 also failed to exhibit a defect in iron transport, but a V . cholerae strain that had chromosomal mutations in both the exbB1 and exbB2 genes displayed a mutant phenotype similar to that of an Escher ichia coli tonB mutant. The genes encoding TonB1, ExbB1, ExbD1 were pa rt of an operon that included three haem transport genes (hutBCD), and all six genes appeared to be expressed from a single Fur-regulated pr omoter upstream of tonB1, A plasmid containing all six genes permitted utilization of haem by an E. coli strain expressing the V, cholerae h aem receptor, HutA, Analysis of the hut genes indicated that hutBCD, w hich are predicted to encode a periplasmic binding protein (HutB) and cytoplasmic membrane permease (HutC and HutD), were required to recons titute the V. cholerae haem transport system in E. coli. In V. cholera e, the presence of hutBCD stimulated growth when haemin was the iron s ource, but these genes were not essential for haemin utilization in V. cholerae.