Vibrio cholerae iron transport: haem transport genes are linked to one of two sets of tonB, exbB, exbD genes

Abstract

SummaryVibrio cholerae was found to have two sets of genes encoding TonB, ExbB and ExbD proteins. The first set (tonB1, exbB1, exbD1) was obtained by complementation of a V. cholerae tonB mutant. In the mutant, a plasmid containing these genes permitted transport via the known V. cholerae high-affinity iron transport systems, including uptake of haem, vibriobactin and ferrichrome. When chromosomal mutations in exbB1 or exbD1 were introduced into a wild-type V. cholerae background, no defect in iron transport was noted, indicating the existence of additional genes 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 (tonB2, exbB2, exbD2 ). A chromosomal mutation 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 Escherichia coli tonB mutant. The genes encoding TonB1, ExbB1, ExbD1 were part of an operon that included three haem transport genes (hutBCD ), and all six genes appeared to be expressed from a single Fur-regulated promoter upstream of tonB1. A plasmid containing all six genes permitted utilization of haem by an E. coli strain expressing the V. cholerae haem receptor, HutA. Analysis of the hut genes indicated that hutBCD, which are predicted to encode a periplasmic binding protein (HutB) and cytoplasmic membrane permease (HutC and HutD), were required to reconstitute the V. cholerae haem transport system in E. coli. In V. cholerae, the presence of hutBCD stimulated growth when haemin was the iron source, but these genes were not essential for haemin utilization in V. cholerae.

Iron Transport Systems in Neisseria meningitidis

“…(vii) Why is there phase variation of the hemoglobin and haptoglobin-hemoglobin receptors? (viii) What is the mechanism of TonB-independent heme uptake reported for meningococci, and does there exist a second set of TonB-ExbBD proteins, as identified in species such as V. cholerae, P. aeruginosa, and S. marcescens, which functions in this process (52,105,109,157,178)? Answers to these questions will enhance our understanding of iron transport and utilization by N. meningitidis and perhaps lead to the development of an effective vaccine.…”

Genetics and Assembly Line Enzymology of Siderophore Biosynthesis in Bacteria

“…It is unclear why the genes for vibriobactin synthesis and transport are divided into two genetic loci, but the separation of genes that usually map together has been observed for other iron acquisition systems in V. cholerae. For example, the heme receptor gene hutA maps at a distance from the other heme transport genes (80).…”

“…This bacterium can also use iron contained in heme or hemoglobin and also produces an iron-regulated hemolysin which may intervene in iron acquisition in vivo (51,80,81,(137)(138)(139). V. cholerae can use ferri-ferrichrome as well (95).…”

Functional Differences between Heme Permeases: Serratia marcescens HemTUV Permease Exhibits a Narrower Substrate Specificity (Restricted to Heme) Than the Escherichia coli DppABCDF Peptide-Heme Permease

“…It is tempting to speculate that in these organisms heme is taken up by peptide permeases. In addition, hemTUV gene inactivation in various mutants, such as Y. enterocolitica, Yersinia pestis, and Vibrio cholerae mutants, does not abolish heme uptake, suggesting that in these mutants peptide permeases may take over from the HemTUV permeases (13,20,22). Genetic inactivation of each permease and of both types of permeases in these strains should help elucidate the relative role of each type of permease in heme uptake.…”