Vibrio choleraeVciB Promotes Iron Uptake via Ferrous Iron Transporters

Abstract

Vibrio cholerae uses a variety of strategies for obtaining iron in its diverse environments. In this study we report the identification of a novel iron utilization protein in V. cholerae, VciB. The vciB gene and its linked gene, vciA, were isolated in a screen for V. cholerae genes that permitted growth of an Escherichia coli siderophore mutant in low-iron medium. The vciAB operon encodes a predicted TonB-dependent outer membrane receptor, VciA, and a putative inner membrane protein, VciB. VciB, but not VciA, was required for growth stimulation of E. coli and Shigella flexneri strains in low-iron medium. Consistent with these findings, TonB was not needed for VciB-mediated growth. No growth enhancement was seen when vciB was expressed in an E. coli or S. flexneri strain defective for the ferrous iron transporter Feo. Supplying the E. coli feo mutant with a plasmid encoding either E. coli or V. cholerae Feo, or the S. flexneri ferrous iron transport system Sit, restored VciB-mediated growth; however, no stimulation was seen when either of the ferric uptake systems V. cholerae Fbp and Haemophilus influenzae Hit was expressed. These data indicate that VciB functions by promoting iron uptake via a ferrous, but not ferric, iron transport system. VciB-dependent iron accumulation via Feo was demonstrated directly in iron transport assays using radiolabeled iron. A V. cholerae vciB mutant did not exhibit any growth defects in either in vitro or in vivo assays, possibly due to the presence of other systems with overlapping functions in this pathogen.

Nonredundant Roles of Iron Acquisition Systems in Vibrio cholerae

“…Growth of the null strain anaerobically, but not aerobically, suggests that ferrous iron is the likely ligand for the system. Feo also transports only ferrous iron, but unlike the null mutant, vFeo grows in the presence of oxygen, indicating that ferrous iron is available aerobically, possibly due to the presence of the VciB protein, which was proposed to act as a ferric iron reductase under aerobic conditions (49). This suggests that it is not a lack of ferrous iron that prevents the growth of the null mutant in oxygen.…”

Vibrio Iron Transport: Evolutionary Adaptation to Life in Multiple Environments

“…No ferrous iron transporter has been identified in the outer membrane. The ferrous iron may diffuse through porins or other relatively nonspecific channels, consistent with the lack of a requirement for TonB for Feo-mediated iron transport (152).…”

“…The transport of ferrous iron and growth of bacteria are increased when VciB is present in addition to a ferrous iron transport system, including V. cholerae Feo, E. coli Feo, or Shigella flexneri Sit (152). The lack of specificity for V. cholerae Feo suggests that VciB does not recognize and interact with the ferrous iron transporter but may serve a more general function, such as in the reduction of ferric iron to ferrous iron for subsequent transport.…”

“…In V. cholerae, ferrous iron transport is facilitated by a cytoplasmic membrane protein designated VciB (152). The transport of ferrous iron and growth of bacteria are increased when VciB is present in addition to a ferrous iron transport system, including V. cholerae Feo, E. coli Feo, or Shigella flexneri Sit (152).…”

“…Similarly, a mutant defective in heme transport had no effect in this model (120), although a mutant defective in both the siderophore and heme iron uptake systems showed some reduction in colonization (198). A mutant defective in both the TonB1 and TonB2 systems also had reduced fitness in the infant mouse model (135), but testing of mutants defective in each of the known iron transport systems has not revealed any single system that is essential for V. cholerae colonization in the mouse model (148,152,198). Because V. cholerae has so many iron transport systems, there may be compensation for the loss of any one system.…”