FIG. 8.

Model for heme utilization in V. cholerae. Heme crosses the outer membrane through one of three outer membrane receptors, HutA, HutR, or HasR (14, 21), and this transport requires the activity of one of the two TonB systems (24) (not shown). Heme is then transported across the inner membrane most likely by the HutBCD transport system (24). The fate of the heme moiety following transport is not known, although at least some of the heme is likely incorporated into cytochromes and other heme proteins.