The fixA and fixB Genes Are Necessary for Anaerobic Carnitine Reduction in Escherichia coli

In Escherichia coli, the use of carnitine as a terminal electron acceptor depends on a functional caiTABCDE operon. It had been suggested that the adjacent but divergent fixABCX operon is also required for carnitine metabolism, perhaps to provide electrons for carnitine reduction. We have constructed E. coli fixA and fixB mutants and find that they are unable to reduce carnitine to γ-butyrobetaine under anaerobic conditions.