Passage of vitamin B12 across the outer and cytoplasmic membranes of Escherichia coli occurs in two steps, each involving independent transport systems. Since the vitamin accumulated in btuC or btuD mutants is readily released from the cell by chase or osmotic shock and does not undergo the usual metabolic conversions, the products of these genes might participate in transport across the cytoplasmic membrane. Mutations Several genetic loci in Escherichia coli are required for the high-affinity and irreversible uptake of vitamin B12 (cyanocobalamin) across the outer and cytoplasmic membranes (4). The btuB gene encodes the 66,000-Mr outer membrane receptor protein for cobalamin and cobinamide, the E colicins, and bacteriophage BF23 (6,9,15,18). The tonB product is necessary for the energy-dependent transfer of cobalamin from the receptor into the periplasm (3, 25). This protein is also required for siderophore-mediated iron uptake (reviewed in reference 22). These iron uptake systems also employ specific receptors in the outer membrane as well as other gene products that mediate transport steps after receptor binding (10,24,29).The btuCD locus, located next to himA at min 37.4 on the E. coli chromosome map (2)
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