Transport of Vitamin B 12 in Escherichia coli : Genetic Studies

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Abstract
The chromosomal location of two genetic loci involved in the transport of cyanocobalamin (B 12 ) in Escherichia coli K-12 was determined. One gene, btuA , is believed to code for the transport protein in the cytoplasmic membrane, because a mutant with an alteration in this gene has lost the ability to accumulate B 12 within the cell although normal levels of the surface receptors for B 12 are present. The other locus, btuB , apparently codes for the surface receptor on the outer membrane. These mutants have lost the ability to bind B 12 and have greatly reduced transport activity, although growth experiments have shown that they can utilize B 12 for growth, but with decreased efficiency. This surface receptor for B 12 also appears to function as the receptor for the E colicins, because btuB mutants are resistant to the E colicins, and mutants selected for resistance to colicin E1 are defective in B 12 binding and transport. The gene order was determined by transduction analysis to be cyc-argH-btuA-btuB-rif-purD . In addition, mutations in metH , the gene for the B 12 -dependent homocysteine methylating enzyme, were obtained in this study. This gene was localized between metA and malB .