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Transport of Vitamin B₁₂ in Escherichia coli: Genetic Studies

¹ Laboratory of Microbial Genetics, Department of Microbiology, University of Virginia School of Medicine, Charlottesville, Virginia 22901

ABSTRACT

The chromosomal location of two genetic loci involved in the transport of cyanocobalamin (B₁₂) 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 B₁₂ within the cell although normal levels of the surface receptors for B₁₂ are present. The other locus, btuB, apparently codes for the surface receptor on the outer membrane. These mutants have lost the ability to bind B₁₂ and have greatly reduced transport activity, although growth experiments have shown that they can utilize B₁₂ for growth, but with decreased efficiency. This surface receptor for B₁₂ 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 B₁₂ 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 B₁₂-dependent homocysteine methylating enzyme, were obtained in this study. This gene was localized between metA and malB.

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