Nucleotide sequence of the gene for the vitamin B12 receptor protein in the outer membrane of Escherichia coli.

The nucleotide sequence of a 2220-base-pair fragment containing the btuB gene of Escherichia coli was determined. There was a single open reading frame which was translated into a 614-amino-acid polypeptide, the first 20 amino acids of which comprised a typical leader sequence. The putative mature or processed form had a molecular weight (66,400) and a composition in close agreement with that determined for the purified receptor. The distribution of amino acids in the receptor protein was similar to that of other outer membrane proteins, showing a fairly even distribution of charged residues and the absence of extensive hydrophobic stretches. The btuB451 mutation appears to alter the receptor to eliminate its ability to function in vitamin B12 uptake without affecting its ligand binding properties. The sequence of the DNA from this mutant was determined and revealed a leucine-to-proline (C-to-T transition) change in the eighth amino acid of the mature form.