Vitamin B12 transport in Escherichia coli K12 does not require the btuE gene of the btuCED operon.

Transport of vitamin B12 across the cytoplasmic membrane of Escherichia coli requires the products of btuC and btuD, two genes in the btuCED operon. The role of btuE, the central gene of this operon, was examined. Deletions within btuE were constructed by removal of internal restriction fragments and were crossed onto the chromosome by allelic replacement. In-frame deletions that removed 20% or 82% of the btuE coding region did not affect expression of the distal btuD gene. These nonpolar deletions had little effect on vitamin B12 binding (whole cells or periplasmic fraction) and transport. They did not affect the utilization of vitamin B12 or other cobalamins for methionine biosynthesis, even in strains with decreased outer membrane transport of vitamin B12. The btuE mutations did not impair adenosyl-cobalamin dependent catabolism of ethanolamine or repression of btuB expression. Thus, despite its genetic location in the transport operon, the btuE product plays no essential role in vitamin B12 transport.