Repression by vitamin B12 of the cobalamin transport protein BtuB in the outer membrane of Escherichia coli operates at both the transcriptional and translational levels and is controlled by transcribed sequences within the leader and proximal portion of the btuB coding sequence. The effects of deletions from either end of this region on repression and expression were determined with lac fusions. An element at the 5' end of the transcript and the putative attenuator within the coding sequence were required for transcriptional repression. The presence of either element caused a marked reduction in btuB-lacZ expression which was reversed by the presence of a conserved sequence element in the leader, suggesting the importance of long-range interactions in the btuB leader for expression and regulation.