FIG. 1.

Biotin regulatory system of E. coli. BirA is represented by green ovals, biotin is represented by black circles, the AMP moiety is represented by red pentagons, AccB is represented by dark blue ovals, and AccC is represented by light blue crescents. The arrows indicate transcription from the leftward and rightward bio promoters. (A to C) BirA switches from the biotin ligation function to the repressor function in response to the intracellular biotin requirement, which is indicated by the level of unbiotinylated AccB. If unbiotinylated AccB levels are high, the protein functions as a biotin ligase. Once the unbiotinylated AccB has been converted to the biotinylated form, the biotinoyl-5′-AMP is no longer consumed and remains bound to BirA. The liganded form of BirA accumulates to levels sufficiently high that the bio operator is fully occupied, resulting in transcriptional repression of the biotin biosynthetic genes. (D) Overproduction of AccC ties up unbiotinylated AccB in a complex that is a poor biotinylation substrate. Therefore, high levels of the liganded form of BirA accumulate, resulting in repression of bio operon transcription.