Figure 5.

The biotin regulatory system of E. coli. BirA is represented by green ovals, biotin by black circles, the AMP moiety by red pentagons, AccB by dark blue ovals and AccC by light blue crescents. The arrows denote transcription from the leftward and rightward bio promoters. (A to C) BirA switches from biotin ligation function to repressor function in response to the intracellular biotin requirement which is monitored by the level of unbiotinylated AccB. If the levels unbiotinylated AccB are high, the protein functions as a biotin ligase. Once the unbiotinylated AccB has been converted to the biotinylated form, the bio-AMP is no longer consumed and remains bound to BirA. This liganded form of BirA accumulates to levels sufficiently high to form dimers that fully occupy the bio operator iresulting in transcriptional repression of the biotin biosynthetic genes. (D) Overproduction of AccC ties up unbiotinylated AccB into 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.