tightly controlled. In the model bacteria E. coli, the balance of biotin
demand versus supply is maintained through the action of the biotin
retention protein A (BirA); a bi-functional protein that is not only
a transcriptional repressor but also serves as the biotin ligase that
catalyzes the attachment of biotin onto the biotin-dependent car-
boxylases. In other microorganisms, such as Corynebacterium
glutamicum and Agrobacterium tumefaciens, there is no BirA homolog
to regulate biotin synthesis and transport. Instead, alternative DNA-
binding proteins perform this function, namely BioQ and BioR
respectively. The mechanisms by which BirA, BioQ and BioR regu-
late biotin biosynthesis and transport will be discussed in this review.