MioC is an FMN-binding protein that is essential for Escherichia coli biotin synthase activity in vitro.

Biotin synthase is required for the conversion of dethiobiotin to biotin and requires a number of accessory proteins and small molecule cofactors for activity in vitro. We have previously identified two of these proteins as flavodoxin and ferredoxin (flavodoxin) NADP(+) reductase. We now report the identification of MioC as a third essential protein, together with its cloning, purification, and characterization. Purified MioC has a UV-visible spectrum characteristic of a flavoprotein and contains flavin mononucleotide. The presence of flavin mononucleotide and the primary sequence similarity to flavodoxin suggest that MioC may function as an electron transport protein. The role of MioC in the biotin synthase reaction is discussed, and the structure and function of MioC is compared with that of flavodoxin.