  The flavin reductase ActVB is involved in the last step of actinorhodin
biosynthesis in Streptomyces coelicolor. Although ActVB can be isolated with
some FMN bound, this form was not involved in the flavin reductase activity. By
studying the ferric reductase activity of ActVB, we show that its FMN-bound
form exhibits a proper enzymatic activity of reduction of iron complexes by
NADH. This shows that ActVB active site exhibits a dual property with regard to
the FMN. It can use it as a substrate that goes in and off the active site or
as a cofactor to provide an electron transferase activity to the polypeptide.
