The ability of two different ferredoxin reductases from Streptomyces coelicolor, to enhance the amount of active recombinant Streptomyces griseus soyC (CYP105D1) was investigated in both Escherichia coli and Streptomyces lividans. In E. coli a two-plasmid system and a single operon construct were used for expression of the CYP105D1 and the ferredoxin reductase(s) under the control of T7 promoters. Expression levels of CYP105D1 were found to range between 85 and 280 nmol l(-1) cell culture after prolonged growth. In S. lividans the CYP105D1 and its ferredoxin were cloned downstream of the Pact1 promoter in the E. coli/Streptomyces shuttle vector pBW160. The recombinant E. coli and S. lividans cells converted 7-ethoxycoumarin into 7-hydroxycoumarin efficiently. Expression of a ferredoxin reductase as an operon with CYP105D1 and its ferredoxin enhances the o-dealkylation of 7-ethoxycoumarin. Ferredoxin NADPH reductase was found to enhance the level of the active form of CYP105D1 monooxygenase when no substrate was present. (C) 2003 Elsevier Science Inc. All rights reserved.