To investigate the flavin utilization by dibenzothiophene monooxygenase (DszC), DszC of a desulfurizing bacterium Mycobacterium goodii X7B was purified from the recombinant Escherichia coli. It was shown to be able to utilize either FMNH2 or FADH(2) when coupled with a flavin reductase that reduces either FMN or FAD. Sequence analysis indicated that DszC was similar to the C-2 component of p-hydroxyphenylacetate hydroxylase from Acinetobacter baumannii, which can use both FADH(2) and FMNH2 as substrates. Both flavins at high concentrations could inhibit the activity of DszC due to autocatalytic oxidation of reduced flavins. The results suggest that DszC should be reclassified as an FMNH2 and FADH(2) both-utilizing monooxygenase component and the flavins should be controlled at properly reduced levels to obtain optimal biodesulfurization results. (C) 2008 Elsevier Ltd. All rights reserved.