Involved in the dimethyl sulfide degradation pathway (PubMed:12835925, PubMed:12686641). Catalyzes the NADH-dependent reduction of FMN (PubMed:12835925, PubMed:15661012). FMNH2 + NAD(+) = FMN + 2 H(+) + NADH Under sulfate limitation conditions, it is transcriptionally activated by the LysR-type transcriptional regulator, CysB. Belongs to the SsuE family.