The Escherichia coli sulfur starvation utilization (ssu) operon includes a two-component monooxygenase system made up of an NADPH-dependent FMN reductase, SsuE, and a monooxygenase, SsuD. SsuE is part of the flavodoxin-like superfamily, and we report here the crystal structures of its apo, FMN-bound and FMNH₂-bound forms at ~2 Å resolution....