The nonheme iron oxygenase VioC fromStreptomyces vinaceuscatalyzes Fe(II)-dependent and a-ketoglutarate-dependent Cb-hydroxylation of l-arginine during the biosynthesis of the tuberactinomycin antibiotic vio-mycin. Crystal structures of VioC were determined in complexes with the cofactor Fe(II), the substrate l-arginine, the product (2S,3S)-hydroxyargi-nine and the coproduct succinate at – A˚ resolution.