NAD(P)H:FMN oxidoreductase component of a two-component system involved in the biosynthesis of the side chain of the aminoglycoside antibiotics in the biosynthetic pathway of butirosin. Together with BtrO, mediates hydroxylation of gamma-L-Glu-GABA-S-BtrI. 4-(gamma-L-glutamylamino)butanoy-[BtrI ACP] + FMNH2 + O2 = 4-(gamma-L-glutamylamino)-(2S)-2-hydroxybutanoyl-[BtrI ACP] + FMN + H(+) + H2O kcat is 1243.13 min(-1) with NADH as substrate in the presence of FMN. kcat is 887.48 min(-1) with NADH as substrate in the presence of FAD. Antibiotic biosynthesis; butirosin biosynthesis. Homotetramer.