Aminoglycoside acyltransferase that attaches the (S)-4-amino-2-hydroxybutyrate (AHBA) side chain from the acyl carrier protein BtrI to the aminoglycoside ribostamycin in the biosynthetic pathway of butirosin. The AHBA side chain protects the antibiotic from several common resistance mechanisms. 4-(gamma-L-glutamylamino)-(2S)-2-hydroxybutanoyl-[BtrI ACP] + ribostamycin = gamma-L-glutamyl-butirosin B + H(+) + holo-[BtrI ACP] Antibiotic biosynthesis; butirosin biosynthesis.