Pyridoxal phosphate-dependent decarboxylase that catalyzes 1 step in the biosynthesis of the side chain of the aminoglycoside antibiotics in the biosynthetic pathway of butirosin. Able to decarboxylate L-ornithine, L-arginine, L-lysine, but not L-glutamate or any D-amino acids. Has low activity with substrates not bound to an acyl-carrier protein. gamma-L-glutamyl-[BtrI ACP] + H(+) = 4-aminobutanoyl-[BtrI ACP] + CO2 Antibiotic biosynthesis; butirosin biosynthesis. Homodimer. Belongs to the Orn/Lys/Arg decarboxylase class-II family. Extended N-terminus.