Catalyses two steps in the biosynthesis of the side chain of the aminoglycoside antibiotics of the butirosin family. The enzyme adds one molecule of?L-glutamate to a dedicated acyl-carrier protein, and following decarboxylation of the product by EC?4.1.1.95,?L-glutamyl-[BtrI acyl-carrier protein] decarboxylase, adds a second?L-glutamate molecule. Requires Mg2+?or Mn2+, and activity is enhanced in the presence of Mn2+.