GabT and GabD activities in wild-type, ΔgabC, and Δnac strains. Cells were grown with each of the indicated nitrogen sources at 0.2%. The errors bars are the standard errors of three to five determinations. Abbreviations: NH3, ammonia; S, serine; A, alanine; P, proline; D, aspartate; GABA+D, GABA plus aspartate; Q, glutamine; R, arginine; PUTR, putrescine.