Regulation of a common amidotransferase subunit

In Bacillus subtilis the trpX locus specifies a glutamine-binding protein designated subunit X, which forms a complex with subunit E to constitute the anthranilate synthase enzyme aggregate (EX) and subunit A to constitute the p-aminobenzoate synthase enzyme aggregate (AX). Subunit X confers upon these enzyme complexes the ability to utilize glutamine as a substrate. The trpX locus has been examined to determine its map position and control. (i) The trpX locus was found to be cotransformed with the lysS and pabA loci. The results of three-factor transformation analyses suggest the following order of these markers: lysS-sul-trpX-pabA. (ii) Mutation to constitutivity of the tryptophan operon resulted in a 50- to 60-fold increase in the level of subunit X when the mutant contained functional trE and abA gene products; however, in the absence of subunit E there was only a 4- to 5-fold increase in the glutamine-binding protein. (iii) Formation of subunit X was derepressed under conditions that allow for the derepression of the trpE and/or pabA loci. (iv) Subunit X synthesis was derepressed to a greater extent in mutants that contain a functional trpE gene product than in mutants that contain a nonsense mutation in the trpE locus. These results are consistent with the hypothesis that the trpE and pabA gene products affect the expression and control of the trpX locus.