Regulation of the tryptophan synthetic enzymes in Clostridium butyricum

Homologous and hybrid complexes of anthranilate synthase from Bacillus species

The subunits of anthranilate synthase were separated and partially purified by Sephadex G-100 gel filtration from the following six species of Bacillus: Bacillus subtilis, Bacillus licheniformis, Bacillus alvei, Bacillus coagulans, Bacillus pumilus, and Bacillus mascerans. Our data suggest that the enzyme from B. alvei is unique among these species. First, the anthranilate synthase complexes are readily dissociated during gel filtration in the absence of glutamine into a large component (aminotransferase), subunit E, and a small component subunit X (glutamine-binding protein), whereas a higher salt concentration is required to dissociate the complex from B. alvei. Second, the aminotransferase activity from all six species is stimulated by glycerol and inhibited by tryptophan; however, only the large component from B. alvei is stimulated by 2-mercaptoethanol. Finally, the large component can be titrated with the small component to yield a complex which can utilize glutamine as a substrate (amidotransferase). The homologous complexes have an amidotransferase to aminotransferase ratio of 1.4 to 2.3, but the B. alvei complex has a ratio of 0.9. Except for complexes that involve the large component from B. alvei, hybrid complexes can be formed which have ratios as good as the homologous complexes. These data are consistent with the hypothesis that B. alvei is unique among the bacilli with respect to some enzymes in the aromatic amino acid biosynthetic pathway.

Regulation of a ligand-mediated association-dissociation system of anthranilate synthesis in Clostridium butyricum

The anthranilate synthetase of Clostridium butyricum is composed of two nonidentical subunits of unequal size. An enzyme complex consisting of both subunits is required for glutamine utilization in the formation of anthranilic acid. Formation of anthranilate will proceed in the presence of partially pure subunit I provided ammonia is available in place of glutamine. Partially pure subunit II neither catalyzes the formation of anthranilate nor possesses anthranilate-5-phosphoribosylpyrophosphate phosphoribosyltransferase activity. The enzyme complex is stabilized by high subunit concentrations and by the presence of glutamine. High KCl concentrations promote dissociation of the enzyme into its component subunits. The synthesis of subunits I and II is coordinately controlled with the synthesis of the enzymes mediating reactions 4 and 5 of the tryptophan pathway. When using gel filtration procedures, the molecular weights of the large (I) and small (II) subunits were estimated to be 127,000 and 15,000, respectively. Partially pure anthranilate synthetase subunits were obtained from two spontaneous mutants resistant to growth inhibition by 5-methyltryptophan. One mutant, strain mtr-8, possessed an anthranilate synthetase that was resistant to feedback inhibition by tryptophan and by three tryptophan analogues: 5-methyl-tryptophan, 4- and 5-fluorotryptophan. Reconstruction experiments carried out by using partially purified enzyme subunits obtained from wild-type, mutant mtr-8 and mutant mtr-4 cells indicate that resistance of the enzyme from mutant mtr-8 to feedback inhibition by tryptophan or its analogues was the result of an alteration in the large (I) subunit. Mutant mtr-8 incorporates [(14)C]tryptophan into cell protein at a rate comparable with wild-type cells. Mutant mtr-4 failed to incorporate significant amounts of [(14)C]tryptophan into cell protein. We conclude that strain mtr-4 is resistant to growth inhibition by 5-methyltryptophan because it fails to transport the analogue into the cell. Although mutant mtr-8 was isolated as a spontaneous mutant having two different properties (altered regulatory properties and an anthranilate synthetase with altered sensitivity to feedback inhibition), we have no direct evidence that this was the result of a single mutational event.

Enzymes of the tryptophan pathway in three Bacillus species

The tryptophan synthetic pathway was characterized in three species of Bacillus, B. subtilis, B. pumilus, and B. alvei. They share the common features of a pathway which is subject to tryptophan repression, contains no unexpected complexes among the five enzymes, exhibits dissociable anthranilate synthase enzymes which do not require phosphoribosyl transferase for amidetransfer activity, contains separate indoleglycerol phosphate synthase and phosphoribosylanthranilate isomerase enzymes, and contains similar tryptophan synthetase multimers. In looking at these characteristics in detail however, differences among the three species became apparent, as, for example, in the complementation observed between the alpha and beta(2) components of tryptophan synthetase, and the dissociation patterns of the large and small components of anthranilate synthase. The results demonstrate some pitfalls in attempting to compare multimeric enzymes in crude extracts from different organisms.