REGULATION OF CHEMOAUTOTROPHIC METABOLISM .3. DAHP SYNTHETASE IN THIOBACILLUS-NEAPOLITANUS

DAHP synthetase (PODH lyase, EC4.1.2.15.) activity was demonstrated in undialysed and dialysed extracts of the wild type strain C and phenylalanine-resistant variant P4 of T. neapolitanus. Activity at pH 6.4 in extracts of both strains was inhibited at least 50% by 10-5 M phenylalanine. Strain C enzyme was inhibited at least 80% by 10-3M tyrosine, but was relatively unaffected by tryptophan. Tryptophan stimulated the P4 enzyme threefold at 10-5-10-4M. Inhibition of the P4 enzyme by phenylalanine could be virtually completely prevented by tyrosine or tryptophan, but these acids and histidine were much less effective in preventing inhibition of the strain C enzyme. Maximum activity in extracts of both strain C and P4 was obtained at pH 8.9, at which pH DAHP synthesis was 8 times greater than at pH 6.4. Activity at pH 8.9 in dialysed extracts of P4 showed KM values of 1.43×10-3 M and 4×10-3 M for E-4-P and PEP respectively. Ki values for competitive inhibition by l-phenylalanine were 5.4×10-6 M and 1.45×10-5 M respectively for ranges of concentration of E-4-P and PEP. Inhibition of the growth of strain C by phenylalanine was concluded to be due to prevention of tyrosine and tryptophan synthesis through inhibitiom of DAHP synthesis. Resistance to phenylalanine was conferred on the mutant P4 by its possession of a system by which inhibition of DAHP synthesis by phenylalanine was prevented by tyrosine and tryptophan. © 1969 Springer-Verlag.