FALSE FEEDBACK INHIBITION OF AROMATIC AMINO ACID BIOSYNTHESIS BY BETA-2-THIENYLALANINE

1. 1. In Escherichia coli , β-2-thienylalanine drastically inhibited protein synthesis within 5 min. Shikimic acid reversed the inhibition non-competitively. The non-competitive nature of the inhibition was confirmed both by measuring the internal concentration of thienylalanine and by estimating it from the rate of thienylalanine incorporation into protein. 2. 2. The 3-deoxy- d - arabino -heptulusonic acid 7-phosphate (DAHP) synthetase normally inhibited by phenylalanine is the principal one present in exponentially growing cells in minimal medium. It was strongly inhibited by thienylalanine. There was no inhibition of the tyrosine-controlled DAHP synthetase. 3. 3. A mutant selected for resistance to thienylalanine had a normal amount of DAHP synthetase. About half of the enzyme was the tyrosine-repressible species, and the other half was insensitive to inhibition by phenylalanine or tyrosine, or repression by tyrosine or tryptophan. 4. 4. The K m for phenylalanine with phenylalanyl-RNA synthetase was 6·10 −6 M. The K i for thienylalanine inhibition of phenylalanine activation was approx. 2·10 −4 M. Although thienylalanine was incorporated into protein, transfer of thienylalanine to transfer RNA by the phenylalanyl-RNA synthetase could not be demonstrated in vitro . 5. 5. The data indicate that false feedback inhibition of the phenylalanine-sensitive DAHP synthetase was responsible for the immediate inhibition of protein synthesis by thienylalanine.