Abstract
An enzyme activity of Bacillus subtilis has been found that catalyzes the dephosphorylation and deamination of phosphohomoserine to α-ketobutyrate, resulting in a bypass of threonine in isoleucine biosynthesis. In crude extracts of a strain deficient in the biosynthetic isoleucine-inhibitable threonine dehydratase, phosphohomoserine was converted to α-ketobutyrate. Phosphohomoserine conversion to α-ketobutyrate was shown not to involve a threonine intermediate. Single mutational events affecting threonine synthetase also affected the phosphohomoserine-deaminating activity, suggesting that the deamination of phosphohomoserine was catalyzed by the threonine synthetase enzyme. It was demonstrated in vivo, in a strain deficient in the biosynthetic threonine dehydratase, that isoleucine was synthesized from homoserine without intermediate formation of threonine.
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