SUMMARY: possesses a biochemical pathway for tyrosine and phenylalanine biosynthesis, while and do not. The detection of 7-phospho-2-dehydro-3-deoxy---heptonate (DAHP) synthase (EC, dehydro-shikimate reductase (EC and 3--pyruvoylshikimate-5-phosphate synthase (EC activities in cell-free extracts established the presence in of a functional shikimate pathway. -Phenylalanine synthesis occurs solely through the phenylpyruvate route via prephenate dehydratase (EC, no arogenate dehydratase activity being found. Although arogenate dehydrogenase was detected, -tyrosine synthesis appears to occur mainly through the 4-hydroxyphenylpyruvate route, via prephenate dehydrogenase (EC, which utilized NAD as a preferred coenzyme substrate. -Tyrosine was found to be the key regulatory molecule governing aromatic biosynthesis. DAHP synthase was feedback inhibited by -tyrosine, but not by -phenylalanine or -tryptophan; -tyrosine was a potent feedback inhibitor of prephenate dehydrogenase and an allosteric activator of prephenate dehydratase. Chorismate mutase (EC was sensitive to product inhibition by prephenate. Prephenate dehydratase was feedback inhibited by -phenylalanine. It was also activated by hydrophobic amino acids (-valine, -isoleucine and -methionine), similar to results previously found in a number of other genera that share the Gram-positive line of phylogenetic descent. Aromatic-pathway-encoded cistrons present in saprophytic large-genome mycoplasmas may have been eliminated in the parasitic small-genome mycoplasmas.


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