Homocysteine and Cysteine Synthetases of Neurospora crassa

The enzymatic synthesis of homocysteine (from homoserine and H2S) and cysteine (from serine and H2S) has been demonstrated in Neurospora crassa. Purification and properties of the enzyme, or enzymes, are described. Both homocysteine and cysteine synthetase are subject to end product feedback control by methionine, which appears to be noncompetitive inhibition. Repression of both activities by methionine and repression of cysteine synthetase by cysteine and homocysteine synthetase by homocysteine were observed. Regulation of both activities by cystathionine via its cleavage products, cysteine or homocysteine, is described. Cystathionineless and homocysteineless mutants grown with limiting methionine have both homocysteine and cysteine synthetase, or synthetases, with specific activities much lower than that of wild type grown in the presence of equivalent methionine. When the mutants and wild type are grown in the presence of limiting homocysteine, the specific activities for cysteine synthetase in both mutants are lower than that in wild type, and homocysteine synthetase is nearly totally repressed in both mutants. Cysteine mutants have cysteine synthetase and homocysteine synthetase with specific activities of the same magnitude as those for wild type grown with equivalent cysteine supplement. The demonstration of homocysteine and cysteine synthetase, or synthetases, permits the concept of direct and separate biosynthetic pathways for cysteine and methionine in Neurospora and obviates the requirement for cystathionine for forward transsulfuration.