Synchronization of Converging Metabolic Pathways: Activation of the Cystathionine γ-Synthase of Neurospora crassa by Methyltetrahydrofolate

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Abstract
Methyltetrahydrofolate synchronizes the activities of the two branches of the pathway of methionine biosynthesis in Neurospora crassa by serving as an essential activator of cystathionine gamma-synthase and antagonizing the feedback inhibition of this enzyme by S-adenosylmethionine. Activation is specific for the methylated form of folate and increases with increasing glutamate content. The inability of extracts of me-1 and me-6 mutants to form cystathionine that has been previously reported is due to the absence of N(5)-methyltetrahydrofolate from these preparations. Extracts of me-1 mutants lack methyltetrahydrofolate because the organisms are deficient in methylenetetrahydrofolate reductase, and those of me-6 because their methyltetrahydrofolate is quantitatively removed by the procedure employed in the preparation of extracts. The folate of the me-6 organisms differs from that of wild type strains in consisting largely of the monoglutamate rather than higher conjugates.