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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 -y-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-i and me-6 mutants to form cystathionine that has been previously reported is due to the absence of N5-methyltetrahydrofolate from these preparations. Extracts of me-i 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.The pathway of methionine biosynthesis in Neurospora crassa, shown in Scheme 1, consists of two converging branches. In one, CH3-THF is formed, and in the other, homocysteine. The two branches are joined with the methylation of homocysteine by CH3-THF to produce methionine. Studies of genes specifying enzymes of the pathway (Scheme I) have produced intriguing results. Mutants of me-1 and me-6, which were found to be deficient in CH2=THF reductase and folyl polyglutamate respectively (1, 2), were later found to be lacking also in the activity of an enzyme coded by the me-t and me-7 genes, cystathionine Sy-synthase3, an enzyme that catalyzes the replacement of the acetyl group of O-acetyl homoserine by cysteine. The present communication describes a study of the relationship between the me-1 and me-6 genes and cystathionine formation. One possible explanation of the failure of extracts of me-i mutants to synthesize cystathionine was that the type of folate that is missing from these strains, CH3-THF, is required by the enzyme catalyzing this process. This explanation was suggested by the observations that a) cystathionine y-synthase activity was reconstituted by pairwise mixing of sephadex-filtered extracts of me-i, me-3, me-6, and me-7 mutants in all combinations except me-i + me-6 (3), and b) trypsin inactivated the components needed to reactivate me-S and me-7, but not me-i. In the case of the me-6 mutants, it was necessary to assume that their CH3-THF was removed during the preparation of the extracts (crude extracts were filtered through Sephadex G-25 to remove S-adenosylmethionine, a feedback inhibitor of cystathionine 'y-synthase). The folate of the me-6 organisms differs from that of other strains, in that it consists largely of the monoglutamate rather than higher (5 or more glutamic acid residues) polyglutainates (2). The results of this investigation demonstrate that the cystathionine y-synthase of N. crassa has an absolute requirement for CH3-THF that is satisfied by either the monoglutamate or polyglutamate forms.

The presence in Escherichia coli extracts of a B12-independent methyltransferase capable of methylating homocysteine with methyl-H4-PteGlu3* was demonstrated by D. D. Woods and his associates (Guest, Friedman, Foster, Tejerina & Woods, 1964b). Similar enzymes have been reported to occur in other microorganisms (Woods,

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