Methionine synthesis in Neurospora crassa.

Horowitz and co-workers (Horowitz, 1947; Teas, Horowitz & Fling, 1948; Fling & Horowitz, 1951) have determined the pathway of methionine synthesis in Neurospora crassa, from genetic analysis of methionine-requiring mutant strains; it involves conversion of cysteine, via cystathionine, into homocysteine, which is methylated to methionine. A similar scheme has been suggested for methionine synthesis in Escherichia coli (Simmonds, 1948; Davis & Mingioli, 1950). The involvement of vitamin B12 in the last step, the methylation of homocysteine, has been clearly indicated in E. coli (Davis & Mingioli, 1950; Gibson & Woods, 1960) as well as in the vitaminB12-requiring protozoan Ochromonas malhamensis (Johnson, Holdsworth, Potter & Kon, 1957; Dalal, Rege & Sreenivasan, 1961). As vitamin B12 is not known to be involved in the metabolism of N. crassa, it was considered of interest to study the biosynthesis of methionine from homocysteine in this organism.