Methionine synthesis in Neurospora. The isolation of cystathionine

Abstract

Among artificially produced biochemical mutants of Neurospora, those which have lost the ability to synthesize methionine form the largest class. At the present writing 87 occurrences of the methionineless character have been observed in this laboratory following treatment of wild type spores with high frequency radiations (1) or mustard gas (2). Methionineless mutants differ from wild type Neurospora in that they fail to grow on a medium containing only sugar, inorganic salts, and biotin, but do grow if, in addition to these constituents, methionine is supplied. In many of the mutants failure of methionine synthesis results from a block in the reduction of sulfate, which, except for a trace of biotin, is the sole source of sulfur in the basal medium. These strains can utilize reduced forms of inorganic sulfur for growth, as well as methionine and other organic sulfur compounds. On the other hand, some of the mutants require organically bound sulfur for growth, an indication that in these strains the block in methionine synthesis comes at a later stage than sulfate reduction. Similar classes of methionine-requiring mutants have been reported in the mold Ophiostoma by Fries (3) and in Escherichia coli by Lampen et al. (4-6).