Prototrophic revertants of a metH2 strain of Aspergillus nidulans which is impaired in the regulation of synthesis of folate-dependent enzymes were isolated and six of them analysed. In three of the isolates reversion was the result of an intragenic suppressor mutation in the metH locus. In the remaining strains suppressor mutations occurred in independent genes. These genes, designated folA, folB and folC, are linked and located in chromosome VI. Mutations in these genes render synthesis of some folate enzymes, particularly folylpolyglutamate synthetase, insensitive to methionine-mediated repression.
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