Regulation of folate-dependent enzyme levels in Aspergillus nidulans: studies with regulatory mutants.

The synthesis of folate-dependent enzymes in Aspergillus nidulans appears to be regulated by intracellular pools of homocysteine and methionine. The results are consistent with the view that homocysteine acts as an inducer and methionine as a corepressor, but the molecular mechanism of the regulation is still unknown. Methionine-requiring mutants, metH2 and metD10, apparently allelic, show deregulation of folate-dependent enzymes. Most characteristic of the mutants is a repressed level of folylpolyglutamate synthetase. New mutations suppressing the metH2 lesion which render folate enzymes insensitive to methionine-mediated repression have been isolated. These mutations are likely to identify new regulatory genes in folate metabolism.