Polyglutamylfolate synthesis in Neurospora crassa: changes in pool size following growth in glycine- and methionine-supplemented media

The effect of media supplements on total and polyglutamylfolate concentrations has been examined in Neurospora crassa wild type (FGSC 853), an ethionine-resistant mutant (FGSC 1212), and a methionine auxotroph (FGSC 1330) which lacks folylpolyglutamate synthetase. When the culture medium contained 1 mm glycine, folate concentrations in the wild type were increased by over 90% and more p-[³H]aminobenzoate was incorporated into folates. Growth in l-methionine-supplemented media (1–5 mm) decreased folate levels and labeling in all three strains. In the wild type, this effect of l-methionine was reversed on transfer to unsupplemented media but p-[³H]aminobenzoate pulse-chase experiments suggested that exogenous methionine did not increase the turnover of labeled folates. At 1 mm, d-methionine did not affect polyglutamylfolate labeling but l-methionine reduced ³H incorporation by 65% in the wild type. Ion-exchange chromatography showed that p-[³H]aminobenzoate was incorporated in formyl- and methyltetrahydrofolates which in the wild type, were principally hexaglutamyl derivatives. Glycine-supplemented growth yielded labeled folates that were 24% heptaglutamates but these and pentaglutamates were lacking when l-methionine was supplied. The specific activity of GTP cyclohydrolase was not significantly affected by culture in l-methionine-containing media. Dialysis and gel filtration both lowered enzyme activities and product formation was not changed when up to 10 μmol of l-methionine was added to the reaction system. The data suggest that methionine or its metabolic products exerts some control over folate production which is distinct from the established inhibition of methylenetetrahydrofolate reductase by AdoMet.