One-carbon metabolism in Neurospora crassa wild-type and in mutants partially deficient in serine hydroxymethyltransferase

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Abstract
The concentrations of folate-dependent enzymes in N. crassa Lindegren A wild type (FGSC no. 853), Ser-l mutant, strain H605a (FGSC no. 118), and for mutant, strain C-24(FGSC no. 9), were compared during exponential growth on defined minimal media. Both mutants were partially lacking in serine hydroxymethyltransferase, but contained higher concentrations of 10-formyltetrahydrofolate synthetase than did the wild type. Mycelia of the mutants contained higher concentrations of these enzymes when growth media were supplemented with 1 mM-glycine. In the wild-type, this glycine supplement also increased the specific activities of 5,10-methylenetetrahydrofolate dehydrogenase and 5,10-methylenetetrahydrofolate reductase. During growth, total folate and polyglutamyl folate concentrations were greatest in the wild-type. Methylfolates were not detected in mutant Ser-l, and were only present in the for mutant after growth in glycine-supplemented media. Exogenous glycine increased folate concentration 3-fold in the wild type, mainly owing to increases in unsubstituted polyglutamyl derivatives. Feeding experiments using 14C-labeled substrates showed that C1 units were generated from formate, glycine and serine in the wild type. Greater incorporations of 14C occurred when mycelia were cultured in glycine-supplemented media. Formate and serine were precursors of C2 units in the mutants, but the ability to cleave glycine was slight or lacking.