Role of cytosolic serine hydroxymethyltransferase in one-carbon metabolism in Neurospora crassa.

Conidiospores of wild type and two mutant strains of Neurospora crassa were grown on [3-13C]serine, [2-13C]glycine, or [13C]formate. Acid extracts of the mycelia were analyzed by 13C NMR for incorporation of the 13C label into choline, serine, and adenine. The goal was to elucidate the function of cytosolic serine hydroxymethyltransferase by comparison of a mutant strain lacking this activity and requiring formate for optimal growth (for mutant strain) to the wild-type strain and the ser3 strain which cannot convert glucose to serine. The results for both the wild-type and ser3 strains showed that the one-carbon adduct of the cytosolic pool of methylenetetrahydrofolate is formed primarily and preferably from C3 of serine. Both organisms could form methylenetetrahydrofolate from formate in the absence of serine and glycine. However, the for mutant strain was restricted in its ability to form methylenetetrahydrofolate from C3 of serine, preferring formate as the one-carbon source. All three strains had an active glycine cleavage complex and mitochondrial serine hydroxymethyltransferase. The formate requirement of the for mutant strain appears to be the result of the inability to form formate in the mitochondria from serine or glycine at a rate sufficient to sustain the biosynthetic pools of methylenetetrahydrofolate and 10-formyltetrahydrofolate in the cytosol. All three strains rapidly accumulated serine from the media to form high intracellular levels of this substrate. However, these strains did not accumulate either glycine or formate from the media at levels that could be detected by 13C NMR.