Glycine synthesis in a Neurospora mutant deficient in serine hydroxymethyltransferase

Serine hydroxymethyltransferase (SHMT), commonly implicatedin the glycine synthesis of eucaryotes, was examined in Neurosporacrassa, wild type (FGSC 853) and a formate-requiring mutant(FGSC 9). The mutant was SHMT-deficient, containing only 15%of the total activity found in the wild type. Differential anddensity gradient centrifugations showed the mutant to be deficientin soluble SHMT activity. Both strains contained particulateSHMT which sedimented with mitochondrial marker enzymes. The origins of glycine were examined by a combination of enzyme,growth and ¹⁴C feeding experiments. Growth of the mutant wasstrongly inhibited by the isocitrate lyase-directed inhibitoritaconate. This inhibition was reduced when exogenous glycinewas supplied. Itaconate (up to 30 mM) did not inhibit growthof the wild type but in both strains isocitrate lyase activitieswere reduced. The mutant contained more lyase and glyoxylateaminotransferase than the wild type. In feeding experiments,[2-¹⁴C]acetate and [l-¹⁴C]glyoxylate were more readily incorporatedinto glycine in the mutant than the wild type. Itaconate (30mM) reduced the flow of acetate carbon into glycine by up to70% in the mutant. It is concluded that deficiency in solubleSHMT necessitates glycine synthesis via an isocitrateglyoxylateglycinesequence. (Received December 24, 1979; )