A phosphoserine phosphatase mutant of Neurospora crassa

Three pathways of serine biosynthesis are postulated to exist in Neurospora crassa. The three pathways are the phosphorylated, the nonphosphorylated, and the glyoxylate-serine pathway. Evidence now exists that the phosphorylated pathway may be the major pathway of serine biosynthesis in Neurospora crassa. A serine requiring mutant, ser(JBM 5), was found to have a lower specific activity of extractable phosphosering phosphatase, one of the enzymes in the phosphorylated pathway, than its isogenic wildtype. In this investigation, the serine requiring mutant, ser(JBM 5), and its isogenic wild-type strain of Neurospora crassa were tested for phosphoserine phosphatase activity. The Ames’ assay procedure and two modifications thereof were used to assay the extracts for phosphatase activity. The mutant had lower phosphoserine phosphatase activity per gm extract than the wild-type, suggesting that the phosphorylated pathway is the major pathway of serine biosynthesis.