title: Homocysteine accumulation causes a defect in purine biosynthesis: further characterization of Schizosaccharomyces pombe methionine auxotrophs.
abstract: Methionine synthase (EC2.1.1.14) catalyses the final step in methionine synthesis, i.e. methylation of homocysteine. A search of the Schizosaccharomyces pombe genomic database revealed a gene designated SPAC9.09, encoding a protein with significant homology to methionine synthase. Disruption of SPAC9.09 caused methionine auxotrophy, and thus the gene was identified as a methionine synthase and designated met26. The met26 mutant was found to exhibit a remarkable growth defect in the absence of adenine even in medium supplemented with methionine. This phenotype was not observed in other methionine auxotrophs. In the budding yeast Saccharomyces cerevisiae, which has been reported to utilize homocysteine in cysteine synthesis, lack of a functional methionine synthase did not cause a requirement for adenine. The introduction of genes from Sac. cerevisiae constituting the cystathionine pathway (CYS4 and CYS3) into Sch. pombe Deltamet26 cells restored growth in the absence of adenine. HPLC analysis showed that total homocysteine content in Deltamet26 cells was higher than in other methionine auxotrophs and that introduction of the Sac. cerevisiae cystathionine pathway decreased total homocysteine levels. These data demonstrate that accumulation of homocysteine causes a defect in purine biosynthesis in the met26 mutant.
authors.affiliation: Research Center, Asahi Glass Co. Ltd, Yokohama, Kanagawa 221-8755, Japan.|Department of Life Sciences, Faculty of Agriculture, Kagawa University, Miki-cho, Kagawa 761-0795, Japan.
authors.forename: Yasuko
authors.initials: Y
authors.lastname: Fujita
authors.affiliation: National Research Institute of Brewing, Higashi-Hiroshima, Hiroshima 739-0046, Japan.
authors.forename: Eiko
authors.initials: E
authors.lastname: Ukena
authors.affiliation: National Research Institute of Brewing, Higashi-Hiroshima, Hiroshima 739-0046, Japan.
authors.forename: Haruyuki
authors.initials: H
authors.lastname: Iefuji
authors.affiliation: Research Center, Asahi Glass Co. Ltd, Yokohama, Kanagawa 221-8755, Japan.
authors.forename: Yuko
authors.initials: Y
authors.lastname: Giga-Hama
authors.affiliation: Department of Life Sciences, Faculty of Agriculture, Kagawa University, Miki-cho, Kagawa 761-0795, Japan.
authors.forename: Kaoru
authors.initials: K
authors.lastname: Takegawa
mesh_terms: D015966:Gene Expression Regulation, Fungal; D006710:Homocysteine / Q000096:biosynthesis*; D006711:Homocystine / Q000378:metabolism; D008715:Methionine / Q000096:biosynthesis*; D011687:Purines / Q000096:biosynthesis*; D012568:Schizosaccharomyces / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D029702:Schizosaccharomyces pombe Proteins
chemical_list: D011687:Purines; D029702:Schizosaccharomyces pombe Proteins; D006710:Homocysteine; D006711:Homocystine; D008715:Methionine; C030985:purine