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Microbiology 152 (2006), 397-404; DOI  10.1099/mic.0.28398-0
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Microbiology 152 (2006), 397-404; DOI  10.1099/mic.0.28398-0
© 2006 Society for General Microbiology

Homocysteine accumulation causes a defect in purine biosynthesis: further characterization of Schizosaccharomyces pombe methionine auxotrophs

Yasuko Fujita1,2, Eiko Ukena3, Haruyuki Iefuji3, Yuko Giga-Hama2 and Kaoru Takegawa1

1 Department of Life Sciences, Faculty of Agriculture, Kagawa University, Miki-cho, Kagawa 761-0795, Japan
2 Research Center, Asahi Glass Co. Ltd, Yokohama, Kanagawa 221-8755, Japan
3 National Research Institute of Brewing, Higashi-Hiroshima, Hiroshima 739-0046, Japan

Correspondence
Kaoru Takegawa
takegawa{at}ag.kagawa-u.ac.jp

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 {Delta}met26 cells restored growth in the absence of adenine. HPLC analysis showed that total homocysteine content in {Delta}met26 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.






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Copyright © 2006 Society for General Microbiology.