Gene sam1 encoding adenosylmethionine synthetase: effects of its expression in the fission yeast Schizosaccharomyces pombe

By screening gene libraries of Schizosaccharomyces pombe with a DNA fragmen t encoding part of the Saccharomyces cerevisiae S-adenosylmethionine synthe tase (SAMS), we isolated the fission yeast sam1 gene. Its sequence exhibits good homology to SAMSs of other organisms and reveals the motifs character istic for SAMSs. SAMS activity and sam1 mRNA levels decrease when cells ent er stationary phase. In haploid strains, gene sam1 is essential for growth; if weakly expressed, cells mate and sporulate at a reduced rate. Strains o verexpressing sam1 exhibit methionine-sensitive growth. This methionine-ind uced growth inhibition is partially relieved by adenine. We assume that met hionine reduces the level of one or several adenine nucleotides by a SAMS-m ediated mechanism. Intracellular SAM levels increase drastically by exogeno usly added methionine. This increase predicts that mutants exhibiting methi onine revertible phenotypes can be indicative for mutations in proteins exh ibiting SAM-dependent functions. In agreement with this prediction, we show that mutant pmt2-5 has this phenotype and that gene pmt2 encodes a potenti al SAM-dependent enzyme. Copyright (C) 2000 John Wiley & Sons, Ltd.