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

Abstract

By screening gene libraries of Schizosaccharomyces pombe with a DNA fragment encoding part of the Saccharomyces cerevisiae S-adenosylmethionine synthetase (SAMS), we isolated the ®ssion yeast sam1 gene. Its sequence exhibits good homology to SAMSs of other organisms and reveals the motifs characteristic for SAMSs. SAMS activity and sam1 mRNA levels decrease when cells enter stationary phase. In haploid strains, gene sam1 is essential for growth; if weakly expressed, cells mate and sporulate at a reduced rate. Strains overexpressing sam1 exhibit methionine-sensitive growth. This methionine-induced growth inhibition is partially relieved by adenine. We assume that methionine reduces the level of one or several adenine nucleotides by a SAMS-mediated mechanism. Intracellular SAM levels increase drastically by exogenously added methionine. This increase predicts that mutants exhibiting methionine revertible phenotypes can be indicative for mutations in proteins exhibiting SAM-dependent functions. In agreement with this prediction, we show that mutant pmt2±5 has this phenotype and that gene pmt2 encodes a potential SAM-dependent enzyme.

Fungal S-adenosylmethionine synthetase and the control of development and secondary metabolism in Aspergillus nidulans

“…The related A. flavus or A. oryzae possess two putative isoenzymes with only 55% amino acid identity, whereas the two S. cerevisiae isoenzymes are closely related with an identity of 91%. The single SAM synthetase gene sasA of A. nidulans is essential as it was also shown for the corresponding single sam1 gene of the fission yeast S. pombe (Hilti et al, 2000). In organisms with two SAM synthetase encoding genes, like S. cerevisiae, the deletion of one gene can be compensated by the other, and both genes need to be deleted to achieve lethality (Cherest and Surdin-Kerjan, 1978).…”

“…Only organisms which possess SAM feedback-inhibited SAM synthetases showed no overexpression phenotypes Corrales et al, 1992Corrales et al, , 1991Duce et al, 1988;Markham et al, 1983;Thomas and Surdin-Kerjan, 1991;Yarlett et al, 1993). The corresponding A. nidulans or S. pombe enzymes are not feedback-regulated (Hilti et al, 2000;Pieniazek et al, 1973). Exogenously added methionine induced sasA expression in A. nidulans and drastically increased the intracellular SAM levels in S. pombe.…”

“…This was surprising because overexpression of the SAM synthetase encoding gene ETH1 of the filamentous fungus N. crassa led to no phenotypical changes in growth rate or morphogenesis (Mautino et al, 1996). In the yeast S. pombe, low expression of sam1 reduced growth, mating and sporulation, while overexpression led to methionine sensitive growth which could be partially suppressed by addition of adenine (Hilti et al, 2000). Misregulated SAM synthetase encoding genes also affect plants where overexpression and suppression of Arabidopsis thaliana sam1 in tobacco led to yellow leaves, reduced plant height, necrotic lesions, and stunted plants with an altered secondary metabolism.…”

“…In contrast, the filamentous fungus Neurospora crassa shows no phenotypical changes when expression of SAM synthetase encoding genes is changed (Belbahri et al, 2000;Hilti et al, 2000;Mautino et al, 1996;Newman et al, 1998;Ochi and Freese, 1982).…”

Characterization of S-adenosylmethionine Synthetase Gene from Red Algae, Pyropia tenera

“…It plays a central role in most methylation of DNA, RNA, proteins, lipids, and polysaccharides in numerous enzymecatalyzed reactions (Hilti et al 2000).…”