S-Adenosylmethionine Synthetase Is Required for Cell Growth, Maintenance of GO Phase, and Termination of Quiescence in Fission Yeast

S-adenosylmethionine is an important compound, because it serves as the methyl donor in most methyl transfer reactions, including methylation of proteins, nucleic acids, and lipids. However, cellular defects in the genetic disruption of S-adenosylmethionine synthesis are not well understood. Here, we report the isolation and characterization of temperature-sensitive mutants of fission yeast S-adenosylmethionine synthetase (Sarni). Levels of S-adenosylmethionine and methylated histone H3 were greatly diminished in saml mutants. saml mutants stopped proliferating in vegetative culture and arrested specifically in G2 phase without cell elongation. Furthermore, saml mutants lost viability during nitrogen starvation-induced G0 phase quiescence. After release from the G0 state, saml mutants could neither increase in cell size nor re-initiate DNA replication in the rich medium. Sarni is thus required for cell growth and proliferation, and maintenance of and exit from quiescence. saml mutants lead to broad cellular and drug response defects, as expected, since S. pombe contains more than 90 S-adenosylmethionine-dependent methyltransferases.