Mutations affecting the biosynthesis of S-adenosylmethionine cause reduction of DNA methylation in Neurospora crassa.
A temperature-sensitive methionine auxotroph of Neurospora crassa was found in a collection of conditional mutants and shown to be deficient in DNA methylation when grown under semipermissive conditions. The defective gene was identified as met-3, which encodes cystathionine-gamma-synthase. We explored the possibility that the methylation defect results from deficiency of S-adenosylmethionine (SAM), the presumptive methyl group donor. Methionine starvation of mutants from each of nine complementation groups in the methionine (met) pathway (met-1, met-2, met-3, met-5, met-6, met-8, met-9, met-10 and for) resulted in decreased DNA methylation while amino acid starvation, per se, did not. In most of the strains, including wild-type, intracellular SAM peaked during rapid growth (12-18 h after inoculation), whereas DNA methylation continued to increase. In met mutants starved for methionine, SAM levels were most reduced (3-11-fold) during rapid growth while the greatest reduction in DNA methylation levels occurred later. Addition of 3 mM methionine to cultures of met or cysteine-requiring (cys) mutants resulted in 5-28-fold increases in SAM, compared with wild-type, at a time when DNA methylation was reduced approximately 40%, suggesting that the decreased methylation during rapid growth in Neurospora is not due to limiting SAM. DNA methylation continued to increase in a cys-3 mutant that had stopped growing due to methionine starvation, suggesting that methylation is not obligatorily coupled to DNA replication in Neurospora.
Roberts
C J
CJ
Institute of Molecular Biology, University of Oregon, Eugene 97403, USA.
Selker
E U
EU
D000596:Amino Acids; D003470:Culture Media / Q000737:chemistry; D004271:DNA, Fungal / Q000378:metabolism*; D005838:Genotype; D008715:Methionine / Q000096:biosynthesis / Q000172:deficiency; D008745:Methylation; D009154:Mutation; D009492:Neurospora crassa / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D012436:S-Adenosylmethionine / Q000096:biosynthesis* / Q000172:deficiency; D013696:Temperature
D000596:Amino Acids; D003470:Culture Media; D004271:DNA, Fungal; D012436:S-Adenosylmethionine; D008715:Methionine