Mutations affecting the biosynthesis of S-adenosylmethionine cause reduction of DNA methylation inNeurospora crassa

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Abstract
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 cystathionlne-γ-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 (mef) 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 mef 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 cystelne-requiring (cys) mutants resulted in 5–28-fold increases in SAM, compared with wild-type, at a time when DNA methylation was reduced −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.