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 de fective gene was identified as met-3, which encodes cystathionine-gamm a-synthase. We explored the possibility that the methylation defect re sults from deficiency of S-adenosylmethionine (SAM), the presumptive m ethyl 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 decreas ed DNA methylation while amino acid starvation, per se, did not. In mo st of the strains, including wild-type, intracellular SAM peaked durin g rapid growth (12-18 h after inoculation), whereas DNA methylation co ntinued 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 m ethionine to cultures of met or cysteine-requiring (cys) mutants resul ted in 5-28-fold increases in SAM, compared with wild-type, at a time when DNA methylation was reduced similar to 40%, suggesting that the d ecreased methylation during rapid growth in Neurospora is not due to l imiting SAM. DNA methylation continued to increase in a cys-3 mutant t hat had stopped growing due to methionine starvation, suggesting that methylation is not obligatorily coupled to DNA replication in Neurospo ra.