A comparison of DNA methylation levels in selected isolates of higher fungi

The extent of cytosine methylation in the DNAs of four basidiomycetes and two ascomycetes has been determined by RP-HPLC. The highest levels of 5-methyldeoxycytidine (5mdC) were found in the homobasidiomycetes Agaricus bisporus and Armillaria bulbosa (with 4.01 ± 0.93% and 4.34% ± 1.03% of the cytosine residues methylated, respectively). The two heterobasidiomycetes examined also contained 5mdC, but at lower levels; (2.26 ± 0.31% for Ustilago maydis and 2.14 ± 0.21% for U. violaceae). The DNA of the dimorphic ascomycete Ophiostoma novo-ulmi showed a 5mdC content ranging from 1.37 ± 0.11% to 1.67 ± 0.39%, depending on the isolate and the morphological stage. Methylated bases were not detected in Saccharomyces cerevisiae. No general correlation between strain degeneration and enhanced or decreased methylation could be found in the commercial mushroom A. bisporus. The possible implication of extensive DNA methylation in genetic transformation of homobasidiomycetes using the bacterial gene for hygromycin phosphotransferase as a selectable marker is discussed.