Active genes in budding yeast display enhanced in vivo accessibility to foreign DNA methylases: a novel in vivo probe for chromatin structure of yeast.

Unlike higher eukaryotes, where an inverse correlation has been generally observed between gene expression and methylation of CpG sites, the budding yeast Saccharomyces cerevisiae lacks DNA methylation. Gene regulatory mechanisms can function independently of DNA methylation in yeast, and yeast strains expressing foreign DNA methylases that modify adenine and CpG residues have been found to be viable. We have used such strains to determine whether the transcriptional status of genes can influence the level of their DNA methylation in vivo. Several genes were tested, for example, GAL1, -7, and -10, PHO5, HMRa and HML alpha, and STE2 and STE3. Surprisingly, we found that all the genes displayed severalfold more methylation in the expressed state as compared to the repressed state. This procedure serves as a novel in vivo probe for the chromatin structure of yeast and potentially for higher eukaryotes.