DNA Methyltransferases as Probes for Chromatin Structure in Yeast

Expression of DNA methyltransferases (MTases) in yeast, which has a naturally unmethylated genome, enables the study of chromatin structure in intact cells. Initial studies, employing in vivo expression of dam MTase, which catalyzes the production of N⁶ -methyladenine (^6me A) at GATC sites, were limited by the occurrence of a target site only once every ∼300 bp in native yeast sequences (1 ,2 ). This limited resolution of dam MTase, although possible to circumvent through the introduction of additional target sites (3 ), poses an obvious barrier to its usefulness. In addition, with no existing genomic sequencing methodology for detection of ^6me A, the methylation status of potential target sites must be ascertained with methylation-sensitive restriction endonucleases (e.g., Dpn I and Dpn II).