Mammalian DNA-cytosine 5-methyltransferases methylate cytosines in deoxyinosine containing DNA polymers more rapidly than in other synthetic or naturally occurring DNAs. The initial methylation rate of poly(dI-dC)·poly(dI-dC) is about 10-times higher than that of poly-(dG-dC)·poly(dG-dC) or of the native Micrococcus luteus DNA. In competitive binding experiments, DNA methyltransferase has about 10-fold higher affinity for the dI-containing alternating DNA polymer than for poly(dG-dC) · poly(dG-dC). The observed high methyl accepting capacity of poly(dI-dC) · poly(dI-dC) may be a useful methodological advance to determine de novo DNA methyltransferase activity in extracts of mammalian cells.
