DNA methylase activity was detected in nuclei from pea shoots. The enzyme can only be extracted by low-salt treatment if the nuclei are pretreated with micrococcal nuclease. Only a single enzyme was detected, and it was purified to a specific activity of 1620 units/mg of protein. It has an Mr of 160,000 on gel filtration and SDS/PAGE. Pea DNA methylase methylates cytosine in all four dinucleotides, and this is interpreted to show that it acts on CNG trinucleotides. Although it shows a strong preference for hemi-methylated double-stranded DNA, it is also capable of methylation de novo. Homologous DNA is the best natural substrate. In vitro the enzyme interacts with DNA to form a salt-resistant complex with DNA that is stable for at least 4 h.

No abstract

DNA methylases from mouse and pea have been purified and characterized. Both are high molecular mass enzymes that show greater activity with hemimethylated than unmethylated substrate DNA. Both methylate cytosines in CpG preferentially, but not exclusively and show similar kinetics of methylation, which makes it difficult to saturate all possible sites on the DNA, but procedures are described that circumvent this problem.