Mouse DNA methylase: methylation of native DNA.

An improved method of purification of DNA methylase from Krebs II ascites cells is reported. The enzyme sediments at 8.3 S on glycerol-gradients and a major band on SDS polyacrylamide gel electrophoresis has a molecular weight of 184 000. Aggregation occurs at low salt and this may interfere with enzymic activity. The preferred double stranded DNA substrate is that rendered partially unmethylated by an in vitro repair mechanism or by isolation from methionine starved cells. Methylation of native partially methylated DNA is favoured under conditions of low salt and high temperature; conditions which encourage 'breathing' of the DNA. Methylation of native, unmethylated DNA also involves breathing but results in formation of a salt resistant tight binding complex between the enzyme and the DNA.