Characterization of DNA methyltransferase from bovine thymus cells.

A DNA methyltransferase was partially purified from bovine thymus heavy cells. The enzyme has Mr 130 000, and introduces methyl groups from S-adenosylmethionine into the 5 position of cytosines in DNA. Sequence specificity analysis revealed that about 60% of the total methylation occurred in the 5'd(C-G)3' doublet. Single-stranded and hemi-methylated DNAs were methylated at an elevated rate by the enzyme. The kinetic analysis showed that the reaction obeys a random sequential mechanism. These results suggest that the enzyme serves primarily as a maintenance DNA methyltransferase.