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.