Deoxyribonucleic acid methyltransferase from the eukaryote, Chlamydomonas reinhardi.

DNA methyltransferase was purified 310-fold from a green alga, Chlamydomonas reinhardi vegetative cells. The native enzyme of molecular weight 55 000--58 000 catalyzed the transfer of methyl groups from S-adenosylmethionine to the 5 position of cytosine in DNA. Native DNA accepted methyl groups 10-fold more than did denatured DNA. The sequence specificity analysis of methylated deoxycytidine in vitro revealed that the enzyme introduces methyl groups preferentially into sequences containing 5'd(T-mC-R)3'. Kinetic analysis of the reaction indicated that the enzyme obeys a random sequential mechanism. The extent of saturation with methyl groups depends upon the species from which the DNA was obtained. Kinetic analysis of the reaction catalyzed by RNA polymerase II has indicated that DNA methylation decreases the rate of initiation of RNA synthesis, but does not affect the rate of RNA chain elongation.