Differential activity of DNA methyltransferase in the life cycle of Chlamydomonas reinhardi.

Abstract

Two MW forms of DNA (cytosine-5-)-methyltransferase [S-adenosyl-L-methionine:DNA (cytosine-5-)-methyltransferase, EC 2.1.1.37], both active in assays in vitro, were isolated from the green alga, C. reinhardi, at various stages of the life cycle. The enzyme with MW 60,000 was found in vegetative cells and gametes of male (mt-) and female (mt+) mating types. The enzyme with MW 200,000 was specific to gametic cells and zygotes, which are the only stages at which methylation of chloroplast DNA occurs in vivo. Chloroplast DNA from gametes was methylated on both strands at most, if not all, methylation sites and the MW 200,000 enzyme methylated both unmethylated and hemimethylated sites, the latter at an elevated rate. Micrococcus luteus DNA showed the same nearest-neighbor frequencies of methylation after methylation by each MW component. The MW 200,000 enzyme is apparently the active multimeric form of the MW 60,000 enzyme; it probably acts as both initiation and maintenance methylase. Methylation of chloroplast DNA in female gametes and zygotes is regulated by assembly of the multimeric MW 200,000 active enzyme, which in turn determines the material inheritance of chloroplast DNA.