Abstract
Chloroplast gene is inherited from only maternal gametes in the green akga, Chlamydomonas reinhardtii. During zygote formation, maternal chloroplast nucleoids are maintained whereas paternal ones completely disapper. Indirect immunofluorescence analyses with anti 5-methylcytosine monoclonal antibodies and methylation mapping directly revealed that, before mating, chloroplast DNA of maternal gametes is heavily methylated whereas that of paternal gametes is not. To investigate the female-specific methylation mechanism, we isolated and characterized a DNA methyltransferase, CrMET1. GFP epifluorescence microscopy analyses indicated it to localyze to chloroplasts. Baculovirus-expressed CrMET1 showed de novo methylation activity. CrMET1 transcripts were absent in vegetative cells of both mating types, but increased in maternal gametes. These experiments suggest that CrMET1 specifically methylates cytosines of chloroplast DNA in maternal gametes, thereby participating in maternal inheritance of chloroplast DNA .
