Role of methylation in the modification and restriction of chloroplast DNA in Chlamydomonas.

The different metabolic paths followed by homologous chloroplast DNAs of maternal and paternal origins in zygotes of Chlamydomonas were examined by prelabeling parental cells, before mating them, with [3H]adenine, [3H]thymidine, and [3H]deoxycytidine. Within 6 hr after mating, maternal chloroplast DNA was extensively methylated to 5-methylcytosine and its bouyant density decreased. Paternal chloroplast DNA was largely degraded. Some radioactivity from deoxycytidine of maternal origin reappeared in thymine, and residual paternal DNA contained radioactivity in a base tentatively identified as uracil. These results confirm and extend our previous findings and support our hypothesis that modification (methylation) and restriction enzymes determine maternal inheritance of chloroplast DNA and that the two parental DNAs have different metabolic fates within the zygote.