REGULATED SYNTHESIS AND LOCALIZATION OF DNA METHYLTRANSFERASE DURING SPERMATOGENESIS

Differences in the methylation patterns of male and female gamete DNA are likely to be involved in genomic imprinting. However, little is kn own of the mechanisms that regulate de novo methylation and demethylat ion during gametogenesis. We report here that the well-characterized M (r) 190 000 form of DNA methyltransferase erase (the only known form) is present in isolated mitotic, meiotic, and postmeiotic male germ cel ls, with the exception of meiotic pachytene spermatocytes, where the p rotein is undetectable by immunoblot analysis and a novel 6.2-kb DNA m ethyltransferase transcript is present. Whereas replication and methyl ation are coupled in somatic cells, the presence of DNA methyltransfer ase in postreplicative male germ cells is consistent with previously o bserved de novo methylation events in these cells. Immunofluorescence experiments revealed that DNA methyltransferase is localized to the nu clei of male germ cells, with a subset of spermatogonia and postreplic ative leptotene/zygotene spermatocytes displaying prominent nuclear fo ci that are strongly enriched in DNA methyltransferase. The data sugge st that down-regulation of DNA methyltransferase expression during the pachytene stage of meiosis utilizes an unusual mechanism that is asso ciated with the production of a larger mRNA, and that de novo methylat ion in leptotene/zygotene spermatocytes may take place in spatially re stricted nuclear domains that are enriched in DNA methyltransferase.