SummaryTissue-specific patterns of methylated deoxycytidine residues in the mammalian genome are preserved by postreplicative methylation of newly synthesized DNA. DNA methyltransferase (MTase) is here shown to associate with replication foci during S phase but to display a diffuse nucleoplasmic distribution in non-S phase cells. Analysis of DNA MTase-P-galactosidase fusion proteins has shown that association with replication foci is mediated by a novel targeting sequence located near the N-terminus of DNA MTase. This sequence has the properties expected of a targeting sequence in that it is not required for enzymatic activity, prevents proper targeting when deleted, and, when fused to P-galactosidase, causes the fusion protein to associate with replication foci in a cell cycle-dependent manner.

Preimplantation mouse embryos contain very high levels of DNA methyltransferase activity. We show here that the form of DNA methyltransferase {DNA MTase) in early embryos differs from the form found in other cells and tissues by a slightly higher mobility on gel electrophoresis. Levels of DNA MTase were found to be very high throughout preimplantation development even though levels of 5-methylcytosine (mSC) in nuclear DNA are known to undergo a substantial decline in the same period. Confocal laser scanning microscopy of mouse embryos stained with DNA MTase-specific antibodies showed striking developmentally regulated changes in the distribution of DNA MTase. From the oocyte stage to the four-cell-stage, most DNA MTase was concentrated in peripheral cytoplasm, and nuclei did not contain detectable DNA MTase. In four-and eight-cell embryos, DNA MTase was seen in cytoplasmic granules; and in eight-cell embryos, DNA MTase was also present in large amounts in nuclei. Nuclei of blastocysts stained only faintly, whereas the cytoplasmic granules remained prominent. Paradoxically, DNA MTase was found to be at its highest levels in nuclei at a developmental stage where levels of mSC in DNA are decreasing most rapidly. Changes in methylation patterns in preimplantation embryos are therefore proposed to be under the control of unidentified regulatory factors rather than DNA MTase itself; these regulatory factors could be members of the group that contains the products of the Ssm-1 and Imp-1 genes, which are involved in the regulation of genomic imprinting.

Key meiotic events in many organisms are controlled at the translational level. In this study, we examine the role of translational regulation in the meiotic cell cycle of Drosophila. In order to address this question, we developed a system for activating Drosophila oocytes in vitro. With this method, hundreds of mature oocytes can be activated to resume and complete meiosis. The stages of meiosis are normal by cytological criteria, and the timing of the meiotic divisions is similar to that of eggs activated in vivo. We use this system to examine the role of protein synthesis in regulating the progression of meiosis and the maintenance of the metaphase I arrest. We find that synthesis of new proteins after metaphase I is not required for anaphase I, meiosis II, or the decondensation of the meiotic products. Also, continued protein synthesis is not required to maintain the metaphase I arrest. New protein synthesis is required, however, for proper chromatin recondensation after meiosis.

The Drosophila MEI-S332 protein has been shown to be required for the maintenance of sister-chromatid cohesion in male and female meiosis. The protein localizes to the centromeres during male meiosis when the sister chromatids are attached, and it is no longer detectable after they separate. Drosophila melanogaster male meiosis is atypical in several respects, making it important to define MEI-S332 behavior during female meiosis, which better typifies meiosis in eukaryotes. We find that MEI-S332 localizes to the centromeres of prometaphase I chromosomes in oocytes, remaining there until it is delocalized at anaphase II. By using oocytes we were able to obtain sufficient material to investigate the fate of MEI-S332 after the metaphase II–anaphase II transition. The levels of MEI-S332 protein are unchanged after the completion of meiosis, even when translation is blocked, suggesting that the protein dissociates from the centromeres but is not degraded at the onset of anaphase II. Unexpectedly, MEI-S332 is present during embryogenesis, localizes onto the centromeres of mitotic chromosomes, and is delocalized from anaphase chromosomes. Thus, MEI-S332 associates with the centromeres of both meiotic and mitotic chromosomes and dissociates from them at anaphase.