Maid: a maternally transcribed novel gene encoding a potential negative regulator of bHLH proteins in the mouse egg and zygote.
The metazoan egg and newly formed embryo are transcriptionally inert. The completion of meiosis, initiation of the first mitoses and activation of transcription, which occur after visible differentiation in the invertebrate embryo, are under the control of genes whose mRNAs and protein products were synthesized and stored in the growing oocyte (Wilt, 1973; Rosenthal and Ruderman, 1987). In mammals, de novo synthesis of transcripts in the oocyte also ceases as it enters meiosis and begins maturation. Although new transcription from the embryonic genome is activated at the 2-cell up to the 8to 16-cell stage, depending on the species (Telford et al., 1990), this process takes time. In mice, resumption of meiosis I, as measured by germinal vesicle breakdown (GVB), occurs approximately 12 hours before ovulation while complete activation of the embryonic genome has taken place within approximately 35 hours of fertilization, by the end of the 2-cell stage (Schultz, 1993; Wassarman et al., 1979). Taken together, this transcriptionally quiescent period can last almost 2 days. During this time the differentiated oocyte and sperm nuclei become reprogrammed to be the active genome of the now totipotent embryo. Transcription and translation of maternal transcripts has been extensively investigated in Xenopus. In contrast to the mammalian egg, the frog egg contains localized cytoplasmic determinants and the frog embryo undergoes significant morphological differentiation and multiple cell divisions within the several hours that elapse between fertilization and activation of their embryonic genomes (Newport and Kirschner, 1982a,b). Translation of maternal transcripts in this organism during the time of transcriptional quiescence is associated with their polyadenylation in the cytoplasm, a process controlled by a protein-RNA complex assembled at an UA-rich sequence in the 3′ untranslated regions (UTR) proximal to the nuclear polyadenylation signal, AAUAAA (Richter, 1996; Wickens et al., 1996). It has been suggested that cap ribose methylation is facilitated during the process of polyadenylation, and that the interaction between the modification machinery at the 3′and 5′-UTRs of these transcripts underlies translational control at this time in development (Kuge and Richter, 1995). Delayed translation of specific stored maternal messages could provide the temporal coordination of protein synthesis necessary for the mammalian oocyte-embryo transition. Compelling experiments performed in mice have shown 3795 Development 127, 3795-3803 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV2575