Although a clamor arose over the cloning of mammals, the fact that very little is known about how normal mammalian development initiates has gone unrecognized. We have developed a multi-faceted approach to define the molecular control of this time in development and stand ready to address this issue. We prepared a large and representative cDNA library from the 2-cell mouse embryo that is being sequenced by the mouse EST project. Our analysis suggests these cDNAs are, in the main, from maternal transcripts that are targeted for timely translation during the oocyte to embryo transition. The principle that maternal transcripts govern the physiology of the newly formed embryo is derived from studying organisms with readily accessible oocytes and embryos. However, these organisms produce mosaic eggs and their embryos become motile within hours of fertilization, whereas mammalian eggs are nonpolar and their embryos implant in the uterus several days after fertilization. In Xenopus maternal transcripts, UA-rich sequences located at specific sites in the 3'-UTR, determine when they are translated. One of our objectives is to understand how temporal translation of maternal transcripts is controlled in mammals. We find similar UA-rich sequences, albeit with different spatial constraints, may control translation of mammalian maternal transcripts. This knowledge is of predictive value. The presence of such a sequence within a given region of the 3'-UTR suggests when its protein will be biosynthesized. We plan to: 1) define the cis-sequences and siting requirements for transcript translation in the mouse oocyte and embryo; and 2) determine whether biosynthesis of the encoded proteins follows the predicted pattern. Our second objective is to understand the functional role and critical nature of the just-in-time availability of specific molecules. We plan to make oocyte- and stage-specific mutations of two genes functioning within biochemical pathways known to be active in the mouse oocyte and embryo. Our goal is to produce a clear picture of the molecules and pathways necessary for embryogenesis in mammals. In the long range, the understanding gained from this study may lead to new therapies for infertility/fertility and diseases of aberrant progression through the cell cycle, such as cancer.
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