Expression of the rig gene in mouse oocytes and early embryos

The molecular basis of oocyte growth and development

The mammalian oocyte is formed early in foetal life and may remain suspended at diplotene of the first meiotic prophase until several decades later. When, following follicular recruitment, the oocyte enters a growth phase where it increases in volume, and undergoes replication and redistribution of the cytoplasmic organelles. These modifications, including secretion of the zona pellucida, reflect a period of intensive RNA synthesis as the oocyte grows and accumulates the molecular program for embryogenesis. Throughout oocyte development follicle cell support is fundamental to provide the germ line cell with nutrients and growth regulators to ensure progression through the protracted growth phase. Conversely, the oocyte actively promotes growth and differentiation of the follicular cells. Finally, re-initiation of meiosis in mature oocytes results in the production of haploid gametes which are capable of supporting early embryo development. Our present knowledge of the molecular biology of mammalian oogenesis is discussed in this review.

Growth and development of the mammalian oocyte

The oocyte is not only the rarest and the largest cell in the body, but it also has one of the most remarkable life histories. Formed in the fetal ovary and suspended at diplotene of meiosis, it may wait for years before beginning to grow, and not until this process is complete can it resume meiosis and undergo fertilisation. Major changes in the number, morphology and distribution of cytoplasmic organelles occur during growth, and a molecular program for embryogenesis is formed. Specific yolk proteins are absent and much of the RNA and some of the protein are degraded by the cleavage stage. The zona pellucida has been intensively studied, but knowledge of oocyte-specific genes is otherwise surprisingly patchy given the significance of this cell type and the expansion of reproductive technology. Finally, it is now clear that oocytes are not mere passengers which depend on granulosa cells for nutrition and regulation but actively promote the growth and differentiation of their follicles.

Sequence of a mouse embryo cDNA clone encoding proteolipid subunit 9 (P1) of the mitochondrial H(+)-ATP synthase

A cDNA clone to an abundantly expressed mRNA in cleavage stage mouse embryos has been sequenced and identified as encoding subunit 9 (P1) of the mitochondrial H(+)-ATP synthase. The deduced amino acid sequence of the mature subunit 9 protein differs in a single residue from the corresponding rat, ovine, bovine and human subunits.

Structural determination of Saccharomyces cerevisiae rig gene and identification of its product as ribosomal protein S21

rig was originally isolated from a rat insulinoma-derived cDNA library. The 145 amino acid sequence of the rig protein is invariant in mammalian cDNAs. In this paper, we have isolated the cDNA and genomic clones for yeast (Saccharomyces cerevisiae) rig, determined their nucleotide sequences, and identified the gene product. The gene and the mRNA encode a basic protein of 142 amino acids which has 61.3% amino acid identity with mammalian rig protein. On two-dimensional gel electrophoresis, the in vitro transcription/translation product of yeast rig cDNA co-migrated with yeast ribosomal protein S21. These results led to the conclusion that yeast rig ribosomal protein S21 and to the determination of the previously unknown primary structure of yeast S21 protein. Unlike most ribosomal protein genes of S. cerevisiae, the gene exists as a single copy in a haploid set of the yeast genome and has no intron, locating at chromosome VII or XV.

Expression of ribosomal protein genes in mouse oocytes and early embryos

The quantitative changes in the mRNAs for ribosomal proteins L7a, L18a, and S15 were assayed in slot hybridization experiments using labeled cRNA probes with total RNA from late growth-phase oocytes, ovulated eggs, and early embryos through the blastocyst stage. All three mRNAs showed a similar developmental pattern of prevalence, but their copy numbers per oocyte or embryo fluctuated according to developmental stage. There are on an average about 17,000 copies of each mRNA in the late growth-phase oocyte; this number drops to one-fifth to one-tenth in the ovulated egg and two-cell embryo but increases rapidly during cleavage to bout 25,000 in the eight-cell embryo and about 42,000 in the blastocyst. A comparison of the levels of these mRNAs with the reported rates of ribosomal protein synthesis (LaMarca and Wassarman, 1979) suggests that, in late growth-phase oocytes, ribosomal protein synthesis is regulated primarily at the translational level and is kept low by some factor limiting mRNA utilization. On the other hand, the high rate of ribosome biosynthesis during early embryogenesis from the two-cell stage onward appears to involve the coordinate activation and transcription of ribosomal RNA and ribosomal protein genes coupled with the immediate translational utilization of ribosomal protein mRNAs.

The genetic program for preimplantation development

This review summarizes information on accumulation profiles of individual gene transcripts in preimplantation development. Most of the information is from the mouse, but some data from other species are reviewed as well. The principal finding is that the transcription of most genes is not temporally linked with any of the three morphogenetic transitions (compaction, cavitation, and blastocoel expansion) that characterize this period. Most genes that are expressed during preimplantation development of the mouse are already being transcribed in the 4-cell stage, and some clearly begin as early as the 2-cell stage. Once activated, a gene continues to be transcribed at least into the blastocyst stage, resulting in continuous mRNA accumulation. Thus the pattern of gene transcription established at the time of genomic activation in the 2-cell stage is perpetuated into the blastocyst, with a few additions along the way. This information is interpreted in light of previous findings concerning the sensitivity of morphogenetic transitions to inhibition of gene expression. The lack of a clear relationship between the timing of expression of most genes and the schedule of morphogenesis leads one to conclude that temporal regulation is imposed downstream of transcription and translation. This conclusion is substantiated by a consideration of factors controlling the events of compaction.