Paris J, Le Guellec R, Couturier A, Le Guellec K, Omilli F, Camonis J, MacNeill S, Philippe M
Centre National de la Recherche Scientifique Unité Associée 256, Université de Rennes I, France.
Proc Natl Acad Sci U S A. 1991 Feb 1;88(3):1039-43. doi: 10.1073/pnas.88.3.1039.
Fertilization of Xenopus laevis eggs triggers a period of rapid cell division comprising 12 nearly synchronous mitoses. Protein synthesis is required for these divisions, and new proteins appear after fertilization. Others proteins however, which are synthesized in the unfertilized egg, are no longer made in the early embryo. To identify such proteins, a differential screen of an egg cDNA library gave nine clones corresponding to mRNAs that are deadenylylated soon after fertilization. The sequence of one of these clones (Eg1) revealed a high homology to p34cdc2, the kinase subunit of maturation-promoting factor. Only 12 amino acids in the deduced amino acid sequence were unique to Eg1 when its sequence was compared to all other known examples of cdc2. Despite this strong similarity, however, Eg1 was unable to complement a yeast cdc2- mutant in Schizosaccharomyces pombe or a cdc28 mutant of Saccharomyces cerevisiae. Four Eg1 transcripts, two major and two minor, were found in Xenopus oocytes and early embryos. These RNAs appeared very early (stage I) in oogenesis and their level remained constant until the midblastula transition, at which time they declined. Eg1 RNA is found in the poly(A)+ fraction of oocytes only between the time of meiotic maturation and fertilization--that is to say, in the unfertilized egg. At fertilization the RNA loses its poly(A) tail and at the same time leaves the polyribosomes.
非洲爪蟾卵的受精引发了一个快速细胞分裂期，该时期包含12次近乎同步的有丝分裂。这些分裂需要蛋白质合成，并且受精后会出现新的蛋白质。然而，其他在未受精卵中合成的蛋白质在早期胚胎中不再产生。为了鉴定此类蛋白质，对卵cDNA文库进行差异筛选得到了9个与受精后不久即去腺苷酸化的mRNA相对应的克隆。其中一个克隆（Eg1）的序列显示与成熟促进因子的激酶亚基p34cdc2具有高度同源性。将其推导的氨基酸序列与所有其他已知的cdc2实例进行比较时，Eg1的推导氨基酸序列中只有12个氨基酸是独特的。尽管有这种强烈的相似性，然而，Eg1无法互补粟酒裂殖酵母中的酵母cdc2 - 突变体或酿酒酵母的cdc28突变体。在非洲爪蟾卵母细胞和早期胚胎中发现了四种Eg1转录本，两种主要的和两种次要的。这些RNA在卵子发生的很早阶段（I期）就出现了，并且它们的水平一直保持恒定，直到囊胚中期转变，此时它们下降。Eg1 RNA仅在减数分裂成熟和受精之间的时间段内在卵母细胞的聚腺苷酸加尾（poly(A)+）部分中被发现——也就是说，在未受精卵中。受精时，RNA失去其聚腺苷酸尾，同时离开多核糖体。
