About half of human conceptions are estimated not to be implanted in the uterus, resulting in unrecognizable spontaneous abortions, and about 5% of human births have a recognizable malformation. In order to find clues to the mechanisms of malformation and abortion, we compared the incidences of radiation-induced malformations and abortions in p53 null (p53-/-) and wild-type (p53+/+) mice. After X-irradiation with 2 Gy on day 9.5 of gestation, p53-/- mice showed a 70% incidence of anomalies and a 7% incidence of deaths, whereas p53+/+ mice had a 20% incidence of anomalies and a 60% incidence of deaths. Similar results were obtained after irradiation on day 3.5 of gestation. This reciprocal relationship of radiosensitivity to anomalies and to embryonic or fetal lethality supports the notion that embryonic or fetal tissues have a p53-dependent "guardian" of the tissue that aborts cells bearing radiation-induced teratogenic DNA damage. In fact, after X-irradiation, the number of cells with apoptotic DNA fragments was greatly increased in tissues of the p53+/+ fetuses but not in those of the p53-/- fetuses.

We have cloned two different human cDNAs that can complement cdc28 mutations of budding yeast Saccharomyces cerevisiae. One corresponds to a gene encoding human p34CDC2 kinase, and the other to a gene (CDK2; cell division kinase) that has not been characterized previously. The CDK2 protein is highly homologous to p34C2 kinase (65% identical) and more significant ly is homologous to Xenopus Egl kinase (89% identical), suggesting that CDK2 is the human homolog of Egl. The human CDC2 and CDK2 genes were both able to complement the inviability ofa null allele ofS. cerevisiae CDC28. This result indicates that the CDK2 protein has a biological activity closely related to the CDC28 and p34CDC2 (1, 2). Beyond their central role in the regulation of mitosis, the CDC28/CDC2 kinases are also involved in the regulation of G1/S phase transition. This G1 role has been established through genetic analysis in budding and fission yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe, respectively (3, 4). In budding yeast, it is primarily G1 arrest that is observed with cdc28 mutants. Furthermore, in vertebrate cells, there is evidence that p34CDC2 kinase in some form is involved in the initiation of DNA replication in addition to its mitotic role (5-7). However, the mouse FT210 cell line carrying a temperaturesensitive cdc2 mutation arrested in G2 and showed no defect in initiation of S phase (8, 9). This result is consistent with the fact that, in mammalian cells, microinjection of antibodies to p34CDC2 caused cells to arrest in G2 but had no effect on DNA replication (10). Thus, a possible G1 role of the p34CDC2 kinase in mammalian cells remains uncertain.Whereas S. pombe cdc2 mutants arrest in both G1 and G2 (4), most temperature-sensitive alleles of S. cerevisiae cdc28 arrest in G1 at restrictive temperatures (3, 11). The G1 bias of cell cycle arrest by cdc28 mutations has facilitated the investigation of the G1 function of the CDC28 kinase. In fact, G1-specific cyclins, which activate the CDC28 kinase for its function required at the G1/S phase transition, were first identified in S. cerevisiae (12-14). Therefore, G, cyclins perform an analogous function to that described for mitotic cyclins in the G2/M phase transition.By taking advantage of the G1 bias of S. cerevisiae cdc28 mutants, we sought to identify mammalian genes involved in regulation of G1 as suppressors of cdc28 mutants. We report here a human gene that we have sequenced 11 and named CDK2 for cell division kinase that can complement cdc28 mutations. CDK2 encodes a protein that is highly homologous to the p34CDC2 (15) and the Xenopus Egl kinases (16). Furthermore, we discuss a possible role for the CDK2 protein in cell cycle control. MATERIALS AND METHODSStrains and Growth Media. S. cerevisiae mutants used were cdc28-13 and a disruption of CDC28 (cdc28::LEU2). These strains were congenic with l5Dau (MATa ura3Ans adel his2 leu2-3, 112 trpl-la) (13). S. pombe cdc2 mutants cdc2-L7 and cdc2-M26 have been described (4). Standard culture media for S....

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The yeast GPA1, STE4, and STE18 genes encode proteins homologous to the respective alpha, beta and gamma subunits of the mammalian G protein complex which appears to mediate the response to mating pheromones. Overexpression of the STE4 protein by the galactose‐inducible GAL1 promoter caused activation of the pheromone response pathway which resulted in cell‐cycle arrest in late G1 phase and induction of the FUS1 gene expression, thereby suppressing the sterility of the receptor‐less mutant delta ste2. Disruption of STE18, in turn, suppressed activation of the pheromone response induced by overexpression of STE4, suggesting that the STE18 product is required for the STE4 action. However, overexpression of both the STE4 and STE18 proteins did not generate a stronger pheromone response than overexpression of STE4 in the presence of wild‐type levels of STE18. These results suggest that the beta subunit is the limiting component for the pheromone response and support the idea that beta and gamma subunits act as a positive regulator. Furthermore, overexpression of GPA1 prevented cell‐cycle arrest but not FUS1 induction mediated by overexpression of STE4. This implies that the alpha subunit acts as a negative regulator presumably through interacting with beta and gamma subunits in the mating pheromone signaling pathway.

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