Human p53 protein was found to be functional in fission yeast in terms of growth repression and checkpoint control. Expression of wild-type p53 or the hot spot mutant p53His273 results in dramatic morphological changes and loss of viability of recipient yeast cells. Overexpression of cdc25C phosphatase, the mitotic activator of cdc2, results in suppression of a p53-induced growth arrest. In order to understand the interplay between p53 and cdc25C in mammalian cells we isolated and sequenced cdc25C cDNA from the epidermoid carcinoma cell line A431, which is known to carry the p53His273 mutation. Two different transcripts of the human cdc25C gene were detected by RT-PCR analysis - one full-length transcript and a shortened version (cdc25Cdm) that carries two deletions in the 5'-region of the gene. In normal human skin fibroblasts only one full-length cdc25C transcript was detected. The two different transcripts code for proteins with a molecular weight of 55 kDa and 46 kDa, respectively. Both cdc25C cDNAs from A431 cells were found to complement a conditional lethal cdc25.22 mutant strain as well as a cdc25 deletion strain of Schizosaccharomyces pombe indicating that functional proteins were translated. Expression of cdc25Cdm variant leads to a stronger uncoupling of DNA replication from mitosis than expression of cdc25C suggesting that the deletion within the amino-terminus of cdc25C leads to a protein which might contribute some potential for oncogenic transformation. As with cdc25C, uncoupling of the DNA synthesis checkpoint by cdc25Cdm was reversed by coexpression of wild-type p53.

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