THE cellular phosphoprotein p53 inhibits progression through the mammalian cell cycle1,2. Both p53 alleles are frequently mutated in human tumours3,4, indicating that p53 is a tumour suppressor. Recent studies have suggested that p53 functions as a transcrip-tional activator 5-8, but the significance of this activity in cell-cycle control has not been established. The adenovirus 2 (Ad2) early IB (E1B) 55K protein binds to p53 in transformed cells9 and contributes to oncogenic transformation by Ad2 (refs 10-12). Here we report that mutants of E1B 55K and wild-type Adl2 E1B 54K proteins show a strong correlation between their ability to inhibit p53-mediated transcriptional activation and their ability to cooperate with adenovirus El A protein in the transformation of primary cells. These results indicate that p53 probably inhibits cell cycling by functioning as a transcription factor.
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