During normal cell cycles, the function of mitotic cyclin-cdk1 complexes, as well as of cdc25C phosphatase, is required for G 2 phase progression. Accordingly, the G 2 arrest induced by DNA damage is associated with a down-regulation of mitotic cyclins, cdk1, and cdc25C phosphatase expression. We found that the promoter activity of these genes is repressed in the G 2 arrest induced by DNA damage. We asked whether the CCAATbinding NF-Y modulates mitotic cyclins, cdk1, and cdc25C gene transcription during this type of G 2 arrest. In our experimental conditions, the integrity of the CCAAT boxes of cyclin B1, cyclin B2, and cdc25C promoters, as well as the presence of a functional NF-Y complex, is strictly required for the transcriptional inhibition of these promoters. Furthermore, a dominantnegative p53 protein, impairing doxorubicin-induced G 2 arrest, prevents transcriptional down-regulation of the mitotic cyclins, cdk1, and cdc25C genes. We conclude that, as already demonstrated for cdk1, NF-Y mediates the transcriptional inhibition of the mitotic cyclins and the cdc25C genes during p53-dependent G 2 arrest induced by DNA damage. These data suggest a transcriptional regulatory role of NF-Y in the G 2 checkpoint after DNA damage.

Experiments previously performed on 32D and C2C12 cell lines indicated that wild type p53 (wtp53) protein has a role in granulocyte and myotube di erentiation. Since these are immortal cells, we asked whether the inhibition of di erentiation induced by the expression of dominantnegative p53 (dnp53) proteins was dependent on the immortalization-determined microenvironment. Thus, we evaluated the e ects produced by interfering with the endogenous p53 gene in murine primary hemopoietic and muscle cells. Expression of dnp53 protein reduced the di erentiation of bone marrow cells into granulocytes and macrophages. Moreover, p53 activation was measurable during the di erentiation process of primary myoblasts, while interference with this activation led to a consistent slow down of terminal di erentiation. Since the impairment of the di erentiation was not accompanied by alterations in the cell cycle withdrawal and in the rate of apoptosis which are coupled with these types of di erentiation, the data here reported support a speci®c role for p53 in the di erentiation process. However, the di erence in the intensity of inhibition between immortal and primary cells, i.e., complete versus slow down, respectively, suggests that the immortalization process might render the cells more sensitive to the loss of wtp53 activity for the di erentiation process.

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