The DNA damage checkpoint in eye imaginal disc cells requires Cdk1 phosphorylation. J.K. Stumpff , T.T. Su. Dept MCDB, University of Colorado, Boulder, CO.

   Cell cycle regulation changes during Drosophiladevelopment. Cells of the syncytial embryo undergo rapid cell cycles containing only M and S phases. Cells of the gastrula acquire a G2 phase, while cells of the larval imaginal discs have canonical cell cycles with G2, M, G1 and S phases. It has been shown previously that cellular responses to DNA damage change as cell cycle regulation changes during Drosophila embryonic development. Cells of the syncytial embryo arrest in mitosis following damage while cells of the gastrula arrest in G2. The mechanisms controlling these arrests are also different (Fogarty et al. 1997, Su et al. 2000 and Sibon et al. 2000). Here, we have addressed the affect of damage on the cells of the eye imaginal disc. Following X-ray treatment, cells of the eye disc have previously been shown to arrest in interphase. This arrest requires both the grapes and mei-41 checkpoint genes (Hari et al. 1995 and Brodsky et al. 2000). We have established that this arrest also requires inhibitory phosphorylation of the Cdk1 mitotic kinase. Expression of a mutant form of Cdk1 (Cdk1AF), which cannot be phosphorylated, causes cells to bypass the normal checkpoint arrest and enter mitosis after DNA damage. However, overexpression of wild-type Cdk1 does not affect checkpoint activation. From this data, we hypothesize that the phosphorylation-state of Cdk1 is regulated by the DNA damage checkpoint in eye disc cells.