Studies on CDKB2 Degradation and Transition to the Endocycle in Response to DNA Double-Strand Breaks in <I>Arabidopsis thaliana </I>

During cell cycle progression, it is essential that DNA damage is repaired before cell division. However, in plants, such regulatory mechanisms remain unknown. Recentaly we found that CDKB2 is degraded in response to DNA double-strand breaks (DSBs), which also trigger transition from the mitotic cell cycle to the endocycle.<br>The first exon of CDKB2 encodes 75 amino acids, and contains the putative PEST motif in the region of amino acid No. 46-59 I made GUS-fusion constructs, which had mutations or deletions in the first exon and observed GUS expression in root tips. A deletion in the region of No. 46-75 suppressed the proteolysis of CDKB2 in response to DSBs. In contrust, mutations in the putative PEST motif did not affect the CDKB2 expression under genotoxic stress conditions. This suggests that the first exon contains regulatory element(s) that is important for CDKB2 degradation, but the PEST domain does not function in response to DSBs.<br>We are also testing the sensitivity to DSBs by using Arabidopsis mutants that have different levels of DNA ploidy. Based on these results, we shall discuss the physiological significance of endocycle induction in response to DSBs.