The cyclin-dependent kinase inhibitor Dacapo promotes replication licensing during Drosophila endocycles

Abstract

The endocycle is a developmentally programmed variant cell cycle in which cells undergo repeated rounds of DNA replication with no intervening mitosis. In Drosophila , the endocycle is driven by the oscillations of Cyclin E/Cdk2 activity. How the periodicity of Cyclin E/Cdk2 activity is achieved during endocycles is poorly understood. Here, we demonstrate that the p21cip1/p27kip1/p57kip2‐like cyclin‐dependent kinase inhibitor (CKI), Dacapo (Dap), promotes replication licensing during Drosophila endocycles by reinforcing low Cdk activity during the endocycle Gap‐phase. In dap mutants, cells in the endocycle have reduced levels of the licensing factor Double Parked/Cdt1 (Dup/Cdt1), as well as decreased levels of chromatin‐bound minichromosome maintenance (MCM2–7) complex. Moreover, mutations in dup / cdt1 dominantly enhance the dap phenotype in several polyploid cell types. Consistent with a reduced ability to complete genomic replication, dap mutants accumulate increased levels of DNA damage during the endocycle S‐phase. Finally, genetic interaction studies suggest that dap functions to promote replication licensing in a subset of Drosophila mitotic cycles.