1. Rosenstiel Center and Department of Biology, Brandeis University, Waltham, Massachusetts 02454-9110, USA
2. F.I.R.C. Institute of Molecular Oncology Foundation, Via Adamello 16, 20139, Milano, Italy, and Dipartimento di Scienze Biomolecolari e Biotecnologie, Universitá degli Studi di Milano, Italy
3. Department of Biochemistry and Cell Biology, SUNY Stony Brook, Stony Brook, New York 11794-5215, USA
4. These authors contributed equally to this work
5. Present address: Rockefeller University, 1230 York Avenue, New York, New York 10021-6399, USA

Correspondence to: James E. Haber¹Marco Foiani² Correspondence and requests for materials should be addressed to M.F. (Email: marco.foiani@ifom-ieo-campus.it) or J.E.H. (Email: haber@brandeis.edu).

Abstract

A single double-strand break (DSB) induced by HO endonuclease triggers both repair by homologous recombination and activation of the Mec1-dependent DNA damage checkpoint in budding yeast^{1, 2, 3, 4, 5, 6}. Here we report that DNA damage checkpoint activation by a DSB requires the cyclin-dependent kinase CDK1 (Cdc28) in budding yeast. CDK1 is also required for DSB-induced homologous recombination at any cell cycle stage. Inhibition of homologous recombination by using an analogue-sensitive CDK1 protein^{7, 8} results in a compensatory increase in non-homologous end joining. CDK1 is required for efficient 5' to 3' resection of DSB ends and for the recruitment of both the single-stranded DNA-binding complex, RPA, and the Rad51 recombination protein. In contrast, Mre11 protein, part of the MRX complex, accumulates at unresected DSB ends. CDK1 is not required when the DNA damage checkpoint is initiated by lesions that are processed by nucleotide excision repair. Maintenance of the DSB-induced checkpoint requires continuing CDK1 activity that ensures continuing end resection. CDK1 is also important for a later step in homologous recombination, after strand invasion and before the initiation of new DNA synthesis.

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