Connessioni molecolari tra struttura della cromatina, riparazione del DNA e attivazione del checkpoint in S. cerevisiae

Checkpoints are surveillance mechanisms that monitor cell cycle progression and preserve the correct order of events. Checkpoints are also activated in response to genomic insults, or alterations of cellular structures, and lead to temporary cell cycle arrest, slowing down of DNA replication, changes in the cellular transcriptional program and, in some instances, apoptosis. The mechanisms used by checkpoints to identify DNA lesions are poorly understood and may involve the function of repair proteins. Looking for mutants specifically defective in activating the checkpoint following UV lesions, but proficient in the response to methyl methane sulfonate and double-strand breaks, we isolated RAD14, the homolog of human XPA, involved in lesion recognition during nucleotide excision repair (NER). Rad14 was also isolated as a partner of the Ddc1 checkpoint protein in a two-hybrid screening, and physical interaction was proven by co- immunoprecipitation. We show that lesion recognition is not sufficient for checkpoint activation, but processing, carried out by repair factors, is required for recruiting checkpoint proteins to damaged DNA. Mutations affecting the core NER machinery abolish G1 and G2 checkpoint responses to UV, preventing activation of the Mec1 kinase and its binding to chromosomes. Conversely, elimination of transcription coupled or global genome repair alone does not affect checkpoints, suggesting a possible interpretation for the heterogeneity in cancer susceptibility observed in different NER syndrome patients. Moreover we show that in Saccharomyces cerevisiae epigenetic modification of histones is required for checkpoint activity in response to a variety of genotoxic stresses. We demonstrate that ubiquitination of histone H2B on lysine 123 by the Rad6-Bre1 complex, is necessary for activation of Rad53 kinase and cell cycle arrest. We found a similar requirement for Dot1-dependent methylation of histone H3. Loss