The Chromatin Landscape around DNA Double-Strand Breaks in Yeast and Its Influence on DNA Repair Pathway Choice.

DNA double-strand breaks (DSBs) are harmful DNA lesions, which elicit catastrophic consequences for genome stability if not properly repaired. DSBs can be repaired by either non-homologous end joining (NHEJ) or homologous recombination (HR). The choice between these two pathways depends on which proteins bind to the DSB ends and how their action is regulated. NHEJ initiates with the binding of the Ku complex to the DNA ends, while HR is initiated by the nucleolytic degradation of the 5'-ended DNA strands, which requires several DNA nucleases/helicases and generates single-stranded DNA overhangs. DSB repair occurs within a precisely organized chromatin environment, where the DNA is wrapped around histone octamers to form the nucleosomes. Nucleosomes impose a barrier to the DNA end processing and repair machinery. Chromatin organization around a DSB is modified to allow proper DSB repair either by the removal of entire nucleosomes, thanks to the action of chromatin remodeling factors, or by post-translational modifications of histones, thus increasing chromatin flexibility and the accessibility of repair enzymes to the DNA. Here, we review histone post-translational modifications occurring around a DSB in the yeast Saccharomyces cerevisiae and their role in DSB repair, with particular attention to DSB repair pathway choice.

The regulation of the DNA damage response at telomeres: focus on kinases.

The natural ends of linear chromosomes resemble those of accidental double-strand breaks (DSBs). DSBs induce a multifaceted cellular response that promotes the repair of lesions and slows down cell cycle progression. This response is not elicited at chromosome ends, which are organized in nucleoprotein structures called telomeres. Besides counteracting DSB response through specialized telomere-binding proteins, telomeres also prevent chromosome shortening. Despite of the different fate of telomeres and DSBs, many proteins involved in the DSB response also localize at telomeres and participate in telomere homeostasis. In particular, the DSB master regulators Tel1/ATM and Mec1/ATR contribute to telomere length maintenance and arrest cell cycle progression when chromosome ends shorten, thus promoting a tumor-suppressive process known as replicative senescence. During senescence, the actions of both these apical kinases and telomere-binding proteins allow checkpoint activation while bulk DNA repair activities at telomeres are still inhibited. Checkpoint-mediated cell cycle arrest also prevents further telomere erosion and deprotection that would favor chromosome rearrangements, which are known to increase cancer-associated genome instability. This review summarizes recent insights into functions and regulation of Tel1/ATM and Mec1/ATR at telomeres both in the presence and in the absence of telomerase, focusing mainly on discoveries in budding yeast.

Exo1 cooperates with Tel1/ATM in promoting recombination events at DNA replication forks.

Tel1/ataxia telangiectasia mutated (ATM) kinase plays multiple functions in response to DNA damage, promoting checkpoint-mediated cell-cycle arrest and repair of broken DNA. In addition, Saccharomyces cerevisiae Tel1 stabilizes replication forks that arrest upon the treatment with the topoisomerase poison camptothecin (CPT). We discover that inactivation of the Exo1 nuclease exacerbates the sensitivity of Tel1-deficient cells to CPT and other agents that hamper DNA replication. Furthermore, cells lacking both Exo1 and Tel1 activities exhibit sustained checkpoint activation in the presence of CPT, indicating that Tel1 and Exo1 limit the activation of a Mec1-dependent checkpoint. The absence of Tel1 or its kinase activity enhances recombination between inverted DNA repeats induced by replication fork blockage in an Exo1-dependent manner. Thus, we propose that Exo1 processes intermediates arising at stalled forks in tel1 mutants to promote DNA replication recovery and cell survival.