SummaryALC1, a novel PARP1-stimulated chromatin-remodelling enzyme promotes DNA repair.Post-translational modifications play key roles in orchestrating chromatin plasticity. Although various chromatin-remodelling enzymes have been described that respond to specific histone modifications, little is known about the role of poly(ADP-ribose) in chromatin remodelling. Here, we identify a novel chromatin-remodelling enzyme, ALC1 (Amplified in Liver Cancer 1), that is specifically regulated by poly(ADP-ribosyl) ation. ALC1 binds poly(ADP-ribose) via a C-terminal Macro domain and catalyzes PARP1-stimulated nucleosome sliding, conferred by an N-terminal ISWI-related helicase core. Our results define ALC1 as a novel DNA damage-response protein, whose role in this process is sustained by its association with known DNA repair factors and its rapid poly(ADP-ribose)-dependent recruitment to DNA damage sites. Furthermore, we show that depletion or overexpression of ALC1 results in sensitivity to DNA-damaging agents. Collectively, these results provide new insights into the mechanisms by which poly(ADP-ribose) regulates DNA repair.The restricted accessibility of DNA within chromatin presents a barrier to DNA manipulations that require direct protein-DNA interactions (1-3). Processes such as transcription, repair and replication that require efficient DNA recognition are therefore dependent on the appropriate modulation of chromatin structure. Chromatin relaxation is a critical event that occurs during DNA repair and is associated with post-translational poly(ADP-ribose) (PAR) modification (4). PAR is synthesized in a reaction that utilizes NAD+ as a substrate by the PARP family of enzymes, of which PARP1 (and to a lesser extent PARP2) respond to DNA strand breaks (5-7). As a consequence of poly(ADP-

Replicated chromosomes are held together by the chromosomal cohesin complex from the time of their synthesis in S phase onward. This requires the replication fork-associated acetyl transferase Eco1, but Eco1's mechanism of action is not known. We identified spontaneous suppressors of the thermosensitive eco1-1 allele in budding yeast. An acetylation-mimicking mutation of a conserved lysine in cohesin's Smc3 subunit makes Eco1 dispensable for cell growth, and we show that Smc3 is acetylated in an Eco1-dependent manner during DNA replication to promote sister chromatid cohesion. A second set of eco1-1 suppressors inactivate the budding yeast ortholog of the cohesin destabilizer Wapl. Our results indicate that Eco1 modifies cohesin to stabilize sister chromatid cohesion in parallel with a cohesion establishment reaction that is in principle Eco1-independent.

Four-way DNA intermediates, also known as Holliday junctions, are formed during homologous recombination and DNA repair, and their resolution is necessary for proper chromosome segregation. Here we identify nucleases from Saccharomyces cerevisiae and human cells that promote Holliday junction resolution, in a manner analogous to that shown by the Escherichia coli Holliday junction resolvase RuvC. The human Holliday junction resolvase, GEN1, and its yeast orthologue, Yen1, were independently identified using two distinct experimental approaches: GEN1 was identified by mass spectrometry following extensive fractionation of HeLa cell-free extracts, whereas Yen1 was detected by screening a yeast gene fusion library for nucleases capable of Holliday junction resolution. The eukaryotic Holliday junction resolvases represent a new subclass of the Rad2/XPG family of nucleases. Recombinant GEN1 and Yen1 resolve Holliday junctions by the introduction of symmetrically related cuts across the junction point, to produce nicked duplex products in which the nicks can be readily ligated.

Epigenetic mechanisms of gene regulation have a profound role in normal development and disease processes. An integral part of this mechanism occurs through lysine acetylation of histone tails which are recognized by bromodomains. While the biological and structural characterization of many bromodomain containing proteins has advanced considerably, the therapeutic tractability of this protein family is only now becoming understood. This paper describes the discovery and molecular characterization of potent (nM) small molecule inhibitors that disrupt the function of the BET family of bromodomains (Brd2, Brd3, and Brd4). By using a combination of phenotypic screening, chemoproteomics, and biophysical studies, we have discovered that the protein-protein interactions between bromodomains and acetylated histones can be antagonized by selective small molecules that bind at the acetylated lysine recognition pocket. X-ray crystal structures of compounds bound into bromodomains of Brd2 and Brd4 elucidate the molecular interactions of binding and explain the precisely defined stereochemistry required for activity.