A novel architectural role for Top2 in S phase

The maintenance of genome integrity is a process of main importance for cell survival. Cells experiencing DNA damage or replicative stresses show mutations accumulations and chromosomes aberrations that are known to be directly related to genetic disease and cancer formations. DNA replication is a dangerous event by itself. Indeed genome integrity can be impaired by unscheduled recombination events, chromosome lesions, replication on a damage template, or replication stress induced by drugs affecting fork progression. Moreover the separation of the two strands of DNA during replication and the progression of the replication bubble physiologically generate topological problems, as positive supercoiling (helical overwinding) or precatenates formation (interwines in the daughter duplexes). DNA topoisomerases are enzyme able to modify the topological state of DNA molecules mediating a transient breakage of one or both he helices of the molecule. The study of these enzymes is of large interest because most commonly used antitumoral drugs act stabilizing topoisomerase-cleaved DNA structure, mediating a citotoxic effect. During replication stress DNA topoisomerases II have been involved in replicon resetting at mitosis through remodeling of chromosomal loops. Other proteins involved in the chromatin architecture are part of the High Mobility Group. The HMG proteins have been implicated in almost every DNA metabolic process, as transcription regulation and maintenance of chromosomal integrity. Moreover, in human HMGB1 (protein related to the Hmo1 protein of yeast) physically interacts and stimulates TopoIIα. We have previously show that during S phase Top2 localize to intergenic regions. In this project using the budding yeast Saccharomyces cerevisiae as experimental model, we showed a genetic interaction between Top2 and Hmo1. Moreover we were able to mark different sites bind by Top2 and Hmo1 that became fragile in absence of Top2, and to characterize the fork progression at some of these sites, defining a novel architectural role for Top2 in S phase.