Calcium-dependent global chromatin compaction protects DNA from UV inflicted damage

Eukaryotic genomes are packaged into chromatin, which is the physiological substrate for all DNA-mediated functions, including DNA damage repair. At the DNA damage site, chromatin organization undergoes critical rearrangements during the repair process. These rearrangements around the lesion sites accommodate at least three steps: providing access to the repair factors, repair, and restoring the DNA’s pre-lesion. chromatin architecture. However, the global changes to chromatin after UV-irradiation were less explored and understood. To investigate the relationship between chromatin condensation and UV irradiation, HeLa-S3 cells were irradiated and subjected to micrococcal nuclease digestion analysis. The results showed that chromatin globally commenced compaction five minutes after UV-irradiation. Twenty-four hours after irradiation chromatin returned to the pre-UV steady-state. Southwestern blots showed that cells were irradiated twice at 15 J/m2 with a five minutes break had a significantly lower cyclobutane pyrimidine dimers (CPD) and DNA (6-4) photoproduct (6-4PP) rate in comparison to cells subjected to 30 J/m2, and had no significant difference from cells irradiated with a single dose of 15 J/m2. Western blot analysis demonstrated a post-UV core histone deacetylation wave which followed the chromatin condensation. Western blots analysis of caspase-3, which is activated in apoptotic cells both by extrinsic and intrinsic pathways, showed no caspase-3 activation after five and ten minutes post UV-irradiation. Here, we demonstrate that an environmental genotoxic agent, UV radiation, causes immediate and global chromatin compaction in HeLa cells and this compaction results in a robust reduction in the newly formed lesions. Our data suggest an influx of calcium cations after UV irradiation is directly involved in inducing chromatin compaction.