Chromatin Condensation Protects DNA from MMS Damage

Chromatin is the complex of proteins and DNA that makes up eukaryotic chromosomes. Chromatin helps the DNA fit inside the nucleus as well as regulating what can access the DNA. Replication, repair, and transcription are all impeded by chromatin. These processes only occur efficiently when chromatin is disassembled. Just as chromatin limits how proteins can access the DNA, we hypothesized that it would also protect DNA. We used the yeast Saccharomyces cerevisiae to study how chromatin affects cellular sensitivity to the DNA alkylating agent methyl methanesulfonate (MMS). MMS alkylation can lead to DNA mismatches, nicks, and double strand breaks, resulting in mutation. Using growth assays, we found that active transcription significantly increases the sensitivity of a gene to MMS. Consistent with this, we found that increased chromatin compaction due to nocodazole-mediated metaphase arrest reduced the effects of MMS. Because transcription led to an increase in MMS-mediated DNA damage, we proposed that transcribed genes would be subject to increased damage under normal growth conditions, requiring recruitment of DNA repair factors. We found by chromatin immunoprecipitation that the Rad51, Rad53, and Rad18 DNA repair proteins are recruited to the GAL1 gene during transcription, although their occupancy was not stimulated by MMS treatment. This suggests that there is enough damage occurring during normal transcription to require DNA repair factors. Together, these results support our model that chromatin helps protect DNA from damage in addition to regulating other DNA-based processes