During TC-HR, RAD52 is recruited to sites of damage by CSB through an acidic domain (AD). The recruitment of RAD51 to sites of ROS-induced DNA damage is dependent on transcription, as well as Cockayne Syndrome Protein B (CSB) and RAD52 proteins. We found that TC-HR requires the RAD51 recombinase but, surprisingly, not the canonical HR proteins BRCA1 and BRCA2. In this study, we used an inducible system to generate ROS at a transcriptionally active locus and characterized the TC-HR pathway. In particular, whether and how the canonical HR and TC-HR pathways are differentially initiated and regulated remains elusive. Despite these tantalizing features, TC-HR is still poorly understood as a pathway. Notably, we showed that ROS activated TC-HR at a transcriptionally active locus, thereby implicating TC-HR in the repair of ROS-induced DNA damage in transcribed regions. Furthermore, the RNA transcript generated by transcription is required for TC-HR. In contrast to the canonical HR, TC-HR functions in a transcription-dependent manner. We and others showed that transcription-coupled homologous recombination (TC-HR) occurs in human and yeast cells and contributes to DSB repair in transcribed regions 7, 8. Recently, a growing body of evidence suggested that active genes are protected by transcription-coupled DNA repair mechanisms 5, 6. Therefore, it is crucial to understand how cells protect the actively transcribed genome against ROS-induced DNA damage. Furthermore, DNA damage in transcribed regions may lead to mutations, indels, and translocations in critical genes, driving tumorigenesis and neurodegeneration. For example, DNA damage-induced stalling of RNA polymerase II (RNAPII) may directly impair gene expression 4. ROS-induced DNA damage in transcriptionally active regions of the genome may be particularly deleterious to cells. ROS induce multiple types of DNA lesions, including oxidized bases, DNA single-strand breaks (SSBs) and double-strand breaks (DSBs), which are removed by different DNA repair pathways 3. Reactive oxygen species (ROS) arise from both cellular metabolism and environmental insults, presenting a major threat to genomic stability that contributes to tumorigenesis and neurodegenerative diseases 1, 2.
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