Task: Question Answering
Input (English): Context: DNA damage is preferentially repaired in the transcribed strand of many active genes. Although the concept of DNA repair coupled with transcription has been widely accepted, its mechanisms remain elusive. We recently reported that in Chinese hamster ovary cells while ultraviolet light-induced cyclobutane pyrimidine dimers (CPDs) are preferentially repaired in the transcribed strand of dihydrofolate reductase gene, CPDs are efficiently repaired in both strands of adenine phosphoribosyltransferase (APRT) locus, in either a transcribed or nontranscribed APRT gene (1). These results suggested that the transcription dependence of repair may depend on genomic context. To test this hypothesis, we constructed transfectant cell lines containing a single, actively transcribed APRT gene, integrated at different genomic sites. Mapping of CPD repair in the integrated APRT genes in three transfectant cell lines revealed two distinct repair patterns, either preferential repair of CPDs in the transcribed strand or very poor repair in both strands. Similar kinetics of micrococcal nuclease digestion were seen for all three transfectant APRT gene domains and endogenous APRT locus. Our results suggest that both the efficiency and strand-specificity of repair of an actively transcribed gene are profoundly affected by genomic context but do not reflect changes in first order nucleosomal structure. 
 Question: Which gene strand is targeted by transcription-coupled repair (TCR)?
Output (English): the transcribed strand