AbstractThis laboratory has recently reported the occurrence of DNA nicking at the onset of terminal skeletal myogenesis by using the technique of in situ nick translation (Dawon and Lough: Dev.Biol.,727:362–367, 1988). Because 1‐beta‐D‐arabi‐nofuranosylcytosine (araC), a cytocidal agent that is routinely used to remove dividingfibroblasts from myogenic cultures, inhibits DNA repair, it was of interest to determine whether araC treatment resulted in an accumulation of the endogeously created nicks. Thus, we have assessed the accumulation of DNA nicks in myotube cells during a 20 hour araC treatment period at the onset of terminal myogenesis (44–64 hours in vitro) by using three techniques: alkaline sucrose gradient density centrifugation, kinetic in situ nick translation, and cellular in situ nick translation. Although alkaline sucrose gradient centrifugation revealed no detectable nicking after 20 hours, kinetic in situ nick translation analysis revealed subtle but significant increases in DNA nicks caused by araC within 7 hours of drug application, and a 1.5‐fold increase in DNA repair sites after 20 hours of drugtreatment. That theseo bservationsreflectednicking specifically in myotube nuclei was determined by immunocytochemical localization of nicked sites after repair with a biotinyafed nucleotide analog (biotin‐11 ‐dUTP). The effects of araC wereonly incompletely reversible, whetheror not thedrugwas removedfrom the cultures, within 2 days of the treatment period. These results indicate that araC inhibits the repair of endogenously created nicks in the DNA of non‐dividing, terminally differentiating myotube cells, and provide further evidence for DNA nicking and repair as a feature of terminal myoge
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