DNA repair in human cells: methods for the determination of calmodulin involvement.

These results of these assays, therefore, suggested that calmodulin functions by modulating the initial step of excision repair of UV-damaged DNA. The initial step in this process is the endonuclease function. The reasoning follows these lines of thought: Dimer chromatography indicates that incision and excision occur albeit at a decreased rate. If no excision occurs, the dimer region will remain bound to the DNA and there would be no difference between the control and treated samples. Therefore any difference detected should be due to a decrease in the incision step of repair. The photolysis assay also indicates a decrease in the incision step. Since the patch size between the control and treated cultures were identical within experimental error (64 vs 72 bases), there is no inhibition of polymerase activity. If excision were affected during the photolysis assay, it is possible that the free region of the incised strand could be used as a primer strand and the repaired DNA could have a higher molecular weight than the control strand. This was not observed. Finally, the cytosine arabinoside procedures indicated that less cytosine arabinoside molecules were incorporated into the damaged regions. Since the photolysis assay indicated that the polymerase reaction was not affected, this would indicate that less initial sites were available for repair, that is, less nicks were available indicative of decreased endonuclease activity.