A model of the kinetics of photorepair in chick embryo fibroblasts.

Recent measurements of the photorepair of UV-induced dimers in chick embryo fibroblasts have shown that the efficiency of photorepair is a function of time after UV exposure. Photorepair is enhanced if the time between UV exposure and photoreactivation is increased. A model is presented which fits the experimental data within the accuracy of the measurements. The disappearance of damaged sites follows first-order rate kinetics, similar to the kinetics of photorepair shown to exist in E. coli. However, some of the chromosomal DNA is protected from enzymatic activity, presumably by the histones and other proteins associated with eukaryotic DNA. The effective number of damaged sites available for repair increases monotonically with time after UV damage. At 37 degrees C all sites become available after 9-12 h. Immediately after UV exposure, about 75% of the DNA is shielded from photorepair.