The kinetics of thymine dimer excision in ultraviolet irradiated hman cells

We have investigated the kinetics of the loss of thymine dimers from the acid-insoluble fraction of several ultraviolet (UV)-irradiation cultured human cell lines. Our results show that UV fluences between 10 and 40 J/m² produce an average of 21-85 x 10⁵ thymine dimers per cell and an eventual maximal loss per cell of 12-20 x 10⁵ thymine dimers. The time for half-maximal loss of dimers ranged from 12-22 h after UV irradiation. In contrast, the time for half-maximal repair synthesis of DNA measured by autoradiography was 4.5 h. This figure agrees well with reported half-maximal repair synthesis times, which range from 0.5 to 3.6 h based on our analysis. The discrepancy in the kinetics of the loss of thymine dimers from DNA and repair synthesis is discussed in terms of possible molecular mechanisms of thymine dimer excision in vivo and in terms of possible experimental artifacts.