The kinetics of thymine dimer excision in ultraviolet-irradiated human cells

We have investigated the kinetics of the loss of thymine dimers from the acid-insoluble fraction of several ultraviolet (UV)-irradiated cultured human cell lines. Our results show that UV fluences between 10 and 40 J/m2 produce an average of 21-85 x 10(5) thymine dimers per cell and an eventual maximal loss per cell of 12-20 x 10(5) 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.     

Two distinct receptors for ATP can be distinguished in Swiss 3T6 mouse fibroblasts by their desensitization

The stimulation of calcium efflux from Swiss 3T6 mouse fibroblasts by extracellular ATP was studied. It was found that the cells could be desensitized to ATP by a previous exposure to the nucleotide, lending support to the theory that this is a receptor mediated process. Another ATP-receptor mediated process in Swiss 3T6 cells, that is also subject to desensitization, causes the permeabilization of the plasma membrane to nucleotides and other normally impermeant compounds [Gonzalez et al., J. Cell. Physiol. 139:109 (1989)]. Here we demonstrate that selective desensitization of the ATP-dependent calcium mobilization pathway can be achieved without affecting ATP-induced permeabilization. Data are presented in support of the existence of multiple ATP-receptors (purinoceptors) in Swiss 3T6 cells.     

Demonstration That Thymine Dimers are Excised as Oligonucleotides from Specifically Incised Ultraviolet-Irradiated Deoxyribonucleic Acid

The average size of the dimer-containing acid-soluble oligonucleotides after incubation of specifically incised DNA with two dimer excising nuclease activities has been determined to be about 8 nucleotides.     

The ultraviolet endonuclease of bacteriophage T4. Further characterization.

The T4 ultraviolet endonuclease was previously shown to produce strand incisions (nicks) in ultraviolet-irradiated DNA on the 5' side of thymine dimers. The present studies demonstrate that the purified endonuclease creates 3'-OH and 5'-P termini at the sites of nicking. Photoreactivation of ultraviolet-sensitive sites, thereby demonstrating directly endonucleause has a molecular weight of approximately 18,000 and attacks ultraviolet-irradiated single-stranded Escherichia coli and M-13 DNA.