H. Gaston Griggs

TIME COURSE OF PHOTOREACTIVATION OF UV‐INDUCED CHROMOSOMAL ABERRATIONS AND LETHAL DAMAGE IN INTERPHASE XENOPUS CELLS

Abstract— Sets of G1, S, and G2 phase Xenopus cells were exposed to 15.0 Jm−2 UV and their ability to photoreactivate the induced cell killing (loss of colony forming ability) and chromosomal aberrations was determined as a function of time following the UV exposure. Most of the lesions induced in G1 cells that lead to cell death were converted to a non‐photoreactivable state before the cells entered the S phase, while lesions leading to chromosomal aberrations were converted to a non‐photoreactivable state as the cells entered the S phase. In S phase cells the UV‐induced lesions leading to aberrations appeared to be converted to a non‐photoreactivable state at a much faster rate than those leading to cell death. A significant fraction of the lesions induced in G2 cells, that lead to cell death, were converted to a non‐photoreactivable state before the progeny of the exposed cells reached the next succeeding S phase. Few, if any, lesions were induced in G2 cells that were expressed as aberrations at the first mitosis following exposure. Some of the lesions induced in the G2 cells led to aberrations that were observable in the progeny that progressed to the second mitosis following exposure. These lesions were converted to a non‐photoreactivable state as the progeny of the exposed G2 cells progressed through the first S phase following exposure. These results suggest that the intracellular mechanism which expresses photoreactivable UV‐induced lesions as cell death is not identical to the mechanism which expresses such lesions as chromosomal aberrations, and the two mechanisms operate with different efficiencies in different phases of the cell cycle.

CHROMOSOMAL ABERRATIONS RESULTING FROM UV EXPOSURES TO G1XENOPUS CELLS

Abstract— The dose dependence of chromosomal aberration frequencies induced by UV light in G1Xenopus cells has been examined. Total aberration frequencies did not exceed control levels until a dose of approximately 7.5 Jm‐2 was administered. Chromatid type aberrations were prevalent throughout the dose range 7.5–25.0 Jm‐2. Chromosome type aberration frequencies were comparatively low but exceeded control levels throughout the range 10.0–25.0 Jm‐2, Results of supporting experiments suggested that most of the chromosome type aberrations observed were in the second mitosis following exposures and were derived from aberrations which normally appear as chromatid types at the first mitosis following exposures.

Photoreactivation of lethal damage and damage leading to chromatid deletions induced in G1 phase hamster x Xenopus hybrid cells by UV.

A86 Xenopus cells, cloned from a Xenopus line that exhibited a high level ot photoreacti‐vation of UV‐induced lethal damage, and V79M1 hamster cells, cloned from a hamster line that did not exhibit efficient photoreactivation of such damage, were fused to produce the V79M1 A86 cell line ‐ a hybrid line in which approximately 84% of the cells contained the entire V79M1 and A86 genomes. Ultraviolet and UV plus photoreactivation fluence‐survival relations were then determined and compared for hybrid and parental Gl phase cells in a first attempt to elucidate interactions of the parental genetic potentials for photoreactivation in the hybrid. Specifically, it was anticipated that the combined V79M1 and A86 genomes in the hybrid would produce photoreactivating enzymes sufficient to efficiently photoreactivate UV‐induced lethal damage in both A86 and V79M1 DNA and little difference would be observed in the levels of photoreactivation exhibited by V79M1 A86 and A86 Gl phase cells. To the contrary, the level of photoreactivation observed for the hybrid did not closely approach that observed for the A86 line. To assist in the interpretation of this somewhat unexpected observation, three additional studies were performed: (1) comparison of ‘optimal’ schemes for photoreactivation of UV‐induced lethal damage in the hybrid and parental Gl phase cells, (2) comparison of the effects of some different types of growth medium on photoreactivation of UV‐induced lethal damage in hybrid and parental Gl phase cells, and (3) comparison of the levels of photoreactivation of UV‐induced chromatid deletions in the V79M1 and A86 chromosomes of Gl phase hybrid cells. The results suggested that the relatively low level of photoreactivation of UV‐induced lethal damage manifested by the hybrid cells could be attributed, at least in part, to their inability to efficiently photoreactivate pyrimidine dimers induced in the DNA of their V79M1 chromosomes.