Time Course of PR of UV-Induced Chromosomal Aberrations and Lethal Damage in S and G2 Xenopus Cells

Sand G2 phase cells were exposed to 150 ergs mm⁻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. In S phase cells, the lesions leading to cell death and those leading to aberrations were both converted to a non-photoreactivable state shortly after the UV exposure. A significant fraction of the lesions induced in G2 cells, that led 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 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 nonphotoreactivable state as the progeny of the exposed G2 cells progressed through the first S phase following exposure

Time Course of PR of UV-Induced Chromosomal Aberrations and Lethal Damage in G1 Xenopus Cells

Synchronous cultures of early G1 cells were exposed to UV and their ability to photoreactivate lethal and aberrational damage was determined as a function of time following UV exposure. Lesions leading to cell death were converted to a non-photoreactivable state before cells entered S phase, while lesions leading to chromosomal aberrations were converted to a non-photoreactivable state as the cells entered S phase. These results indicate that the intracellular mechanism which expresses photoreactivable UV-induced lesions in G1 cells as cell death is not identical to the mechanism which expresses such lesions as chromosomal aberrations, and the two mechanisms operate primarily in different phases of the cell cycle

Study of Ultraviolet-Induces Chromatid and Chromosome Aberrations as a Function of Dose in G1 Phase Vertebrate Tissue Cultures

G1 phase A8 Xenopus laevis (toad) and V79B Cricetulus griseus (hamster) tissue cultures were used to observe the frequency of ultraviolet-induced chromosomal aberrations as a function of dose. When cultures are irradiated with ultraviolet light, visible aberrations are virtually absent until a threshold of approximately 80 ergs mm⁻¹ is reached. Aberrations then occur as a nonlinear function of dose. Chromatid aberrations are by far the most prevalent until doses in excess of 200 ergs mm⁻¹ are administered, at which point chromosome aberrations become common

Photoreactivation of Lethal Damage Induced in Hamster X Xenopus Hybrid Cells and Their Parentals by UV Light

A85 Xenopus cells that exhibited a high level of photoreactivation (PR) and V79B2 hamster cells that exhibited little PR were fused to produce the V79B2 x A85 cell line — a hybrid line which possessed a relatively stable karyotype, with most cells containing the entire V79B2 and A85 genomes. UV and UV plus PR fluence-survival relations were then determined and compared for the hybrid and parental lines in a first attempt to elucidate interactions of the parental PR mechanisms in the hybrid. It was anticipated that the A85 genome in the hybrid would produce PR enzyme in sufficient concentration and of such a nature as to efficiently PR UV-induced lethal damage in both A85 and V79B2 DNA, and little difference would be observed in the levels of PR exhibited by the V79B2 x A85 and A85 lines. To the contrary, the level of PR observed for the hybrid was substantially below that observed for the A85 line. To assist in the interpretation of this unexpected observation, three additional preliminary studies were carried out: 1) Comparison of the optimum PR schemes for the A85 and hybrid lines, 2) examination of relations between the PR and dark UV repair mechanisms possessed by these lines, and 3) comparison of the levels of PR of chromatid deletions induced by UV in selected V79B2 and A85 chromosomes of the hybrid. The results suggested that the relatively low level of PR manifested by the hybrid cells was a consequence of their inability to efficiently PR pyrimidine dimers induced by UV in V79B2 DNA

Photoreactivation of UV-Induced Damage in G1 Phase Xenopus Cells That Leads to a Sister Chromatid Exchanged and Cell Death

Experiments were conducted with the A87 Xenopus tissue culture cell line which centered on use of the line\u27s efficient photoreactivation (PR) mechanism to: (1) determine the extent to which sister chromatid exchanges (SCEs), induced by exposing early G1 phase cells to low UV fluenced, are photoreactivable, and (2) determine the extent to which the photoreactivable SCEs resulting from these low UV fluences constitute lethal lesions. For the first determination, UV fluences - SCE frequency relations and UV fluence + PR fluence - SCE frequency relations were established for UV fluences in the range 0-12 J/m2 and a single PR fluence of 22,000 J/m2. Comparison of these relations indicated that the cells photoreactivated a predominant fraction (near .70) of the induced SCEs. For the second determination, a detailed time course of PR of induced SCEs relation and a time course of PR of induced lethality relation were established for the cells, using a single UV fluence of 5.0 J/m2 and a single PR fluence of 22,000 J/M2.Comparison of these relations indicated that few, if any, photoreactivable SCEs constituted photoreactivable lethal lesions. This comparison also suggested that further high resolution cytological studies of time course of PR of UV-induced SCEs may reveal additional relations between repair of SCEs and changes in vertebrate chromosome structure as cells progress through interphase