DNA-to-protein cross-linking in synchronized HeLa cells exposed to ultra-violet light

The U.V.-induced DNA-to-protein cross-linking in synchronously growing HeLa cells exhibits age-dependent variations. The maximum yield of cross-linking is observed during S-phase and exposure of mid-S cells to 250 ergs/mm2 renders about 22 per cent of DNA unextractable owing to cross-linking with proteins. Cells irradiated in G1 or G2 exhibit much smaller amount of cross-linking. The unextracted DNA was recovered from the protein fraction, and the addition of 0-5 per cent trypsin solution rendered all the DNA extractable. Fluctuations in the age-response pattern for the yield of DNA-protein cross-links varies throughout the cell-cycle in the same way as the efficiency for cell-killing, i.e. cells are resistant in G1, sensitive as they move into and through the S phase, and resistant again in G2. Dose-response curves determined at different times in the cycle reveal that up to about 500 ergs/mm2 the amount of DNA recovered from S cells decreased for about 8 per cent for every 100 ergs/mm2 of U.V. light. The amount of recovered DNA remained unchanged for cells exposed to 500 ergs/mm2 in the middle of S, for as long as 8 hours after exposure, i.e. for as long as the experiments were carried out no recovery from this damage was observed.

Ultraviolet light-induced division delayed in synchronized Chinese hamster cells

The age-dependent, ultraviolet light (UVL) (254 nm)-induced division delay of surviving and nonsurviving Chinese hamster cells was studied. The response was examined after UVL exposures adjusted to yield approximately the same survival levels at different stages of the cell cycle, 60% or 30% survival. Cells irradiated in the middle of S suffered the longest division delay, and cells exposed in mitosis or in G(1) had about the same smaller delay in division. Cells irradiated in G(2), however, were not delayed at either survival level. It was further established, after exposures that yielded about 30% survivors at various stages of the cycle, that surviving cells had shorter delays than nonsurvivors. This difference was not observed for cells in G(2) at the time of exposure; i.e., neither surviving nor nonsurviving G(2) cells were delayed in division. The examination of mitotic index vs. time revealed that most cells reach mitosis, but all of the increase in the number of cells in the population can be accounted for by the increase of the viable cell fraction. These observations suggest strongly that nonsurviving cells, although present during most of the experiment, are stopped at mitosis and do not divide. Cells in mitosis at the time of irradiation complete their division, and in the same length of time as unirradiated controls. Division and mitotic delays after UVL are relatively much larger than after X-ray doses that reduce survival to about the same level.

Sensitivity of synchronized Chinese hamster cells to ultraviolet light

The age response for lethality of Chinese hamster cells to ultraviolet light shows that they are resistant in G(1), sensitive as they move into and through the S phase and resistant again in G(2) and mitosis. Survival curves determined at different times in the cycle reveal that mitotic cells are the most resistant fraction, much more resistant than S cells, and more resistant than either G(1) or G(2) cells. The extent to which the age response is ilfluenced by nucleic acid and protein synthesis was investigated by using inhibitors of these processes. In the presence of inhibitors of DNA or protein synthesis added to G(1) cells before exposure, cell survival neither declines to the minimum survival of S cells nor rises subsequently to the resistance of G(2) cells. If, before exposure, DNA synthesis is arrested in the middle of S, when survival is at a minimum, the subsequent rise in survival during G(2) is not prevented. However when cycloheximide is added before exposure, during the middle of S, this rise is prevented. When actinomycin D, an inhibitor of RNA synthesis is added prior to exposure the age response is affected only slightly. Postirradiation treatment of G(1) and mid-S cells with inhibitors of DNA or protein synthesis maintains survival at a level characteristic of the age of the cells.