Effect of near ultraviolet and visible light on amoeba

The near ultraviolet and visible light (VL) impinging at an intensity of 2.5 x 102Js-1m-2 for 2.5h killed the mitotic and early S-phase (0- to 15-min-old) amoebae. At the mid- and late S-period only a fraction of cells were killed by VL and the G2 phase cells were quite resistant. Amoebae of all cell cycle stages showed a delay in the first mitotic division. DNA synthesis, as measured by [3H] thymidine incorporation, was depressed in the VL-exposed early-S amoebae. A concurrent but temporary inhibition in [3H] leucine incorporation also occurred in these cells. However, no significant change in [3H] uridine incorporation has been found. To localize the site of lethal damage, nuclear transplantation studies were undertaken between the control amoebae and the amoebae treated with VL. The nucleus of VL-exposed early S-phase cell recovered when transplanted immediately after VL exposure into an enucleate G2 cytoplasm but died if grafted into an enucleate S-phase cytoplasm. The therapeutic effect of the G2 cytoplasm, although at a lower level, was also evident even when the treated early S-phase nucleus was implanted 20h later, but not after 48h, into the G2 cytoplasm. The amoeba cytoplasm showed resistance to VL-irradiation, could accept a control nucleus from any cell cycle stage, and function normally. The G2 nucleus also remained apparently unaffected to VL exposure and could survive when it was transferred to the control cytoplasm of any cell-cycle phase. All these findings are discussed in the light of the possible existance of a repair system against VL-induced damage in the G2-phase amoeba. (author)