The interaction of heat and radiation affecting the ability of nuclear DNA to undergo supercoiling changes.

DNA damage (putatively strand breaks) from ionizing radiation inhibits the ability of intercalating dyes to induce right-handed supercoils in the DNA loops of HeLa nucleoids [Cook and Brazelle, J. Cell Sci. 22, 287-302 (1976); Roti Roti and Wright, Cytometry 8, 461-467 (1987)] while heat-induced changes in the nuclear matrix enhance this ability [Roti Roti and Painter, Radiat. Res. 89, 166-175 (1982)]. Since heat and radiation interact synergistically or additively on most cellular functions which they affect, the rewinding of DNA supercoils is unusual in that these agents alone affect it in an antagonistic manner. When HeLa cells were exposed to 45 degrees C for 30 min and immediately irradiated with 10 Gy of 137Cs gamma rays, the rewinding response was intermediate between that for cells which had been exposed to 10 Gy only and control. When repair of this damage was assayed in control cells, 97% of the initial damage had been repaired at 30 min postirradiation; at the same time only 10% of the initial damage had been repaired in the heat-shocked cells. This apparent dose reduction effect and the inhibition of repair were interpreted to indicate that heat-induced changes in nuclear structure were masking DNA damage from the assay and the repair system. These effects correlated with the amount of heat-induced excess protein associated with the nucleus and the nucleoid.