Abstract
The purpose of this work is to investigate a mechanism for synergistic interaction between exposures to radiation and chemicals. The mechanism that we propose is based on the torsional stress that DNA sequences in active genes experience as a consequence of gene expression. Our approach involves experiments on an in vitro plasmid DNA system in conjunction with computer modeling to aid in the interpretation of experimental findings. We have observed that the torsional stress from normal physiological levels of negative supercoiling increases the susceptibility of plasmid pIBI 30 to the induction of single-strand breaks by X rays. Calculations of the conformational equilibrium of pIBI 30 at various levels of torsional stress identified a region of the plasmid that was rich in adenine-thymine (A-T) base pairs and particularly sensitive to melting of the double helix. In this article we present experimental evidence that this region of pIBI 30 is hypersensitive to radiation-induced strand scission. Since bulky DNA adducts in negatively supercoiled DNA tend to produce regions of local denaturation, we speculate that formation of such regions by chemical exposure will sensitize the genome to radiation exposure until the bulky adducts are removed and a proper conformational equilibrium is restored. Experiments with pIBI 30 designed to test this potential mechanism for synergism between radiation and chemical exposure are discussed.