Neutron and gamma-radiation sensitivity of plasmid DNA of varying superhelical density

Several families of negatively supercoiled topoisomers of plasmid pIBI30 were prepared by a modification of the procedure of Singleton and Wells (Anal. Biochem. 122, 253-257, 1982). The average superhelical density (sigma) was determined by two-dimensional agarose gel electrophoresis and varied from -0.010 to -0.067, corresponding to a change in the number of supercoils from 3 to 19 and an effective volume change from 1.6 x 10(8) to 4 x 10(8) A3. Samples were exposed to either fission-neutron or 60Co gamma radiation and assayed for single-strand breaks by agarose gel electrophoresis. Form I DNA for all topoisomers decreased exponentially with increasing dose. The D37 values for both neutron and gamma radiation increased monotonically with increasing magnitude of sigma. Using a branched plectonemic (interwound) form for DNA over the range of sigma studied and standard (single-hit) target theory, a quantitative linear fit to (D37)-1 as a function of the effective DNA radius, S(A), was obtained. The model predicts that both the slope (a) and the intercept (b) of (D37)-1 as a function of S(A) are directly proportional to the length of DNA and the radiation fluence. Furthermore, the ratio b/a (= ro) at sigma = 0 depends only on the ionic strength of the medium and is independent of the radiation source parameters. Our results support the model and we calculate ro = 13.4 +/- 1.4 nm, a value consistent with other investigations. Our results are consistent with studies using 137Cs (Milligan et al., Radiat.(ABSTRACT TRUNCATED AT 250 WORDS)