NEUTRON AND GAMMA-RADIATION SENSITIVITY OF PLASMID DNA OF VARYING SUPERHELICAL DENSITY

Several families of negatively supercoiled topoisomers of plasmid pIBI 30 were prepared by a modification of the procedure of Singleton and W ells (Anal. Biochem. 122, 253-257, 1982). The average superhelical den sity (a) was determined by two-dimensional agarose gel electrophoresis and varied from -0.010 to -0.067, corresponding to a change in the nu mber of supercoils from 3 to 19 and an effective volume change from 1. 6 x 10(8) to 4 x 10(8) Angstrom(3). Samples were exposed to either fis sion-neutron or Co-60 gamma radiation and assayed for single-strand br eaks by agarose gel electrophoresis. Form I DNA for all topoisomers de creased exponentially with increasing dose. The D-37 values for both n eutron and gamma radiation increased monotonically with increasing \si gma\. Using a branched plectonemic (interwound) form for DNA over the range of sigma studied and standard (single-hit) target theory, a quan titative linear fit to (D-37)(-1) as a function of the effective DNA r adius, S(Angstrom), was obtained. The model predicts that both the slo pe (a) and the intercept (b) of (D-37)(-1) as a function of S(Angstrom ) are directly proportional to the length of DNA and the radiation flu ence, Furthermore, the ratio b/a (= r(o)) 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 r(o) = 13.4 +/- 1.4 nm, a value consistent with other investigations. Our results are consistent with studies using Cs-137 (Milligan et al., Rad iat. Res. 132, 69-73, 1992) but disagree with data obtained for X rays (Miller et al., Int. J. Radiat. Biol. 59, 941-949, 1991). (C) 1995 by Radiation Research Society