Critical energies for SSB and DSB induction in plasmid DNA by low-energy photons: action spectra for strand-break induction in plasmid DNA irradiatedin vacuum
Km. Prise et al., Critical energies for SSB and DSB induction in plasmid DNA by low-energy photons: action spectra for strand-break induction in plasmid DNA irradiatedin vacuum, INT J RAD B, 76(7), 2000, pp. 881-890
Purpose: To measure action spectra for the induction of single- strand brea
ks (SSB) and double-strand breaks (DSB) in plasmid DNA by low-energy photon
s and provide estimates for the energy dependence of strand-break formation
important for track-structure simulations of DNA damage.
Materials and methods: Plasmid pMSG-CAT was irradiated as a monolayer, unde
r vacuum, with 7-150 eV photons produced by a synchrotron source. Yields of
SSB and DSB were determined by the separation of the three plasmid forms b
y gel electrophoresis.
Results: The yields of SSB per incident photon increased from 1.4 x 10(-15)
SSB per plasmid per photon/cm(2) at 7 eV to 7.5 x 10(-14) SSB per plasmid
per photon/cm(2) at 150 eV. Direct induction of DSB was also detected incre
asing from 3.4 x 10(-17) DSB per plasmid per photon/cm(2) at 7 eV to 4.1 x
10(-15) DSB per plasmid per photon/cm(2) at 150 eV. When the absorption cro
ss-section of the DNA was considered, the quantum efficiency for break form
ation increased over the energy range studied. Over the entire energy range
, the ratio of SSB to DSB remained constant.
Conclusions: These studies provide evidence for the ability of photons as l
ow as 7 eV to induce both SSB and DSB. The common action spectrum for both
lesions suggests that they derive from the same initial photoproducts under
conditions where the DNA is irradiated in vacuum and a predominantly direc
t effect is being observed. The spectral and dose-effect behaviour indicate
s that DSB are induced predominantly by single-event processes in the energ
y range covered.