WAVELENGTH DEPENDENCE OF THE FORMATION OF SINGLE‐STRAND BREAKS AND BASE CHANGES IN DNA BY THE ULTRAVIOLET RADIATION ABOVE 150 nm

The wavelength dependence of the formation of two types of DNA damage, single‐strand breaks and base changes, was investigated in the UV region from 150 nm to 254 nm using superhelical closed circular (form I) colicin El DNA with synchrotron radiation. Single‐strand breaks were measured by agarose gel electrophoresis as a direct conversion of form I DNA to form II DNA (open circular). Base damages were defined as sensitive sites to a crude extract of endonuclease from Micrococcus luteus. They also were estimated using the same conversion, from form I to form II after the DNA was treated with endonuclease. The fluence‐effect relationship could be fitted by a simple exponential function for both types of damage. Action spectra were constructed based on the reciprocal of the 37% fluence. The action spectrum for strand breaks increased rather monotonically over three decades from 254 nm to 150 nm in a logarithmic scale, while that for base damages showed a breaking point at 190 nm, being relatively flat above 190 nm. The characteristics of the action spectra are compared with the absorption spectra of the DNA and its main chain moiety calculated on the basis of data on calf thymus DNA and synthetic polynucleotides. Our main conclusions are (1) that the majority of single‐strand breaks were induced by the absorption of photon in the sugar‐phosphate group in the vacuum‐UV region and (2) that the base changes were induced equally well by absorption in the vacuum‐UV and in the far‐UV region.