BREAKS IN PLASMID DNA STRAND INDUCED BY L ASER-RADIATION AT A WAVELENGTH OF 193 NM

DNA of plasmid pBR322 irradiated with laser at a wavelength of 193 nm was treated with an extract containing proteins from E.coli K12 AB1157 (wild-type). The enzymes were found to produce single and double-stra nd DNA breaks, which was interpreted as a transformation of a portion of cyclobutane pyrimidine dimers and (6-4) photoproducts into nonrepai rable single-strand DNA breaks. The products resulted from ionization of DNA, in particular, single-strand breaks, transform to double-stran d breaks. A comparison of these data with the data on survival of plas mid upon transformation of E.coli K12 AB1157 enables one to assess the biological significance of single and double-strand breaks. The inact ivation of the plasmid (in AB1157) is mainly determined by the number of directly formed laser-induced single-strand breaks, whereas the con tribution of enzymatically produced single- and double-strand breaks i s insignificant.