DNA molecules of pUC19, pBR322 and PhiX174 were irradiated by various doses of UV light and the irradiated molecules were cleaved by about two dozen type II restrictases. The irradiation generally blocked the cleavage in a dose-dependent way. In accordance with previous studies, the (A + T)-richness and the (PyPy) dimer content of the restriction site belongs among the factors that on average, cause an increase in the resistance of UV damaged DNA to the restrictase cleavage. However, we observed strong effects of UV irradiation even with (G + C)-rich and (PyPy)-poor sites. In addition, sequences flanking the restriction site influenced the protection in some cases (e.g. HindIII), but not in others (e.g. SalI), whereas neoschizomer couples SmaI and AvaI, or SacI and Ecl136II, cleaved the UV-irradiated DNA similarly. Hence the intrastrand thymine dimers located in the recognition site are not the only photoproduct blocking the restrictases. UV irradiation of the A-form generally made the irradiated DNA less resistant to restrictase cleavage than irradiation in the B-form and in some cases, the A-form completely protected the UV-irradiated DNA against the damage recognized by the restrictases. The present results also demonstrate that the UV irradiation approach used to generate partial digests in genomic DNA studies, can be extended to the (G + C)-rich and (PyPy)-poor restriction sites. The present extensive and quantitative data can be used in genomic applications of UV damage probing by restrictases.

Institute of Biophysics Academy of Sciences of the Czech Republic Královopolská 135 CZ 612 65 Brno Czech Republic

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