SUBSTRATE-SPECIFICITY OF ULTRAVIOLET-ENDONUCLEASE FROM MICROCOCCUS-LUTEUS - ENDONUCLEOLYTIC CLEAVAGE OF DEPURINATED DNA

The UV-endonuclease isolated from M. luteus, specific for pyrimidine dimers, is able to attack not only uv-irradiated DNA (leading to 3''OH-5''PO4 single-strand breaks) but also superhelical covalently-closed DNA of phage .lambda. damaged by heating at 70.degree. C, pH 5.93. The number of endonuclease-sensitive defects in the DNA corresponds to the number of alkali-labile bonds (apurinic sites) induced by heating. Competition between uv-induced lesions and apurinic sites for uv-endonuclease is demonstrated; the affinity of the enzyme for pyrimidine dimers is about 3 times that for apurinic sites. Both activities of the uv-endonuclease are inactivated at 50.degree. C at the same rate. The uv-endonculease is able to reduce the infectious activity of depurinated .lambda. DNA towards Ca2+-treated uvr+ and uvrA Escherichia coli cells. Both pyrimidine dimers and apurinic sites can be recognized by the same enzyme (the uv-endonuclease).