Ultraviolet reactivation and ultraviolet mutagenesis of infectious lambda DNA: strong inhibition by treatment of DNA in vitro with UV-endonuclease from Micrococcus luteus.

UV-endonuclease from Microcossuc luteus induces single-stranded breaks in UV-irradiated DNA of phage lambda and the average length of the fragments produced (after UV-doses to DNA of 135 and 675 erg/mm2) is equal to the average spacing between pyrimidine dimers. The plaque-forming ability of UV-irradiated lambda DNA used to infect Ca++-treated uvr A6, uvrB5 or uvrC34 recipient Escherichia coli cells (but not uve+ cells) may be significantly enhanced by treatment of lambda DNA with UV-endonuclease. This enzyme strongly decreases the reactivation of UV-irradiated lambda DNA caused by UV-irradiation of uvr+ or uvrA6 Ca++-treated cells and eliminates most clear-mutations especially if mutations are analysed using Ca++-treated uvr A6 recipient cells. It is concluded that UV-endonuclease switches a significant part of potentially mutagenic pyrimidine dimers from the UV-induced "error-prone" repair pathway to "error-free" excision repair pathway.