Template properties of ultraviolet-irradiated poly(dC) replicated by E. coli DNA polymerase I: indication for a role of apyrimidinic-sites in UV-induced mutagenesis.

Ultraviolet irradiation alters the template properties of poly(dC) when replicated by Escherichia coli DNA polymerase I. These effects are due to base modifications. Some of them are identified as apurinic/apyrimidinic sites (AP-sites) by their sensitivity to AP-endonuclease B purified from Micrococcus luteus, and their template properties. The rate of formation of AP-sites in poly(dC) is estimated at 3 X 10(-7) site per nucleotide per J.m-2. Exposure of supercoiled or relaxed pBR322 DNA to UV light results also in the formation of sites sensitive to AP-endonuclease B. In this case, the rate of formation of AP-sites is the same in relaxed or supercoiled DNA: 0.3 X 10(-7) site per nucleotide per J.m-2. The apyrimidinic sites are generated through the processing of an ultraviolet induced primary lesion. We suggest that this lesion is cytosine hydrate by its rate of decay and preferential formation in single stranded DNA. Our results suggest that AP-sites might be a minor pathway leading to UV-induced mutagenesis.