The most stable DNA-lesions generated by irradiation with ultraviolet light (254 nm) are two kinds of pyrimidine dimers (PDs), cylobutanc pyrimidine dimers (CPDs) and (6-4) photopro-ducts (6-4PP) (1). Blockage of DNA-polymerase Irom Thermits aquaticus(Taq polymera.se) at PDs has been applied in primer extension assays lor photofootprinting (2) and for mapping PDs at nucleotide resolution (3,4). These assays depend on an efficient and precise bloekage at PDs. Using plastnid DNA with a site specific CPD or 6-4PP and irradiated yeast DNA, we report here that Taq DNA polyrnerase is indeed completely blocked at PDs. Furthermore, we show that the signal depends on the local sequence and on whether the DNA was pretreated with T4-endonuclease V (T4-endoV)or with T4-endoV and DNA-phofolyase. These results justify the. use of primer extension protocols for quuntitation of PDs and they point out some technical limitations of this approach.
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