Xeroderma pigmentosum variant (XP-V) correcting protein from HeLa cells has a thymine dimer bypass DNA polymerase activity

Xeroderma pigmentosum variant (XP-V) represents one of the most common form s of this cancer-prone DNA repair syndrome. Unlike classical XP cells, XP-V cells are normal in nucleotide excision repair but defective in post-repli cation repair. The precise molecular defect in XP-V is currently unknown, b ut it appears to be a protein involved in translesion synthesis. Here we es tablished a sensitive assay system using an SV40 origin-based plasmid to de tect XP-V complementation activity. Using this system, we isolated a protei n from HeLa cells capable of complementing the defects in XP-V cell extract s. The protein displays novel DNA polymerase activity which replicates cycl obutane pyrimidine dimer-containing DNA templates. The XPV polymerase activ ity was dependent on MgCl2, sensitive to NEM, moderately sensitive to KCl, resistant to both aphidicolin and ddTTP, and not stimulated by PCNA. In gly cerol density gradients, the activity co-sedimented with a 54 kDa polypepti de at 3,5S, indicating that the monomeric form of this polypeptide was resp onsible for the activity. The protein factor corrected the translesion defe cts of extracts from three XPV cell strains. Bypass DNA synthesis by the XP -V polymerase occurred only in the presence of dATP, indicating that it can incorporate only dATP to bypass a di-thymine lesion.