DNA polymerase η (Polη) functions in error-free by-pass of ultraviolet light-induced DNA lesions, and mutational inactivation of Polη in humans causes the cancer prone syndrome, the variant form of xeroderma pigmentosum (XPV). Both Saccharomyces cerevisiae and human Polη efficiently insert two adenines opposite the two thymines of a cyclobutane pyrimidine dimer. Interestingly, in the fission yeast Schizosaccharomyces pombe, the eso1 + encoded protein is comprised of two domains, wherein the NH 2 terminus is highly homologous to Polη, and the COOH terminus is highly homologous to the S. cerevisiae Ctf7 protein which is essential for the establishment of sister chromatid cohesion during S phase. Here we characterize the DNA polymerase activity of S. pombe GST-Eso1 fusion protein and a truncated version containing only the Polη domain. Both proteins exhibit a similar DNA polymerase activity with a low processivity, and steady-state kinetic analyses show that on undamaged DNA, both proteins misincorporate nucleotides with frequencies of ∼10-2 to 10 -3. We also examine the two proteins for their ability to replicate a cyclobutane pyrimidine dimer-containing DNA template and find that both proteins replicate through the lesion equally well. Thus, fusion with Ctf7 has no significant effect on the DNA replication or damage bypass properties of Polη. The possible role of Ctf7 fusion with Polη in the replication of Cohesin-bound DNA sequences is discussed.
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