DNA Polymerase β can Incorporate Ribonucleotides during DNA Synthesis of Undamaged and CPD-damaged DNA

Overexpression of the error-prone DNA polymerase β (Pol β) has been found to increase spontaneous mutagenesis by competing with the replicative polymerases during DNA replication. Here, we investigate an additional mechanism potentially used by Pol β to enhance genetic instability via its ability to incorporate ribonucleotides into DNA. By using an in vitro primer extension assay, we show that purified human and calf thymus Pol β can synthesize up to 8-mer long RNA. Moreover, Pol β can efficiently incorporate rCTP opposite G in the absence of dCTP and, to a lesser extent, rATP opposite T in the absence of dATP and rGTP opposite C in the absence of dGTP. Recently, Pol β was shown to catalyze in vitro translesion replication of a thymine cyclobutane pyrimidine dimer (CPD). Here, we investigate if ribonucleotides could be incorporated opposite the CPD damage and modulate the efficiency of the bypass process. We find that all four rNTPs can be incorporated opposite the CPD lesion, and that this process affects translesion synthesis. We discuss how incorporation of ribonucleotides into DNA may contribute to the high frequency of mutagenesis observed in Pol β up-regulating cells.