Proliferating cell nuclear antigen (PCNA) has been shown to interact with a variety of DNA polymerases (pol) such as pol ␦, pol ⑀, pol , pol , pol , and pol ␤. Here we show that PCNA directly interacts with the newly discovered pol cloned from human cells. This interaction stabilizes the binding of pol to the primer template, thus increasing its affinity for the hydroxyl primer and its processivity in DNA synthesis. However, no effect of PCNA was detected on the rate of nucleotide incorporation or discrimination efficiency by pol . PCNA was found to stimulate efficient synthesis by pol across an abasic (AP) site. When compared with pol ␦, human pol showed the ability to incorporate a nucleotide in front of the lesion. Addition of PCNA led to efficient elongation past the AP site by pol but not by pol ␦. However, when tested on a template containing a bulky DNA lesion, such as the major cisplatin Pt-d(GpG) adduct, PCNA could not allow translesion synthesis by pol . Our results suggest that the complex between PCNA and pol may play an important role in the bypass of abasic sites in human cells.In the last few years the number of known eukaryotic DNA polymerases (pols), 1 including terminal transferase and telomerase, has increased to at least 19 (1). A particular pol might have more than one functional task in a cell, and a particular DNA transaction may require more than one pol, suggesting that nature has provided various safety mechanisms. This multifunctional feature is especially valid for the variety of novel pols identified in the last 3 years. These are the lesion-replicating enzymes pol , pol , pol , pol , and Rev1, and a group of pols called pol , pol , pol , pol , and pol that fulfill a variety of other tasks. The gene encoding the novel pol was cloned and mapped to mouse chromosome 19 and to human chromosome 10 (2, 3). pol contains all the critical residues involved in DNA binding, nucleotide binding, nucleotide selection, and catalysis of DNA polymerization and has been assigned to family X based on sequence homology with pol ␤, pol , and terminal deoxynucleotidyltransferase. pol has been suggested to play a role in meiotic recombination and DNA repair, and the recent demonstration of an intrinsic 5Ј-deoxyribose-5-phosphate lyase activity in pol supports a function of this enzyme in base excision repair (4, 5). Cloned and purified human (h) pol inserts nucleotides in a DNA template-dependent manner and is processive in small gaps containing a 5Ј-phosphate group. These properties, together with its nucleotide insertion fidelity parameters and lack of proofreading activity, indicate that hpol is a novel pol ␤-like enzyme (5). Recently, pol was purified from calf thymus tissue, and the biochemical properties of the native calf thymus pol were shown to be similar to the recombinant hpol protein (6). In particular, it has been shown that the native calf thymus enzyme was able to synthesize DNA on a template containing abasic (AP) sites with the same efficiency as on undamaged DNA, thus suggesting a potenti...

Human DNA polymerases (pols) beta and lambda could promote template slippage and generate -1 frameshifts on defined heteropolymeric DNA substrates containing a single abasic site. Kinetic data demonstrated that pol lambda was more efficient than pol beta in catalyzing translesion DNA synthesis past an abasic site, particularly in the presence of low nucleotide concentrations. Moreover, pol lambda was found to generate frameshifts in two ways: first, by using a nucleotide-stabilized primer misalignment mechanism, or second, by promoting primer reannealing using microhomology regions between the terminal primer sequence and the template strand. Our results suggest a molecular mechanism for the observed high in vivo rate of frameshifts generation by pol lambda and highlight the remarkable ability of pol lambda to promote microhomology pairing between two DNA strands, further supporting its proposed role in the nonhomologous end joining process.