Polι, a remarkably error-prone human DNA polymerase

The Saccharomyces cerevisiae RAD30 gene encodes DNA polymerase eta. Humans possess two Rad30 homologs. One (RAD30A/POLH) has previously been characterized and shown to be defective in humans with the Xeroderma pigmentosum variant phenotype. Here, we report experiments demonstrating that the second human homolog (RAD30B), also encodes a novel DNA polymerase that we designate pol iota. pol iota, is a distributive enzyme that is highly error-prone when replicating undamaged DNA. At template G or C, the average error frequency was similar to 1 x 10(-2). Our studies revealed, however, a striking asymmetry in misincorporation frequency at template A and T. For example, template A was replicated with the greatest accuracy, with misincorporation of G, A, or C occurring with a frequency of similar to 1 x 10(-4) to 2 x 10(-4). In dramatic contrast, most errors occurred at template T, where the misincorporation of G was, in fact, favored similar to 3:1 over the correct nucleotide, A, and misincorporation of T occurred at a frequency of -6.7 x 10(-1). These findings demonstrate that pol iota is one of the most error-prone eukaryotic polymerases reported to date and exhibits an unusual misincorporation spectrum in vitro.