Beyond translesion synthesis: polymerase κ fidelity as a potential determinant of microsatellite stability

Microsatellite DNA synthesis represents a significant component of human genome replication that must occur faithfully. However, yeast replicative DNA polymerases do not possess high fidelity for microsatellite synthesis. We hypothesized that the structural features of Y-family polymerases that facilitate accurate translesion synthesis may promote accurate microsatellite synthesis. We compared human polymerases kappa (Pol kappa) and eta (Pol eta) fidelities to that of replicative human polymerase delta holoenzyme (Pol delta 4), using the in vitro HSV-tk assay. Relative polymerase accuracy for insertion/deletion (indel) errors within 2-3 unit repeats internal to the HSV-tk gene concurred with the literature: Pol delta 4 >> Pol kappa or Pol eta. In contrast, relative polymerase accuracy for unit-based indel errors within [GT](10) and [TC](11) microsatellites was: Pol kappa epsilon Pol delta 4 > Pol eta. The magnitude of difference was greatest between Pols kappa and delta 4 with the [GT] template. Biochemically, Pol kappa displayed less synthesis termination within the [GT] allele than did Pol delta 4. In dual polymerase reactions, Pol kappa competed with either a stalled or moving Pol delta 4, thereby reducing termination. Our results challenge the ideology that pol kappa is error prone, and suggest that DNA polymerases with complementary biochemical properties can function cooperatively at repetitive sequences.