Replisome-mediated Trans-lesion Synthesis and Leading-strand Template Lesion Skipping Are Competing Bypass Mechanisms [DNA and Chromosomes]

October 9th, 2014 by Gabbai, C. B., Yeeles, J. T. P., Marians, K. J.

A number of different enzymatic pathways have evolved to ensure that DNA replication can proceed past template base damage. These pathways include lesion skipping by the replisome, replication fork regression followed by either correction of the damage and origin-independent replication restart or homologous recombination-mediated restart of replication downstream of the lesion, and bypass of the damage by a trans-lesion synthesis DNA polymerase. We report here that of two trans-lesion synthesis polymerases tested, only DNA Polymerase IV, not DNA Polymerase II, could engage productively with the E. coli replisome to bypass leading-strand template damage despite the fact that both enzymes are shown to be interacting with the replicase. Inactivation of the 3′→5′ proofreading exonuclease of DNA Polymerase II did not enable bypass. Bypass by DNA Polymerase IV required its ability to interact with the β clamp and act as a trans-lesion polymerase but did not require it's "Little Finger" domain, a secondary region of interaction with the β clamp. Bypass by DNA Polymerase IV came at the expense of the inherent leading-strand lesion skipping activity of the replisome, indicating that they are competing reactions.