Escherichia coli RNA and DNA polymerase bypass of dihydrouracil: mutagenic potential via transcription and replication.

Dihydrouracil (DHU) is a DNA base damage product produced in significant amounts by ionizing radiation damage to cytosine under anoxic conditions. DHU represents a model for pyrimidine base damage (ring saturation products) of the type recognized and repaired by Escherichia coli endonuclease III and its homologs in other species. We have built this lesion into synthetic oligonucleotides, with DHU placed at a single location downstream from an E.coli RNA polymerase promoter. This construct was used to determine the effect of DHU when encountered on a DNA template strand by either E.coli RNA or DNA polymerase (Klenow fragment). Single round transcription experiments or primer extension-type replication experiments were conducted in order to make a direct comparison between RNA and DNA polymerases with DHU placed within the same sequence context. Both DNA and RNA polymerase efficiently bypass DHU and insert adenine opposite this lesion. These results suggest that DHU is mutagenic with respect to both replication and transcription and have implications for DNA repair as well the routes leading to generation of mutant proteins in dividing and non-dividing cells.