Lesions in DNA often pose considerable impediments to genome duplication. To overcome this block to DNA replication, cells utilize specialized accessory factors that allow synthesis of nascent DNA chains opposite the blocking lesion. Recent studies suggest that many of the key participants in translesion DNA synthesis belong to a large family of structurally related DNA polymerases that are found in prokaryotes, archaea and eukaryotes. Phylogenetic analysis of these polymerases suggest that they can be broadly subdivided into four groups typified by Escherichia coli UmuC, E. coli DinB, Saccharomyces cerevisiae Rev1 and the S. cerevisiae Rad30 protein. In the past year, the laboratory has focussed on mechanisms of translesion replication at both ends of the evolutionary spectrum. In E. coli, this process only occurs when UmuC physically interacts with UmuD' to form UmuD'2C, (polV). Because polV is s a low-fidelity enzyme, its activities within the cell are strictly controlled. Studies from the lab revealed that the UmuD' subunit of polV is specifically targeted for degradation by the ClpXP serine protease by a novel mechanism of """"""""trans-targeting"""""""": a process whereby the signal for degradation is in fact located in a protein that is not actually degraded by the enzyme. Since cells contain very low levels of polV, there must be a mechanism whereby these molecules are targeted to regions of the chromosome where they can perform translesion replication. Genetic analysis suggested that such targeting is achieved via an interaction with RecA. Indeed, such targeting was visualized by electron microscopy which revealed that when present at low concentrations, polV specifically binds to the tip of a RecA nucleoprotein filament. Biochemical studies with polV also reveal a requirement for the beta and gamma complexes from E. coli pol III holoenzyme as well as the E. coli single stranded-binding protein. Ongoing studies aim to understand the precise mechanism by which all the polV, RecA, beta/gamma complex and Ssb proteins interact to facilitate translesion replication. Studies with human enzymes suggest that most translesion replication is performed by DNA polymerase eta. The lab has, however, discovered a related polymerase, called pol iota. Interestingly, unlike pol eta which replicates thymine-thymine cyclobutane pyrimidine dimers efficiently and accurately, pol iota's ability to bypass pyrimidine dimers is quite limited, but is highly error-prone. Biochemical characterization of pol iota reveals that is also """"""""error-prone"""""""" on undamaged DNA templates. On average, pol iota makes one error every 100 nucleotides replicated. However, the fidelity of the enzyme is also strikingly dependent upon the template base being replicated. For example, guanosine is inserted opposite thymine at least three fold better than the """"""""correct"""""""" base adenosine. Ongoing studies are aimed at elucidating the function of this remarkable polymerase within the cell.
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