Dna Replication, Repair, And Mutagenesis In Eukaryotic A

Child Health And Human Development

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Abstract

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 are phylogenetically related DNA polymerases that have collectively been termed the Y-family of DNA polymerases. In the past year, scientific studies within the section have focussed on understanding the molecular mechanisms of translesion replication in all three kingdoms of life: bacteria, archaea and eukaryotic cells. In E. coli, this process only occurs when UmuC physically interacts with UmuD? to form UmuD?2C, (polV). Because polV is a low-fidelity enzyme, its activities within the cell are strictly controlled. For example, the enzyme is greatly stimulated by interactions with the RecA protein. Interestingly, these studies suggest that two distinct biochemical modes of RecA binding are necessary for pol V-catalyzed translesion replication. One RecA mode is characterized by a strong stimulation in nucleotide incorporation either directly opposite a lesion or at undamaged template sites, but by the absence of lesion bypass. A separate RecA mode is necessary for translesion synthesis Scientist within the section have recently identified and cloned a DinB homolog from the archaeon Sulfolobus solfataricus P2, called DNA polymerase IV (Dpo4). Characterization of the enzyme reveals that the protein possesses many biochemical properties similar to other DinB polymerases including a propensity to make frameshift mutations. S. solfataricus Dpo4 has been overproduced, purified and its structure has been solved by X-ray crystallography. Like all DNA polymerases characterized to date, the enzyme possesses a topology similar to a right hand with domains that resemble ?fingers?, a ?palm? and a ?thumb?. Dpo4 also possesses a unique domain called the ?little finger? that helps the enzyme bind to DNA. Interestingly, the active site of the enzyme is sufficiently large enough to accommodate significant structural rearrangements of the nascent primer terminus including primer-template misalignment and flipping of bases into the minor groove, so as to avoid the lesions in DNA. Studies with human DNA polymerase iota, which was recently discovered by scientist in the section, revealed that in addition to exhibiting a remarkable template-dependent misincorporation spectrum on undamaged DNA in vitro, the enzyme is also highly error-prone when copying a variety of DNA lesions. An exception was at Benzo[a]pyrene diol epoxide adducts of deoxyadenosine, where the enzyme efficiently inserted the correct base, dTMP, opposite the adducted adensosine base. Further elongation was, however, limited and appears to be performed by the related Y-family polymerase, pol kappa. Based upon our in vitro observations, we hypothesize that pol iota and pol kappa act together to facilitate the error-free bypass of diol epoxide-adducted deoxyadenosine in vivo and in doing so, protect humans from the carcinogenic effects of exposure to Beno[a]pyrene diol epoxides. We have also recently examined the sub-cellular localization of pol iota within a living human cell. These studies revealed that despite the fact that pol iota lacks an obvious nuclear localization signal, it is predominantly localized to the nucleus, where it associates with the cell?s normal replication machinery. Following DNA damage, pol iota accumulates into discrete foci at sites of stalled replication forks. Interestingly, the pattern of foci formation was identical to that previously reported for the related Y-family polymerase, pol eta, suggesting that damage-induced pol iota- and pol eta-foci formation is tightly coordinated within the cell. Using the yeast two-hybrid assay, in vitro ?pull-down? assays and Far western analysis, we discovered that pol eta and pol iota interact with each other. Our data suggest, therefore, that human pols eta and iota may coexist in a larger holoenzyme complex whereby their lesion-bypassing activities can be coordinated in response to DNA damage and that both enzymes may play a general role in maintaining genomic integrity, as well as participating in translesion replication.