| DNA is continually threatened by endogenous and environmental agents that result in DNA lesions. Most of these DNA lesions are eliminated by error-free DNA repair mechanisms. But some DNA lesions often persist in the genome during replication, especially under circumstances such as (a) when DNA damage occurs during the S phase of cell growth; or (b) when cells sustain significant DNA damage;(c) when some genomic regions are inefficiently repaired; (d) when a particular damage is poorly repaired. Then DNA polymerases would stall at the DNA lesion, DNA replication cannot proceed and block cellular division. More severely, some cells would die. In response, cells have evolved a damage tolerance system to enable replication of the damaged DNA templates. Translesion synthesis represents one of the damage tolerance mechanisms.Translesion synthesis can be divided into two categories: error-free and error-prone. Translesion synthesis requires a specialized DNA polymerase to copy the damaged DNA template. TLS in prokaryotes and eukaryotes (including human) involves multiple recently discovered DNA polymerases including DNA polymerase etc. These polymerases were evolutionarily conserved, and this group is now referred to as the UmuC superfamily. DNA polymerase 4 is not a member of UmuC superfamily, but dose serve a role in TLS.In Escherichia coli, damage-induced mutagenesis is mainly mediated by the UmuCD pathway under the control of the SOS response, DNA polymerase V composed of the UmuD2C protein complex. UmuC contains polymerase activity. In the presence of the RecA and SSB protein, pol V's TLS is highly stimulated. This mechanism largely results in targeted mutations opposite the DNA lesion. Whenreplicating undamaged DNA template pol V is highly mutagenic and forms preferentially purine-purine and pyrimidine-pyrimidine mismatches, resulting in SOS non(un)targeted mutagenesis. Under genotoxic stress conditions, the SOS system additionally turns on the dinB gene that codes for DNA polymerase IV. This polymerase run shot of 3'->5' exonuclease ability, and is a low fidelity polymerase. pol IV take part in the mutagenesis of nonirradiated phage infecting UV-preirradiated bacterial cells(termed UTM for untargeted mutagenesis). When overexpression of pol IV, an increase in F' episomal frameshift mutations can be observed.Recently, mouse and human homologs of DinB were identified, and the genes were designated Dinb1 and DINB1 respectively. Human DINB1 protein was named Pol K. . Purified Pol K is unable to perform translesion synthesis opposite a cis-syn TT dimmer, a TT(6-4) photoproduct and a cisplatin adduct. But it can efficiently bypassed a template 8-oxoguanine, incorporating mainly A. Human Pol most frequently incorporates A opposite a template AP site. However, efficient further extension from opposite the AP site requires T as the next template base, and is mediated mainly by a -1 deletion mechanism. This polymerase was able to bypass an acetylaminofluorene-modified G in DNA, incorporating C and less efficiently A opposite the lesion. Furthermore, Pol efficiently bypassed a template (-)-trans-anti-benzo[a]pyrene-N2-dG lesion in an error -free manner by incorporating a C opposite th bulky adduct. Transient expression of the mouse Dinbl-cDNA in cultured cells caused a nearly 10-fold increase in the incidence of 6-thioguanine resistance mutations. Therefore, we set out to explore the possible role of Pol K in untargeted mutagenesis.Pol belongs to UmuC superfamily also, and is a translesion synthesis polymerase. Poln is very efficient in error-free lesion bypass of a cis-syn TT dimmer, but it cannot bypass of a (6-4)TT photoproduct. However, this polymerase is able to efficiently incorporate a G opposite the 3' T of the (6-4) TT photoproduct before aborting DNA synthesis. When Pol copy undamaged DNA, its fidelity was muchlower than any other template-dependent DNA polymerase studied. Pol lacks an intrinsic proofreading exonuclease activity and, depending on the mismatch, maks one base s...
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