DNA and Chromosomes
The abundant DNA adduct N7-methyl deoxyguanosine contributes to miscoding during replication by human DNA polymerase ηAside from abasic sites and ribonucleotides, the DNA adduct N7-methyl deoxyguanosine (N7-CH3 dG) is one of the most abundant lesions in mammalian DNA. Because N7-CH3 dG is unstable, leading to deglycosylation and ring-opening, its miscoding potential is not well-understood. Here, we employed a 2′-fluoro isostere approach to synthesize an oligonucleotide containing an analog of this lesion (N7-CH3 2′-F dG) and examined its miscoding potential with four Y-family translesion synthesis DNA polymerases (pols): human pol (hpol) η, hpol κ, and hpol ι and Dpo4 from the archaeal thermophile Sulfolobus solfataricus.
Human DNA polymerase η has reverse transcriptase activity in cellular environmentsClassical DNA and RNA polymerase (pol) enzymes have defined roles with their respective substrates, but several pols have been found to have multiple functions. We reported previously that purified human DNA pol η (hpol η) can incorporate both deoxyribonucleoside triphosphates (dNTPs) and ribonucleoside triphosphates (rNTPs) and can use both DNA and RNA as substrates. X-ray crystal structures revealed that two pol η residues, Phe-18 and Tyr-92, behave as steric gates to influence sugar selectivity.
Human DNA polymerase η accommodates RNA for strand extensionRibonucleotides are the natural analogs of deoxyribonucleotides, which can be misinserted by DNA polymerases, leading to the most abundant DNA lesions in genomes. During replication, DNA polymerases tolerate patches of ribonucleotides on the parental strands to different extents. The majority of human DNA polymerases have been reported to misinsert ribonucleotides into genomes. However, only PrimPol, DNA polymerase α, telomerase, and the mitochondrial human DNA polymerase (hpol) γ have been shown to tolerate an entire RNA strand.
Bypass of DNA-Protein Cross-links Conjugated to the 7-Deazaguanine Position of DNA by Translesion Synthesis PolymerasesDNA-protein cross-links (DPCs) are bulky DNA lesions that form both endogenously and following exposure to bis-electrophiles such as common antitumor agents. The structural and biological consequences of DPCs have not been fully elucidated due to the complexity of these adducts. The most common site of DPC formation in DNA following treatment with bis-electrophiles such as nitrogen mustards and cisplatin is the N7 position of guanine, but the resulting conjugates are hydrolytically labile and thus are not suitable for structural and biological studies.
Structural and Kinetic Analysis of Miscoding Opposite the DNA Adduct 1,N6-Ethenodeoxyadenosine by Human Translesion DNA Polymerase η1,N6-Ethenodeoxyadenosine (1,N6-ϵdA) is the major etheno lesion formed in the reaction of DNA with epoxides substituted with good leaving groups (e.g. vinyl chloride epoxide). This lesion is also formed endogenously in DNA from lipid oxidation. Recombinant human DNA polymerase η (hpol η) can replicate oligonucleotide templates containing 1,N6-ϵdA. In steady-state kinetic analysis, hpol η preferred to incorporate dATP and dGTP, compared with dTTP. Mass spectral analysis of incorporation products also showed preferred purine (A, G) incorporation and extensive −1 frameshifts, suggesting pairing of the inserted purine and slippage before further replication.
Mechanism of Ribonucleotide Incorporation by Human DNA Polymerase ηRibonucleotides and 2′-deoxyribonucleotides are the basic units for RNA and DNA, respectively, and the only difference is the extra 2′-OH group on the ribonucleotide sugar. Cellular rNTP concentrations are much higher than those of dNTP. When copying DNA, DNA polymerases not only select the base of the incoming dNTP to form a Watson-Crick pair with the template base but also distinguish the sugar moiety. Some DNA polymerases use a steric gate residue to prevent rNTP incorporation by creating a clash with the 2′-OH group.
Roles of Residues Arg-61 and Gln-38 of Human DNA Polymerase η in Bypass of Deoxyguanosine and 7,8-Dihydro-8-oxo-2′-deoxyguanosineBackground: Arg-61 and Gln-38 of human DNA polymerase (hpol) η play important roles in the catalytic reaction.Results: Mutations R61M or Q38A/R61A dramatically disrupt the activity of hpol η.Conclusion: Polarized water molecules can mimic and partially compensate for the missing side chains of Arg-61 and Gln-38 in the Q38A/R61A mutant.Significance: The positioning and positive charge of Arg-61 synergistically contribute to the activity of hpol η, with additional effects of Gln-38.
Structural and Kinetic Analysis of Nucleoside Triphosphate Incorporation Opposite an Abasic Site by Human Translesion DNA Polymerase ηBackground: Abasic sites are the most common lesion in DNA.Results: Kinetic and mass spectrometric assays demonstrate that human polymerase (pol) η preferentially inserts A and G opposite an abasic site.Conclusion: Crystal structures reveal H-bonding between incoming ATP and GTP and the 5′-phosphate of the abasic moiety.Significance: Abasic site bypass by pol η follows a “purine rule” for insertion, with formation of frameshifts.