Using a fluorescent cytosine analogue tC(o) to probe the effect of the Y567 to Ala substitution on the preinsertion steps of dNMP incorporation by RB69 DNA polymerase

Quasi-intrinsic fluorescent probes for detecting the DNA adduct (ABPdG) based on an excited-state intermolecular charge transfer mechanism

N'-(2'-Deoxyguanosin-8-yl)-4-aminobiphenyl (^ABPdG) is one of the most representative carcinogenic DNA adducts formed by human exposure to 4-aminobiphenyl (4-ABP) during dye production, rubber-manufacturing processes and cigarette smoke. Accordingly, the ultrasensitive detection of ABP-derived adducts in DNA with minimal interference to the native structures becomes key for elucidating carcinogenesis mechanisms and mitigating the risk of cancer. In view of the lack of efficient optical emission in ^ABPG, we report a theoretical study on the photophysical properties of a set of quasi-intrinsic fluorescent C-analogues, which can form stable W-C base pairs with ^ABPG. It is found that fluorophore replacement and ring-expansion can bring a red-shifted absorption and bright photoluminescence due to additional π-conjugation. In particular, because the tricyclic cytosine analogue 1,3-diaza-2-oxophenoxazine (tC^O) possesses distinct optical properties, it is proposed as a biosensor to identify ^ABPG. The TDDFT-calculated absorption maximum of tC^O is red-shifted by 97 nm in comparison with that of the native C base, which contributes to selective excitation after incorporating into the nucleic acids. Although the fluorescence is insensitive to base pairing with natural guanine, the excited state intermolecular charge transfer (ESICT)-governed "OFF-ON" signal can be observed in the presence and absence of ^ABPG. Moreover, to evaluate the direct availability of the bright C-analogues with high selectivity for the deoxyguanosine adduct ^ABPG in DNA, we further investigated thoroughly the effects of its linking to deoxyribose on its absorption and emission, which shows little difference from that of experiment.

RB69 DNA polymerase structure, kinetics, and fidelity

This review will summarize our structural and kinetic studies of RB69 DNA polymerase (RB69pol) as well as selected variants of the wild-type enzyme that were undertaken to obtain a deeper understanding of the exquisitely high fidelity of B family replicative DNA polymerases. We discuss how the structures of the various RB69pol ternary complexes can be used to rationalize the results obtained from pre-steady-state kinetic assays. Our main findings can be summarized as follows. (i) Interbase hydrogen bond interactions can increase catalytic efficiency by 5000-fold; meanwhile, base selectivity is not solely determined by the number of hydrogen bonds between the incoming dNTP and the templating base. (ii) Minor-groove hydrogen bond interactions at positions n – 1 and n – 2 of the primer strand and position n – 1 of the template strand in RB69pol ternary complexes are essential for efficient primer extension and base selectivity. (iii) Partial charge interactions among the incoming dNTP, the penultimate base pair, and the hydration shell surrounding the incoming dNTP modulate nucleotide insertion efficiency and base selectivity. (iv) Steric clashes between mismatched incoming dNTPs and templating bases with amino acid side chains in the nascent base pair binding pocket (NBP) as well as weak interactions and large gaps between the incoming dNTPs and the templating base are some of the reasons that incorrect dNTPs are incorporated so inefficiently by wild-type RB69pol. In addition, we developed a tC°–tC_nitro Förster resonance energy transfer assay to monitor partitioning of the primer terminus between the polymerase and exonuclease subdomains.

Structure-function analysis of ribonucleotide bypass by B family DNA replicases

Ribonucleotides are frequently incorporated into DNA during replication, they are normally removed, and failure to remove them results in replication stress. This stress correlates with DNA polymerase (Pol) stalling during bypass of ribonucleotides in DNA templates. Here we demonstrate that stalling by yeast replicative Pols δ and ε increases as the number of consecutive template ribonucleotides increases from one to four. The homologous bacteriophage RB69 Pol also stalls during ribonucleotide bypass, with a pattern most similar to that of Pol ε. Crystal structures of an exonuclease-deficient variant of RB69 Pol corresponding to multiple steps in single ribonucleotide bypass reveal that increased stalling is associated with displacement of Tyr391 and an unpreferred C2'-endo conformation for the ribose. Even less efficient bypass of two consecutive ribonucleotides in DNA correlates with similar movements of Tyr391 and displacement of one of the ribonucleotides along with the primer-strand DNA backbone. These structure-function studies have implications for cellular signaling by ribonucleotides, and they may be relevant to replication stress in cells defective in ribonucleotide excision repair, including humans suffering from autoimmune disease associated with RNase H2 defects.

Alteration in the cavity size adjacent to the active site of RB69 DNA polymerase changes its conformational dynamics

Internal cavities are a common feature of many proteins, often having profound effects on the dynamics of their interactions with substrate and binding partners. RB69 DNA polymerase (pol) has a hydrophobic cavity right below the nucleotide binding pocket at the tip of highly conserved L415 side chain. Replacement of this residue with Gly or Met in other B family pols resulted in higher mutation rates. When similar substitutions for L415 were introduced into RB69pol, only L415A and L415G had dramatic effects on pre-steady-state kinetic parameters, reducing base selectivity by several hundred fold. On the other hand, the L415M variant behaved like the wild-type. Using a novel tC^o-tC_nitro Förster Resonance Energy Transfer (FRET) assay, we were able to show that the partition of the primer terminus between pol and exonuclease (exo) domains was compromised with the L415A and L415G mutants, but not with the L415M variant. These results could be rationalized by changes in their structures as determined by high resolution X-ray crystallography.

DNA mismatch synthesis complexes provide insights into base selectivity of a B family DNA polymerase

Current hypotheses that attempt to rationalize the high degree of base selectivity exhibited by replicative DNA polymerases (pols) concur that ternary complexes formed with incorrect dNTPs are destabilized. Knowing what accounts for this destabilization is likely to be the key to understanding base discrimination. To address this issue, we have determined crystal structures of ternary complexes with all 12 mismatches using an engineered RB69 pol quadruple mutant (qm, L415A/L561A/S565G/Y567A) that enabled us to capture nascent mispaired dNTPs. These structures show that mismatches in the nascent base-pair binding pocket (NBP) of the qm pol differ markedly from mismatches embedded in binary pol-DNA complexes. Surprisingly, only 3 of 12 mismatches clash with the NBP when they are modeled into the wild-type (wt) pol. The remaining can fit into a wt pol ternary complex but deviate from normal Watson-Crick base-pairs. Repositioning of the templating nucleotide residue and the enlarged NBP in qm ternary complex play important roles in accommodating incorrect incoming dNTPs. From these structures, we propose additional reasons as to why incorrect dNTPs are incorporated so inefficiently by wt RB69 pol: (i) steric clashes with side chains in the NBP after Fingers closing; (ii) weak interactions or large gaps between the incoming dNTP and the templating base; and (iii) burying a protonated base in the hydrophobic environment of the NBP. All of these possibilities would be expected to destabilize the closed ternary complex so that incorporation of incorrect dNTP would be a rare event.

Probing minor groove hydrogen bonding interactions between RB69 DNA polymerase and DNA

Minor groove hydrogen bonding (HB) interactions between DNA polymerases (pols) and N3 of purines or O2 of pyrimidines have been proposed to be essential for DNA synthesis from results obtained using various nucleoside analogues lacking the N3 or O2 contacts that interfered with primer extension. Because there has been no direct structural evidence to support this proposal, we decided to evaluate the contribution of minor groove HB interactions with family B pols. We have used RB69 DNA pol and 3-deaza-2'-deoxyadenosine (3DA), an analogue of 2-deoxyadenosine, which has the same HB pattern opposite T but with N3 replaced with a carbon atom. We then determined pre-steady-state kinetic parameters for the insertion of dAMP opposite dT using primer/templates (P/T)-containing 3DA. We also determined three structures of ternary complexes with 3DA at various positions in the duplex DNA substrate. We found that the incorporation efficiency of dAMP opposite dT decreased 10(2)-10(3)-fold even when only one minor groove HB interaction was missing. Our structures show that the HB pattern and base pair geometry of 3DA/dT is exactly the same as those of dA/dT, which makes 3DA an optimal analogue for probing minor groove HB interactions between a DNA polymerase and a nucleobase. In addition, our structures provide a rationale for the observed 10(2)-10(3)-fold decrease in the rate of nucleotide incorporation. The minor groove HB interactions between position n - 2 of the primer strand and RB69pol fix the rotomer conformations of the K706 and D621 side chains, as well as the position of metal ion A and its coordinating ligands, so that they are in the optinal orientation for DNA synthesis.