RNA-guided DNA base damage repair via DNA polymerase-mediated nick translation.

DNA repair is mediated by DNA synthesis guided by a DNA template. Recent studies have shown that DNA repair can also be accomplished by RNA-guided DNA synthesis. However, it remains unknown how RNA can guide DNA synthesis to repair DNA damage. In this study, we revealed the molecular mechanisms underlying RNA-guided DNA synthesis and base damage repair mediated by human repair DNA polymerases. We showed that pol ß, pol κ, and pol ι predominantly synthesized one nucleotide, and pol η, pol ν, and pol θ synthesized multi-nucleotides during RNA-guided DNA base damage repair. The steady-state kinetics showed that pol η exhibited more efficient RNA-guided DNA synthesis than pol ß. Using molecular dynamics simulation, we further revealed dynamic conformational changes of pol ß and pol η and their structural basis to accommodate the RNA template and misoriented triphosphates of an incoming nucleotide. We demonstrated that RNA-guided base damage repair could be accomplished by the RNA-guided DNA strand-displacement synthesis and nick translation leading to nick ligation in a double-strand DNA region. Our study revealed a novel RNA-guided base damage repair pathway during transcription and DNA replication.

Insights into the DNA and RNA Interactions of Human Topoisomerase III Beta Using Molecular Dynamics Simulations.

Human topoisomerase III beta (hTOP3B) is the only topoisomerase in the human cell that can act on both DNA and RNA substrates. Recent findings have emphasized the physiological importance of hTOP3B and consolidated it as a valuable drug target for antiviral and anticancer therapeutics. Although type IA topoisomerases of different organisms have been studied over the years, the step-by-step interaction of hTOP3B and nucleic acid substrates is still not well understood. Due to the lack of hTOP3B-RNA structures as well as DNA/RNA covalent complexes, computational investigations have been limited. In our study, we utilized molecular dynamics (MD) simulations to study the interactions between hTOP3B and nucleic acids to get a closer look into the residues that play a role in binding DNA or RNA and facilitate catalysis, along with the differences and similarities when hTOP3B interacts with DNA compared to RNA. For this, we generated multiple models of hTOP3B complexed with DNA and RNA sequences using the hTOP3B crystal structure and 8-mer single-stranded DNA and RNA sequences. These models include both covalent and noncovalent complexes, which are then subjected to MD simulations and analyzed. Our findings highlight the complexes' stability, sequence preference, and interactions of the binding pocket residues with different nucleotides. Our work demonstrates that hTOP3B forms stable complexes with both DNA and RNA and provides a better understanding of the enzyme's interaction with different nucleic acid substrate sequences.

Structure-activity relationship of pyrrolidine pentamine derivatives as inhibitors of the aminoglycoside 6'- N -acetyltransferase type Ib.

Resistance to amikacin and other major aminoglycosides is commonly due to enzymatic acetylation by aminoglycoside 6'- N -acetyltransferase type I enzyme, of which type Ib [AAC(6')-Ib] is the most widespread among Gram-negative pathogens. Finding enzymatic inhibitors could be an effective way to overcome resistance and extend the useful life of amikacin. Small molecules possess multiple properties that make them attractive compounds to be developed as drugs. Mixture-based combinatorial libraries and positional scanning strategy led to the identification of a chemical scaffold, pyrrolidine pentamine, that, when substituted with the appropriate functionalities at five locations (R1 - R5), inhibits AAC(6')-Ib-mediated inactivation of amikacin. Structure-activity relationship (SAR) studies showed that while truncations to the molecule result in loss of inhibitory activity, modifications of functionalities and stereochemistry have different effects on the inhibitory properties. In this study, we show that alterations at position R1 of the two most active compounds, 2700.001 and 2700.003 , reduced inhibition levels, demonstrating the essential nature not only of the presence of an S -phenyl moiety at this location but also the distance to the scaffold. On the other hand, modifications on the R3, R4, and R5 positions have varied effects, demonstrating the potential for optimization. A correlation analysis between molecular docking values (ΔG) and the dose required for two-fold potentiation of compounds described in this and the previous studies showed a significant correlation between ΔG values and inhibitory activity. Highlights: Amikacin resistance in Gram-negatives is mostly caused by the AAC(6')-Ib enzymeAAC(6')-Ib has been identified in most Gram-negative pathogensInhibitors of AAC(6')-Ib could be used to treat resistant infectionsCombinatorial libraries and positional scanning identified an inhibitorThe lead compound can be optimized by structure activity relationship studies.

Structural Heterogeneity of Human Histone H2A.1.

Histones are highly basic chromatin proteins that tightly package and order eukaryotic DNA into nucleosomes. While the atomic structure of the nucleosomes has been determined, the three-dimensional structure of DNA-free histones remains unresolved. Here, we combine tandem nonlinear and linear ion mobility spectrometry (FAIMS-TIMS) coupled to mass spectrometry in parallel with molecular modeling to study the conformational space of a DNA-free histone H2A type 1 (H2A.1). Experimental results showed the dependence of the gas-phase structures on the starting solution conditions, characterized by charge state distributions, mobility distributions, and collision-induced-unfolding pathways. The measured H2A.1 gas-phase structures showed a high diversity of structural features ranging from compact (C) to partially folded (P) and then highly elongated (E) conformations. Molecular dynamics simulations provided candidate structures for the solution H2A.1 native conformation with folded N- and C-terminal tails, as well as gas-phase candidate structures associated with the mobility trends. Complementary collision cross section and dipole calculations showed that the charge distribution in the case of elongated gas-phase structures, where basic and acidic residues are mostly exposed (e.g., z > 15+), is sufficient to induce differences in the dipole alignment at high electric fields, in good agreement with the trends observed during the FAIMS-TIMS experiments.