Analyzing the relationship between single base flipping and strand slippage near DNA duplex termini

Mononucleotide repeat expansions with non-natural polymerase substrates

Replicative strand slippage is a biological phenomenon, ubiquitous among different organisms. However, slippage events are also relevant to non-natural replication models utilizing synthetic polymerase substrates. Strand slippage may notably affect the outcome of the primer extension reaction with repetitive templates in the presence of non-natural nucleoside triphosphates. In the current paper, we studied the ability of Taq, Vent (exo-), and Deep Vent (exo-) polymerases to produce truncated, full size, or expanded modified strands utilizing non-natural 2′-deoxyuridine nucleotide analogues and different variants of the homopolymer template. Our data suggest that the slippage of the primer strand is dependent on the duplex fluttering, incorporation efficiency for a particular polymerase-dNTP pair, rate of non-templated base addition, and presence of competing nucleotides.

Geometric Patterns for Neighboring Bases Near the Stacked State in Nucleic Acid Strands

Structural variation in base stacking has been analyzed frequently in isolated double helical contexts for nucleic acids, but not as often in nonhelical geometries or in complex biomolecular environments. In this study, conformations of two neighboring bases near their stacked state in any environment are comprehensively characterized for single-strand dinucleotide (SSD) nucleic acid crystal structure conformations. An ensemble clustering method is used to identify a reduced set of representative stacking geometries based on pairwise distances between select atoms in consecutive bases, with multiple separable conformational clusters obtained for categories divided by nucleic acid type (DNA/RNA), SSD sequence, stacking face orientation, and the presence or absence of a protein environment. For both DNA and RNA, SSD conformations are observed that are either close to the A-form, or close to the B-form, or intermediate between the two forms, or further away from either form, illustrating the local structural heterogeneity near the stacked state. Among this large variety of distinct conformations, several common stacking patterns are observed between DNA and RNA, and between nucleic acids in isolation or in complex with proteins, suggesting that these might be stable stacking orientations. Noncanonical face/face orientations of the two bases are also observed for neighboring bases in the same strand, but their frequency is much lower, with multiple SSD sequences across categories showing no occurrences of such unusual stacked conformations. The resulting reduced set of stacking geometries is directly useful for stacking-energy comparisons between empirical force fields, prediction of plausible localized variations in single-strand structures near their canonical states, and identification of analogous stacking patterns in newly solved nucleic acid containing structures.