Assembly of polymerase chain reactions at room temperature can sometimes lead to low yields or unintentional products due to mispriming. Mutation of isoleucine 707 to leucine in DNA polymerase I from Thermus aquaticus substantially decreases its activity at room temperature without compromising its ability to amplify DNA. To understand why a conservative change to the enzyme over 20 Å from the active site can have a large impact on its activity at low temperature, we solved the X-ray crystal structure of the large (5′-to-3′ exonuclease-deleted) fragment of Taq DNA polymerase containing the cold-sensitive mutation in the ternary (E–DNA–ddNTP) and binary (E–DNA) complexes. The I707L KlenTaq1 ternary complex was identical to the wild-type in the closed conformation except for the mutation and a rotamer change in nearby phenylalanine 749, suggesting that the enzyme should remain active. However, soaking out of the nucleotide substrate at low temperature results in an altered binary complex made possible by the rotamer change at F749 near the tip of the polymerase O-helix. Surprisingly, two adenosines in the 5′-template overhang fill the vacated active site by stacking with the primer strand, thereby blocking the active site at low temperature. Replacement of the two overhanging adenosines with pyrimidines substantially increased activity at room temperature by keeping the template overhang out of the active site, confirming the importance of base stacking. These results explain the cold-sensitive phenotype of the I707L mutation in KlenTaq1 and serve as an example of a large conformational change affected by a conservative mutation.

The large GTPase dynamin 2 controls both endosomal fission and microtubule acetylation. Here we report that dynamin 2 alters microtubules and regulates the trafficking of human adenovirus type 37. Dynamin 2 knockdown by siRNA in infected cells resulted in accumulation of acetylated tubulin, repositioning of microtubule organizing centers (MTOCs) closer to cell nuclei, increased virus in the cytosol (with a compensatory decrease in endosomal virus), reduced proinflammatory cytokine induction, and increased binding of virus to the nucleoporin, Nup358. These events led to increased viral DNA nuclear entry and viral replication. Overexpression of dynamin 2 generated opposite effects. Therefore, dynamin 2 inhibits adenovirus replication and promotes innate immune responses by the infected cell. MTOC transposition in dynamin 2 knockdown promotes a closer association with nuclear pore complexes to facilitate viral DNA delivery. Dynamin 2 plays a key role in adenoviral trafficking and influences host responses to infection.

Adenoviruses are common human mucosal pathogens of the gastrointestinal, respiratory, and genitourinary tracts and ocular surface. Here, we report finding Chi-like sequences in adenovirus recombination hot spots. Adenovirus coinfection in the presence of bacterial RecA protein facilitated homologous recombination between viruses. Genetic recombination led to evolution of an important external feature on the adenoviral capsid, namely, the penton base protein hypervariable loop 2, which contains the arginine-glycine-aspartic acid motif critical to viral internalization. We speculate that free Rec proteins present in gastrointestinal secretions upon bacterial cell death facilitate the evolution of human adenoviruses through homologous recombination, an example of viral commensalism and the complexity of virus-host interactions, including regional microbiota.