Nucleic acids: how high resolution structural biology help us to understand Darwinian evolution

Abstract

The reviews in this issue of Current Opinion in Structural Biology are organized from the analysis of DNA and RNA structures to that of nucleic acid complexes and assemblies. We selected elements with intricate structures, from the assemblies of G-quadruplexes to the mysterious and awesome complexity of the group II intron fold. The structures range from the exquisitely precise molecular recognition modes displayed by riboswitches bound to their ligands, to the apparently loose folds of the internal ribosome entry site RNAs. However, all these structures present a striking diversity in architectures; for example, the various internal linkages of the G-quadruplexes, the variety of folds of riboswitches for similar size ligands, and sometimes the same ligand like S-adenosylmethionine (SAM). They all illustrate the modularity of the assembly of nucleic acids, as well as the basis of the amazing functional diversity that Darwinian evolution exploits. In protein complexes with nucleic acids, the recurrence of folds, sometimes surprising and rarely suspected, is also the rule. Two reviews contribute to this point. The chapter on the photolyases and the cryptochromes illustrates how structural similarity can be coupled to functional difference. The chapter on the RNA interference (RNAi) machinery adds further evolutionary comparisons on how variations on similar structures can lead to changes in biological function.