Unnatural Bases: The genetic basis of all life on the planet is comprised of deoxyribonucleic acid (DNA) with four nitrogenous nucleotide bases, abbreviated to A, G, C, and T. But there are variations on this theme, and Pinheiro et al. (p. 341 ; see the Perspective by Joyce ) describe the directed evolution of unnatural nucleic acid–like genetic polymers. Variant enzymes were developed that efficiently transcribed DNA to anhydrohexitol (HNA), cyclohexenyl (CeNA), locked (LNA), and threofuranosyl (TNA) nuceic acid analogs. Further variant enzymes were developed to reverse-transcribe these analogs back to DNA. Thus, man-made nucleic acid analogs can be designed and selected that have the potential to operate in a way analogous to the natural process of heredity and evolution.

The use of chemically synthesized short interfering RNAs (siRNAs) is currently the method of choice to manipulate gene expression in mammalian cell culture, yet improvements of siRNA design is expectably required for successful application in vivo. Several studies have aimed at improving siRNA performance through the introduction of chemical modifications but a direct comparison of these results is difficult. We have directly compared the effect of 21 types of chemical modifications on siRNA activity and toxicity in a total of 2160 siRNA duplexes. We demonstrate that siRNA activity is primarily enhanced by favouring the incorporation of the intended antisense strand during RNA-induced silencing complex (RISC) loading by modulation of siRNA thermodynamic asymmetry and engineering of siRNA 3′-overhangs. Collectively, our results provide unique insights into the tolerance for chemical modifications and provide a simple guide to successful chemical modification of siRNAs with improved activity, stability and low toxicity.

Four different XNAs — polymers with backbone chemistries not found in nature, namely, arabino nucleic acids, 2′-fluoroarabino nucleic acids, hexitol nucleic acids and cyclohexene nucleic acids — are found to be able to support the evolution of synthetic enzymes (XNAzymes) that catalyse several chemical reactions. The development of nucleic acid catalysis may have been key to the emergence of life on Earth. RNA enzymes retain key functions in living systems, notably in translation and RNA processing, and a various RNA and DNA enzymes have been created by in vitro evolution approaches. Alexander Taylor et al. have now synthesized a series of catalytic polymers using four different backbone chemistries (XNAs) not found in nature. The most efficient XNAzyme, based on 2′-fluoroarabino nucleic acids, was used to generate an in trans RNA endonuclease, an RNA ligase and an XNA–XNA ligase. These results establish technologies for the discovery of catalysts in a wide range of polymer scaffolds not found in nature. The emergence of catalysis in early genetic polymers such as RNA is considered a key transition in the origin of life1, pre-dating the appearance of protein enzymes. DNA also demonstrates the capacity to fold into three-dimensional structures and form catalysts in vitro2. However, to what degree these natural biopolymers comprise functionally privileged chemical scaffolds3 for folding or the evolution of catalysis is not known. The ability of synthetic genetic polymers (XNAs) with alternative backbone chemistries not found in nature to fold into defined structures and bind ligands4 raises the possibility that these too might be capable of forming catalysts (XNAzymes). Here we report the discovery of such XNAzymes, elaborated in four different chemistries (arabino nucleic acids, ANA5; 2′-fluoroarabino nucleic acids, FANA6; hexitol nucleic acids, HNA; and cyclohexene nucleic acids, CeNA7) directly from random XNA oligomer pools, exhibiting in trans RNA endonuclease and ligase activities. We also describe an XNA–XNA ligase metalloenzyme in the FANA framework, establishing catalysis in an entirely synthetic system and enabling the synthesis of FANA oligomers and an active RNA endonuclease FANAzyme from its constituent parts. These results extend catalysis beyond biopolymers and establish technologies for the discovery of catalysts in a wide range of polymer scaffolds not found in nature8. Evolution of catalysis independent of any natural polymer has implications for the definition of chemical boundary conditions for the emergence of life on Earth and elsewhere in the Universe9.