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.

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.

DNA polymerase substrate specificity is fundamental to genome integrity and to polymerase applications in biotechnology. In the current paradigm, active site geometry is the main site of specificity control. Here, we describe the discovery of a distinct specificity checkpoint located over 25 Å from the active site in the polymerase thumb subdomain. In Tgo, the replicative DNA polymerase fromThermococcus gorgonarius, we identify a single mutation (E664K) within this region that enables translesion synthesis across a template abasic site or a cyclobutane thymidine dimer. In conjunction with a classic “steric-gate” mutation (Y409G) in the active site, E664K transforms Tgo DNA polymerase into an RNA polymerase capable of synthesizing RNAs up to 1.7 kb long as well as fully pseudouridine-, 5-methyl-C–, 2′-fluoro–, or 2′-azido–modified RNAs primed from a wide range of primer chemistries comprising DNA, RNA, locked nucleic acid (LNA), or 2′O-methyl–DNA. We find that E664K enables RNA synthesis by selectively increasing polymerase affinity for the noncognate RNA/DNA duplex as well as lowering theK_mfor ribonucleotide triphosphate incorporation. This gatekeeper mutation therefore identifies a key missing step in the adaptive path from DNA to RNA polymerases and defines a previously unknown postsynthetic determinant of polymerase substrate specificity with implications for the synthesis and replication of noncognate nucleic acid polymers.

Nucleic acids display remarkable properties beyond information storage and propagation. The well-understood base pairing rules have enabled nucleic acids to be assembled into nanostructures of ever increasing complexity. Although nanostructures can be constructed using other building blocks, including peptides and lipids, it is the capacity to evolve that sets nucleic acids apart from all other nanoscale building materials. Nonetheless, the poor chemical and biological stability of DNA and RNA constrain their applications. Recent advances in nucleic acid chemistry and polymerase engineering enable the synthesis, replication, and evolution of a range of synthetic genetic polymers (XNAs) with improved chemical and biological stability. We discuss the impact of this technology on the generation of XNA ligands, enzymes, and nanostructures with tailor-made chemistry.