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Engineered polymerases that can copy genetic information between DNA and xeno-nucleic acids (XNA) hold tremendous value as reagents in future biotechnology applications. However, current XNA polymerases function with inferior activity relative to their natural counterparts, indicating that current polymerase engineering efforts would benefit from new benchmarking assays. Here, we describe a highly parallel, low-cost method for measuring the average rate and substrate specificity of XNA polymerases in a standard qPCR instrument. Our approach, termed polymerase kinetic profiling (PKPro), involves monitoring XNA synthesis on a self-priming template using high-resolution melting (HRM) fluorescent dyes that intercalate into the growing duplex as the template strand is copied into XNA. Since changes in fluorescence are directly proportional to XNA synthesis, quantitative measurements are obtained by calibrating the fluorescent signal against chemically synthesized standards. Using PKPro, we discovered that XNA polymerases function with rates of ∼1-80 nt/min and exhibit substrate specificities of ∼0.1-5-fold for xNTP versus dNTP. Last, we show how PKPro could be used in a highly parallel screen by analyzing 288 different polymerase reaction conditions. On the basis of these results, we suggest that PKPro provides a powerful tool for evaluating the activity of XNA polymerases.

Polymerase engineering is making it possible to synthesize xeno-nucleic acid polymers (XNAs) with diverse backbone structures and chemical functionality. The ability to copy genetic information back and forth between DNA and XNA has led to a new field of science known as synthetic genetics, which aims to study the genetic concepts of heredity and evolution in artificial genetic polymers. Since many of the polymerases needed to synthesize XNA polymers are not available commercially, researchers must express and purify these enzymes as recombinant proteins from E. coli. This unit details the steps needed to express, purify, and evaluate the activity of engineered polymerases with altered substrate recognition properties. The protocol requires 6 days to complete and will produce ∼20 mg of pure, nuclease-free polymerase per liter of E. coli bacterial culture. © 2017 by John Wiley & Sons, Inc., (Copyright © 2017 John Wiley & Sons, Inc.)

The ability to synthesize and propagate genetic information encoded in the framework of xeno-nucleic acid (XNA) polymers would inform a wide range of topics from the origins of life to synthetic biology. While directed evolution has produced examples of engineered polymerases that can accept XNA substrates, these enzymes function with reduced activity relative to their natural counterparts. Here, we describe a biochemical strategy that enables the discovery of engineered polymerases with improved activity for a given unnatural polymerase function. Our approach involves identifying specificity determining residues (SDRs) that control polymerase activity, screening mutations at SDR positions in a model polymerase scaffold, and assaying key gain-of-function mutations in orthologous protein architectures. By transferring beneficial mutations between homologous protein structures, we show that new polymerases can be identified that function with superior activity relative to their starting donor scaffold. This concept, which we call scaffold sampling, was used to generate engineered DNA polymerases that can faithfully synthesize RNA and TNA (threose nucleic acid), respectively, on a DNA template with high primer-extension efficiency and low template sequence bias. We suggest that the ability to combine phenotypes from different donor and recipient scaffolds provides a new paradigm in polymerase engineering where natural structural diversity can be used to refine the catalytic activity of synthetic enzymes.

Polymerases that synthesize artificial genetic polymers hold great promise for advancing future applications in synthetic biology. However, engineering natural polymerases to replicate unnatural genetic polymers is a challenging problem. Here we present droplet-based optical polymerase sorting (DrOPS) as a general strategy for expanding polymerase function that employs an optical sensor to monitor polymerase activity inside the microenvironment of a uniform synthetic compartment generated by microfluidics. We validated this approach by performing a complete cycle of encapsulation, sorting and recovery on a doped library and observed an enrichment of ∼1,200-fold for a model engineered polymerase. We then applied our method to evolve a manganese-independent α-L-threofuranosyl nucleic acid (TNA) polymerase that functions with >99% template-copying fidelity. Based on our findings, we suggest that DrOPS is a versatile tool that could be used to evolve any polymerase function, where optical detection can be achieved by Watson-Crick base pairing.

Threose nucleic acid (TNA) is an unnatural genetic polymer capable of undergoing Darwinian evolution to generate folded molecules with ligand-binding activity. This property, coupled with a nuclease-resistant backbone, makes TNA an attractive candidate for future applications in biotechnology. Previously, we have shown that an engineered form of the Archaean replicative DNA polymerase 9°N, known commercially as Therminator DNA polymerase, can copy a three-letter genetic alphabet (A,T,C) from DNA into TNA. However, our ability to transcribe four-nucleotide libraries has been limited by chain termination events that prevent the synthesis of full-length TNA products. Here, we show that chain termination is caused by tG:dG mispairing in the enzyme active site. We demonstrate that the unnatural base analogue 7-deazaguanine (7dG) will suppress tGTP misincorporation by inhibiting the formation of Hoogsteen tG:dG base pairs. DNA templates that contain 7dG in place of natural dG residues replicate with high efficiency and >99% overall fidelity. Pre-steady-state kinetic measurements indicate that the rate of tCTP incorporation is 5-fold higher opposite 7dG than dG and only slightly lower than dCTP incorporation opposite either 7dG or dG. These results provide a chemical solution to the problem of how to synthesize large, unbiased pools of TNA molecules by polymerase-mediated synthesis.