Xenobiology
The development of synthetic genetic materials (xeno-nucleic acids or XNAs) by systematically engineering DNA polymerases [1] is a clear example of the power of directed evolution for synthetic biology, and the first step towards developing an organism based on a synthetic genetic material.
Directed evolution of DNA polymerases for XNA synthesis was achieved through selection, by compartmentalised self-tagging (CST), and high throughput screening [1]. CST is an emulsion-based selection platform developed to allow the isolation of thermophilic DNA polymerases capable of incorporating modified nucleotides. Reminiscent of a primer extension assay, active polymerase variants incorporate the modified nucleotides provided extending a biotinylated primer against their own plasmids – extension of the primer stabilises its hybridization to the plasmid used as template. Recovery of the biotinylated primers leads to recovery of stably hybridised plasmids, and thus the recovery of the genotype of active polymerase variants.
Two rounds of selection were sufficient to allow the isolation of DNA polymerase variants capable of synthesising a number of XNAs, including hexitol nucleic acids (HNA), ‘locked’ nucleic acids (LNA), fluoroarabino nucleic acids (FANA) as well as a number of 2’-modifications (e.g. RNA, 2’-fluoro-DNA, 2’-azido-DNA) [1,2]. Together with a rationally designed polymerase variant capable of synthesising DNA from an XNA template, a total of eight synthetic genetic systems were established, with potential application in diagnostics and therapeutics through aptamer selection and nanotechnology [4].
This approach not only enabled the development of the first synthetic genetic materials, but also identified a novel region in the DNA polymerase involved in substrate recognition and discrimination [2]. In addition, development of synthetic genetic systems allows exploration of the boundary conditions of chemical information storage [3]. Although the engineered polymerases were capable of synthesising XNAs in a test tube environment, considerable work is still required before XNA polymerases will be capable of functioning within a true biological system.
Our goal is to further engineer biological parts involved in the storage, maintenance and interpretation of chemical information with a view towards learning how these systems emerged (or could emerge) and towards assembling synthetic components orthogonal to (i.e. independent and unable to interact with) biology