Construction of synthetic life forms with genetic materials comprising unnatural nucleic acids

Abstract

<p>One goal in synthetic and engineering biology is to create alternative life forms with novel functions through the design and construction of new biological components and systems. Three main approaches are used to achieve this goal: (1) Constructing a regulatory network through natural biological module assembly; (2) building artificial genome DNA via de novo DNA synthesis; and (3) creating artificial biological systems or living organisms based on synthetic materials (such as chemically modified nucleic acids, proteins or lipids). In the third research area, also referred to as "chemical synthetic biology", scientists investigate alternative genetic systems, other than DNA, with the aim of building synthetic life forms. A variety of chemically modified nucleic acids, also known as xenobiotic-nucleic acids (XNAs), have been designed and synthesized over the past few decades. They can be classified with respect to the modification sites, including nucleobasemodified XNAs, sugar-modified XNAs, phosphodiester-modified XNAs, or nucleic acids containing two or three combinations. Compared with natural DNA/RNA, XNAs possess similar but superior chemical and biological properties, which enable them to function more effectively for biomedical, biotechnology and nanotechnology applications. More importantly, chemical synthetic biologists gain more insight into the chemical etiology of ribofuranosyl nucleic acids DNA/ RNA, and the chemical basis for genetic functions including information storage, heredity and evolution. With the help of laboratory-evolved XNA polymerases, genetic information encoded in XNAs could be transferred back and forth into DNA, suggesting certain XNAs can function as alternative genetic material in vitro. It is not yet known whether these XNAs can replace DNA and function as genetic material in living cells. In nature many variations of nucleobase, sugar and phosphate backbones of DNA and RNA have been observed, demonstrating that integrating alternative genetic sets into living systems is possible. Nevertheless, the structural diversification of XNAs and the complexity of the cell environment make it challenging to insert XNAs into living cells. Initial studies showed that genetic information encoded by the XNAs inside a plasmid can be accurately used to generate DNA copies. Through the insertion of unnatural base pairs (UBPs), semi-synthetic organisms with extended genomes were successfully constructed. All these results highlight the possibility of using alternative XNAs for artificial life. Furthermore, the construction of synthetic life forms will increase our understanding of the fundamental principles of living systems, the origins of life and evolution.</p>