Xenobiotic life

Abstract

The ability to generate living systems that exploit unnatural molecules has expanded greatly. There are a series of unnatural nucleotides, amino acids, and membranes to choose from. However, at this early stage, many challenges remain in implementing the newly built parts inside of living cells. For example, a library of unnatural nucleotides is available with altered heterocycle or sugar units, but aside from one impressive example, no genomes have been stably altered in a way that incorporates these artificial nucleotides. Similar problems exist for proteins in that current technology allows for one or two unnatural amino acids to be incorporated into a single protein inside of a cell but not more. Many of the encountered difficulties arise from interference with multiple cellular pathways that have evolved over billions of years of evolution to be codependent. That is why some success has been found in exploiting the newly made artificial systems in vitro. Just as one example, artificial, nonliving systems can be built with a single gene that can be used to direct the synthesis of two entirely different protein sequences with no limit on the number of unnatural amino acids. However, a different set of challenges exists for nonliving xenobiotic systems that severely limit their utility. Nevertheless, remarkable progress has been made within and outside of living systems.