Synthetic biology: Tailor-made genetic codes.

The translation apparatus is the cell’s factory for protein synthesis, stitching together amino acid substrates into sequence-controlled polypeptides (proteins) from a defined template. With protein elongation rates of up to 20 amino acids per second and an accuracy of 99.99% (ref. 1), prokaryotic translation is faster and more accurate than many enzymatic conversions that require only a single biocatalyst and a single reactant. This extraordinary synthetic capability has driven extensive efforts to harness the translation apparatus for novel functions. Pioneering efforts have demonstrated that it is possible to genetically encode more than the 20 naturally occurring amino acids2–5. This powerful tool can be used to expand the functionality of proteins and peptides. As a result, reprogramming of the genetic code has emerged as one of the defining opportunities in synthetic biology2. Some of the potential goals include providing platforms for the synthesis of polymers containing unnatural amino acids that can be selected from a large catalogue; enabling new biocontainment strategies; expanding the chemical reactions conducted by and within living systems; and developing new materials and therapeutics. So far, the major route to these goals has consisted of engineering the translation machinery of living organisms, such as Escherichia coli, to fabricate cells that can synthesize unnatural polymers. The current approaches, however, usually allow the sitespecific incorporation of just a few unnatural amino acids into proteins6,7 — for instance by reassigning the UAG stop codon. This limits the potential of genetic code reprogramming. Creating tailor-made genetic codes with ‘blank’ codons that can be re-assigned to multiple distinct unnatural monomers would require substantial re-engineering of the entire apparatus to keep cells alive. Freed from the need to sustain life, cell-free systems are more amenable to engineering design. About half a century ago, Nirenberg and co-workers developed one of the first cell-free SYNTHETIC BIOLOGY