虽然化学家能设计具有所期望性能的新颖氨基酸，但这些氨基酸中只有少数几种被细胞机器成功导入蛋白中。
即使在这种情况下，每次也只能将一个非天然氨基酸添加到一个蛋白上。理论上，四倍体密码子的使用（而不是在天然蛋白合成中所用的三倍体）能够提高灵活性，因为这样可以提供额外的空白密码子，以分配给异常的氨基酸。天然核糖体在解码四倍体时效率非常低，并且无法通过演化来更好地完成此项任务，因为这样的话它们会误读整个蛋白组。
Jason Chin及其同事绕过了这个问题，他们所采用的办法是，创造和用合成方法演变并列的或“正交”的核糖体，这样得到的核糖体能利用成对的非天然tRNA合成酵素/tRNA来高效解码四倍体密码子。这个体系有可能允许将多达200个新颖氨基酸导入通过基因编码的人工设计的蛋白中。(生物谷Bioon.com)
生物谷推荐原文出处：
Nature doi:10.1038/nature08817
Encoding multiple unnatural amino acids via evolution of a quadruplet-decoding ribosome
Heinz Neumann1,2, Kaihang Wang1,2, Lloyd Davis1, Maria Garcia-Alai1 & Jason W. Chin1
The in vivo, genetically programmed incorporation of designer amino acids allows the properties of proteins to be tailored with molecular precision1. The Methanococcus jannaschii tyrosyl-transfer-RNA synthetase–tRNACUA (MjTyrRS–tRNACUA)2, 3 and the Methanosarcina barkeri pyrrolysyl-tRNA synthetase–tRNACUA (MbPylRS–tRNACUA)4, 5, 6 orthogonal pairs have been evolved to incorporate a range of unnatural amino acids in response to the amber codon in Escherichia coli 1, 6, 7. However, the potential of synthetic genetic code expansion is generally limited to the low efficiency incorporation of a single type of unnatural amino acid at a time, because every triplet codon in the universal genetic code is used in encoding the synthesis of the proteome. To encode efficiently many distinct unnatural amino acids into proteins we require blank codons and mutually orthogonal aminoacyl-tRNA synthetase–tRNA pairs that recognize unnatural amino acids and decode the new codons. Here we synthetically evolve an orthogonal ribosome8, 9 (ribo-Q1) that efficiently decodes a series of quadruplet codons and the amber codon, providing several blank codons on an orthogonal messenger RNA, which it specifically translates8. By creating mutually orthogonal aminoacyl-tRNA synthetase–tRNA pairs and combining them with ribo-Q1 we direct the incorporation of distinct unnatural amino acids in response to two of the new blank codons on the orthogonal mRNA. Using this code, we genetically direct the formation of a specific, redox-insensitive, nanoscale protein cross-link by the bio-orthogonal cycloaddition of encoded azide- and alkyne-containing amino acids10. Because the synthetase–tRNA pairs used have been evolved to incorporate numerous unnatural amino acids1, 6, 7, it will be possible to encode more than 200 unnatural amino acid combinations using this approach. As ribo-Q1 independently decodes a series of quadruplet codons, this work provides foundational technologies for the encoded synthesis and synthetic evolution of unnatural polymers in cells.
