"Not-so-popular" orthogonal pairs in genetic code expansion

During the past decade, genetic code expansion has been proved to be a powerful tool for protein studies and engineering. As the key part, a series of orthogonal pairs have been developed to site-specifically incorporate hundreds of noncanonical amino acids (ncAAs) into proteins by using bacteria, yeast, mammalian cells, animals, or plants as hosts. Among them, the pair of tyrosyl-tRNA synthetase/tRNA(Tyr) from Methanococcus jannaschii and the pair of pyrrolysyl-tRNA synthetase/tRNA(Pyl) from Methanosarcina species are the most popular ones. Recently, other "not-so-popular" orthogonal pairs have started to attract attentions, because they can provide more choices of ncAA candidates and are necessary for simultaneous incorporation of multiple ncAAs into a single protein. Here, we summarize the development and applications of those "not-so-popular" orthogonal pairs, providing guidance for studying and engineering proteins.