Abstract
We recently characterized a new class of pyrrolysyl-tRNA synthetase (PylRS)/PyltRNA pairs from Methanomassiliicocales that are active and orthogonal in Escherichia coli. The aminoacyl-tRNA synthetases (aaRSs) of these pairs lack the N-terminal domain that is essential for tRNA recognition and in vivo activity in the Methanosarcina mazei (Mm) PylRS but share a homologous active site with MmPylRS; this facilitates the transplantation of mutations discovered with existing PylRS systems into the new PylRS systems to reprogram their substrate specificity for the incorporation of noncanonical amino acids (ncAAs). Several of the new PylRS/PyltRNA pairs, or their evolved variants [including Methanomethylophilus alvus (Ma) PylRS/MaPyltRNA(6)CUA], are mutually orthogonal to the MmPylRS/MmPyltRNA pair, and the active sites of the Mm pair and Ma pair can be diverged to enable the incorporation of distinct ncAAs in response to distinct codons via orthogonal translation in E. coli. Here we demonstrate that MaPylRS/MaPyltRNA(6)CUA is orthogonal to the aaRSs and tRNAs in mammalian cells and directs efficient incorporation of ncAAs into proteins. Moreover, we confirm that the MaPylRS/MaPyltRNA(6) and MmPylRS/MmPyltRNA pairs are mutually orthogonal in mammalian cells and demonstrates that these pairs can be used to encode distinct ncAAs into a protein in mammalian cells. Thus, the MaPylRS/MaPyltRNA(6)CUA pair provides an additional pair that is orthogonal in both E. coli and mammalian systems and is mutually orthogonal to the most widely used system for genetic code expansion. Our results provide a foundation for expanding the scope of genetic code expansion and may also facilitate strategies for proteome-wide ncAA tagging with mutually orthogonal systems.
Highlights
We recently characterized a new class of pyrrolysyl-tRNA synthetase (PylRS)/PyltRNA pairs from Methanomassiliicocales that are active and orthogonal in Escherichia coli
The ability to incorporate noncanonical amino acids (ncAAs) into proteins in mammalian cells has been further expanded by strategies that replace the genomically encoded EcTrpRS/TrptRNA pair in E. coli with the Saccharomyces cerevisiae (Sc) TrpRS/TrptRNA pair.[12−14] Because the ScTrpRS/TrptRNA is orthogonal in E. coli, suppressor derivatives of the EcTrpRS/TrptRNA pair can be introduced into the resulting E. coli strains and evolved for ncAA incorporation
The level of amber codon readthrough mediated by the MaPylRS/ MaPyltRNA(6)CUA pair is comparable to that mediated by the highly active MmPylRS/MmPyltRNACUA pair (Figure 2a); this demonstrates that the MaPylRS/MaPyltRNA(6)CUA pair is highly active in mammalian cells
Summary
We recently characterized a new class of pyrrolysyl-tRNA synthetase (PylRS)/PyltRNA pairs from Methanomassiliicocales that are active and orthogonal in Escherichia coli. Recent work has extended this strategy to the EcTyrRS/ TyrtRNA pair.[15] In several cases, aaRS/tRNA pairs that are active and orthogonal with respect to the endogenous aaRSs and tRNAs in mammalian cells have Pair is active in mammalian cells and that both MaPylRS and MaPyltRNA(6)CUA are orthogonal with respect to the endogenous tRNAs and the aaRSs in human cells.
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