With the objective to functionalize proteins for the understanding of their biological roles and developing protein-based biosensors, I have been developing methods to synthesize proteins with defined modifications and applying them to study protein functional roles and generate proteins with new properties. These methods rely on the read-through of an in-frame stop codon in mRNA by a nonsense suppressor tRNA specifically acylated with a noncanoncial amino acid (NAA) by a unique aminoacyl-tRNA synthetase and the genetic incorporation of this NAA at the stop codon site. NAAs either provide chemical handles for site-specific manipulation or mimic the posttranslational modifications, which are critical for understanding cellular regulations and signal transduction. The pyrrolysine synthetase (PylRS) has been wildly used to incorporate NAAs into proteins in E. coli. Taking advantage of PylRS, I have developed method to genetically incorporate ketone-containing N--acetyl-L-lysine analog, 2-amino-8-oxononanoic acid (KetoK), into proteins for their site-specific modifications and used it to mimic the protein lysine acetylation process. I have also modified the ribosome in order to improve the amber suppression efficiency and therefore to achieve incorporation of multiple copies of NAA into one protein. By overexpressing a truncated ribosomal protein, L11C, I have demonstrated 5-fold increase of amber suppression level in E. coli, leading to higher expression levels for proteins incorporated with NAAs. I have also demonstrated this method can be applied successfully to incorporate at least 3 NAAs into one protein in E. coli. With the success of incorporating multiple NAAs into one protein, I have further introduced two distinct NAAs into one protein simultaneously. This is done by using a wild type or evolved PylRS-pylTUUA pair and an evolved M. jannaschii tyrosyl-tRNA synthetase (MjTyrRS)-tRNACUA pair. By suppressing both UAG and UAA stop codons in one mRNA, a protein incorporated with two NAAs is synthesized with a decent yield. There is of great interest to incorporate new NAAs into proteins, which is done by library selection. By introducing both positive and negative selective markers into one plasmid, I have developed a one-plasmid selection method. In this method, the positive and negative selections are accomplished by in a single type of cells hosting a single selection plasmid.

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Systematic studies of basal nonsense suppression, orthogonality of tRNAPyl variants, and cross recognition between codons and tRNA anticodons are reported. E. coli displays detectable basal amber and opal suppression but shows a negligible ochre suppression. Although detectable, basal amber suppression is fully inhibited when a pyrrolysyl-tRNA synthetase (PylRS)-tRNAPyl_CUA pair is genetically encoded. trnaPyl_CUA is aminoacylated by an E. coli aminoacyl-tRNA synthetase at a low level, however, this misaminoacylation is fully inhibited when both PylRS and its substrate are present. Besides that it is fully orthogonal in E. coli and can be coupled with PylRS to genetically incorporate a NAA at an ochre codon, tRNAPyl_UUA is not able to recognize an UAG codon to induce amber suppression. This observation is in direct conflict with the wobble base pair hypothesis and enables using an evolved M. jannaschii tyrosyl-tRNA synthetase-tRNAPyl_UUA pair and the wild type or evolved PylRS-tRNAPyl_UUA pair to genetically incorporate two different NAAs at amber and ochre codons. tRNAPyl_UCA is charged by E. coli tryptophanyl-tRNA synthetase, thus not orthogonal in E. coli. Mutagenic studies of trnaPyl_UCA led to the discovery of its G73U form which shows a higher orthogonality. Mutating trnaPyl_CUA to trnaPyl_UCCU not only leads to the loss of the relative orthogonality of tRNAPyl in E. coli but also abolishes its aminoacylation by PylRS.