Amino acids attach to specific sites on tRNA molecules during protein synthesis.
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Scientific literature and textbooks establish that aminoacyl-tRNA synthetases specifically attach amino acids to corresponding tRNA molecules during protein synthesis.
The aminoacyl-tRNA synthetases arose early in evolution and established the rules of the genetic code through their specific interactions with amino acids and RNA molecules. About half of these tRNA charging enzymes are class I synthetases, which contain similar N-terminal nucleotide-fold-like structures that are joined to variable domains implicated in specific protein-tRNA contacts. Here, we show that a bacterial synthetase gene can be split into two nonoverlapping segments. We split the gene for Escherichia coli methionyl-tRNA synthetase (a class I synthetase) at several sites near the interdomain junction, such that one segment codes for the nucleotide-fold-containing domain and the other provides determinants for tRNA recognition. When the segments are folded together, they can recognize and charge tRNA, both in vivo and in vitro. We postulate that an early step in the assembly of systems to attach amino acids to specific RNA molecules may have involved specific interactions between discrete proteins that is reflected in the interdomain contacts of modern synthetases.
There is considerable interest among protein scientists in methods that permit unnatural amino acids to be incorporated at specific sites in proteins. Such methods would facilitate studies of protein structure and function and would allow for the synthesis of proteins having novel properties. Current methods use nonsense-suppressing tRNAs (1) to incorporate an unnatural amino acid at the desired site in a protein and chemical (rather than enzymatic) means to attach the unnatural amino acid to the nonsense-suppressing tRNA (2). However, protein yields are typically modest because the suppressor tRNA participates in only one round of translation. Protein expression could be increased by the development of in vivo systems, but this poses significant challenges. It requires a synthetase that activates an unnatural amino acid and only attaches it to a designated tRNA. In addition, it requires that the designated tRNA is not aminoacylated by any other synthetase in the cell and that it efficiently translates a nonsense codon. A “21st cognate pair” is essentially created if all of these criteria are met (Fig. 1). The paper by Kowal et al. (3) in this issue of PNAS makes an important contribution by describing a novel approach for creating tRNA-synthetase pairs that efficiently incorporate natural amino acids at specific sites in proteins in vivo. By taking a novel approach in their choice of tRNAs and synthetases for membership in a new cognate pair, RajBhandary's group has created
Attachment to tRNA by Aminoacyl-tRNA Synthetase Activates the Amino Acid 389 AA~tRNA Formation … of mRNA Synthesis Before the Structural Genes RNA Synthesis and Protein Synthesis Are Coupled … taught and did research on RNA synthesis and protein synthesis until 1976. While at Harvard
Attachment to tRNA by Aminoacyl-tRNA Synthetase Activates the Amino Acid 389 AA—tRNA Formation … mRNA Synthesis Before the Structural Genes 486 RNA Synthesis and Protein Synthesis Are Coupled … taught and did research on RNA synthesis and protein synthesis until 1976. While at Harvard
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