Once there were twenty.

Abstract

Aminoacyl-tRNA synthetases play a central role in protein synthesis by covalently linking the correct amino acid to the correct tRNA (1). Each aminoacyl-tRNA is then carried to the ribosome where it interacts with the cognate trinucleotide codon on the mRNA and transfers the amino acid onto a growing polypeptide chain. Work on tRNAs and aminoacyl-tRNA synthetases from bacteria, fungi, plants, and mammals led to the general notion of the occurrence of 20 aminoacyl-tRNA synthetases in all organisms, one for each of the 20 amino acids. Early indication of an exception came from the finding that three Gram-positive bacteria lacked the enzyme glutaminyl-tRNA synthetase (GlnRS),† which covalently links glutamine to the glutamine tRNA (tRNAGln) (2). Instead the glutamine tRNA is first aminoacylated with glutamic acid using glutamyl-tRNA synthetase (GluRS) to form Glu-tRNAGln. In a second step, the Glu-tRNAGln is converted to Gln-tRNAGln by an amidotransferase now called Glu-AdT (Fig. 1). The latter reaction requires ATP, which activates the side chain carboxyl group of glutamic acid on the tRNA to form a carboxyphosphate anhydride and glutamine or asparagine, which provides the amino group for transamidation (3). Thus, depending on the organism, there are two pathways for the synthesis of Gln-tRNAGln, a direct pathway involving GlnRS and an indirect pathway involving GluRS and Glu-AdT (Fig. 1). Further work showed that also in archaebacteria (4, 5), cyanobacteria, mitochondria, and chloroplasts (6, 7), Gln-tRNAGln is formed not directly but in a two-step process as outlined above. In archaebacteria, such as halobacteria, Asn-tRNAAsn is also synthesized in an analogous two-step process involving Asp-tRNAAsn as an intermediate (8). The widespread utilization of the Glu-AdT pathway for incorporation of glutamine into proteins and its possible similarity to other glutamine dependent amidotransferase reactions involved in a …