Domain-specific recruitment of amide amino acids for protein synthesis

The formation of aminoacyl-transfer RNA is a crucial step in ensuring the a ccuracy of protein synthesis. Despite the central importance of this proces s in all living organisms, it remains unknown how archaea and some bacteria synthesize Asn-tRNA and Gln-tRNA. These amide aminoacyl-tRNAs can be forme d by the direct acylation of tRNA, catalysed by asparaginyl-tRNA synthetase and glutaminyl-tRNA synthetase, respectively. A separate, indirect pathway involves the formation of mis-acylated Asp-tRNA(Asn) or Glu-tRNA(Gln), and the subsequent amidation of these amino acids while they are bound to tRNA , which is catalysed by amidotransferases(1,2). Here we show that all archa ea possess an archaea-specific heterodimeric amidotransferase (encoded by g atD and gatE) for Gln-tRNA formation. However, Asn-tRNA synthesis in archae a is divergent: some archaea use asparaginyl-tRNA synthetase, whereas other s use a heterotrimeric amidotransferase (encoded by the gatA, gatB and gatC genes). Because bacteria primarily use transamidation(3), and the eukaryal cytoplasm uses glutaminyl-tRNA synthetase, it appears that the three domai ns use different mechanisms for Gln-tRNA synthesis; as such, this is the on ly known step in protein synthesis where all three domains have diverged. C loser inspection of the two amidotransferases reveals that each of them rec ruited a metabolic enzyme to aid its function; this provides direct evidenc e for a relationship between amino-acid metabolism and protein biosynthesis .