Ammonia Channel Couples Glutaminase with Transamidase Reactions in GatCAB

The formation of glutaminyl transfer RNA (Gln-tRNA Gln ) differs among the three domains of life. Most bacteria employ an indirect pathway to produce Gln-tRNA Gln by a heterotrimeric glutamine amidotransferase CAB (GatCAB) that acts on the misacylated Glu-tRNA Gln . Here, we describe a series of crystal structures of intact GatCAB from Staphylococcus aureus in the apo form and in the complexes with glutamine, asparagine, Mn 2+ , and adenosine triphosphate analog. Two identified catalytic centers for the glutaminase and transamidase reactions are markedly distant but connected by a hydrophilic ammonia channel 30 Å in length. Further, we show that the first U-A base pair in the acceptor stem and the D loop of tRNA Gln serve as identity elements essential for discrimination by GatCAB and propose a complete model for the overall concerted reactions to synthesize Gln-tRNA Gln .