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Ammonia Channel Couples Glutaminase with Transamidase Reactions in GatCAB

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Science  30 Jun 2006:
Vol. 312, Issue 5782, pp. 1954-1958
DOI: 10.1126/science.1127156

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Abstract

The formation of glutaminyl transfer RNA (Gln-tRNAGln) differs among the three domains of life. Most bacteria employ an indirect pathway to produce Gln-tRNAGln by a heterotrimeric glutamine amidotransferase CAB (GatCAB) that acts on the misacylated Glu-tRNAGln. 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, Mn2+, 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 tRNAGln serve as identity elements essential for discrimination by GatCAB and propose a complete model for the overall concerted reactions to synthesize Gln-tRNAGln.

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