The formation of aminoacyl-transfer RNA is a crucial step in ensuring the accuracy of protein synthesis. Despite the central importance of this process in all living organisms, it remains unknown how archaea and some bacteria synthesize Asn-tRNA and Gln-tRNA. These amide aminoacyl-tRNAs can be formed 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. Here we show that all archaea possess an archaea-specific heterodimeric amidotransferase (encoded by gatD and gatE) for Gln-tRNA formation. However, Asn-tRNA synthesis in archaea is divergent: some archaea use asparaginyl-tRNA synthetase, whereas others use a heterotrimeric amidotransferase (encoded by the gatA, gatB and gatC genes). Because bacteria primarily use transamidation, and the eukaryal cytoplasm uses glutaminyl-tRNA synthetase, it appears that the three domains use different mechanisms for Gln-tRNA synthesis; as such, this is the only known step in protein synthesis where all three domains have diverged. Closer inspection of the two amidotransferases reveals that each of them recruited a metabolic enzyme to aid its function; this provides direct evidence for a relationship between amino-acid metabolism and protein biosynthesis.
Dispersed fluorescence spectra following the excitation of the CBr2A1B1-X1A1 2 and 2 bands at visible wavelengths were acquired in a discharge supersonic free jet expansion using an intensified charge-coupled device (ICCD) detector. The dispersed fluorescence spectra show signal-to-noise ratios of up to 60 and extend out to a maximum red shift frequency of 6000 cm(-1). Complete and detailed vibrational structure of the ground singlet state (X1A1) was observed. Vibrational parameters including fundamental frequencies, anharmonicities, and coupling constants were determined for the CBr2 X1A1 state. Additional vibrational structure starting at approximately 3650 cm(-1) was observed and this indicates the singlet-triplet energy gap to be >10 kcal mol(-1).
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