GLUTAMYL-TRNA(GLN) AMIDOTRANSFERASE IN DEINOCOCCUS RADIODURANS MAY BECONFINED TO ASPARAGINE BIOSYNTHESIS

Asparaginyl-tRNA (Asn-tRNA) and glutaminyl-tRNA (Gln-tRNA) are essenti al components of protein synthesis, They can be formed by direct acyla tion by asparaginyl-tRNA synthetase (AsnRS) or glutaminyl-tRNA synthet ase (GlnRS), The alternative route involves transamidation of incorrec tly charged tRNA. Examination of the preliminary genomic sequence of t he radiation-resistant bacterium Deinococcus radiodurans suggests the presence of both direct and indirect routes of Asn-tRNA and Gln-tRNA f ormation. Biochemical experiments demonstrate the presence of AsnRS an d GlnRS, as well as glutamyl-tRNA synthetase (GluRS), a discriminating and a nondiscriminating aspartyl-tRNA synthetase (AspRS), Moreover, b oth Gln-tRNA and Asn-tRNA transamidation activities are present. Surpr isingly, they are catalyzed by a single enzyme encoded by three ORFs o rthologous to Bacillus subtilis gatCAB. However, the transamidation ro ute to Gln-tRNA formation is idled by the inability of the discriminat ing D, radiodurans GluRS to produce the required mischarged Glu-tRNA(G ln) substrate. The presence of apparently redundant complete routes to Asn-tRNA formation, combined with the absence from the D, radiodurans genome of genes encoding tRNA-independent asparagine synthetase and t he lack of this enzyme in D, radiodurans extracts, suggests that the g atCAB genes may be responsible for biosynthesis of asparagine in this asparagine prototroph.