Structure of the Pseudomonas aeruginosa transamidosome reveals unique aspects of bacterial tRNA-dependent asparagine biosynthesis

Many prokaryotes lack a tRNA synthetase to attach asparagine to its cognate tRNA^Asn, and instead synthesize asparagine from tRNA^Asn-bound aspartate. This conversion involves two enzymes: a nondiscriminating aspartyl-tRNA synthetase (ND-AspRS) that forms Asp-tRNA^Asn, and a heterotrimeric amidotransferase GatCAB that amidates Asp-tRNA^Asn to form Asn-tRNA^Asn for use in protein synthesis. ND-AspRS, GatCAB, and tRNA^Asn may assemble in an ∼400-kDa complex, known as the Asn-transamidosome, which couples the two steps of asparagine biosynthesis in space and time to yield Asn-tRNA^Asn. We report the 3.7-A resolution crystal structure of the Pseudomonas aeruginosa Asn-transamidosome, which represents the most common machinery for asparagine biosynthesis in bacteria. We show that, in contrast to a previously described archaeal-type transamidosome, a bacteria-specific GAD domain of ND-AspRS provokes a principally new architecture of the complex. Both tRNA^Asn molecules in the transamidosome simultaneously serve as substrates and scaffolds for the complex assembly. This architecture rationalizes an elevated dynamic and a greater turnover of ND-AspRS within bacterial-type transamidosomes, and possibly may explain a different evolutionary pathway of GatCAB in organisms with bacterial-type vs. archaeal-type Asn-transamidosomes. Importantly, because the two-step pathway for Asn-tRNAAsn formation evolutionarily preceded the direct attachment of Asn to tRNAAsn, our structure also may reflect the mechanism by which asparagine was initially added to the genetic code.