Book of Abstracts: Albany 2003
June 17-21 2003
RNA Recognition by Threonyl-tRNA Synthetase
E. coli threonyl-tRNA synthetase (ThrRS) is a homodimeric enzyme which has the ability to bind its own mRNA upstream of the initiation codon at a site called the operator. It represses its translation by preventing the ribosome binding. The operator (113 nucleotides) folds into 4 domains, two stem-loops displaying threonine specific anticodon sequences, a linker domain, and the Shine-Dalgarno region indispensable for the control. Extensive structural mapping as well as point mutations have shown that each anticodon domain of the dimeric enzyme recognizes one of the stem-loops (1).
ThrRS is built upon the canonical class II catalytic domain and is flanked by an anticodon-binding domain typical of the IIa subgroup and a characteristic Nterminal region that folds into two domains, N1 and N2. The tRNA is strongly anchored at three levels: the anticodon, the CCA end and the acceptor arm, which is clamped on its minor groove side by the N2 domain (2).
The ThrRS:tRNA complex can be compared with the crystal structure of the complex between a truncated form of the ThrRS, deprived of its N-terminal region, and domain 2 of the operator (37 nucleotides). The operator recognition proceeds through the same principal elements as the tRNA anticodon stem-loop recognition, including an extensive binding of the anticodon bases and cross-subunit interactions between the ribose-phosphate chain of the RNA stem and the second subunit of the protein, thus giving a structural proof of the molecular mimicry between the two partners of ThrRS. The two systems are however not strictly identical. Structural analysis of the two complexes shows how an internal loop and other specific features allows the RNA to fulfil efficiently its particular function (3).