ENZYME STRUCTURE WITH 2 CATALYTIC SITES FOR DOUBLE-SIEVE SELECTION OFSUBSTRATE

High-fidelity transfers of genetic information in the central dogma ca n be achieved by a reaction called editing. The crystal structure of a n enzyme with editing activity in translation is presented here at 2.5 angstroms resolution. The enzyme, isoleucyl-transfer RNA synthetase, activates not only the cognate substrate L-isoleucine but also the min imally distinct L-valine in the first, aminoacylation step. Then, in a second, ''editing'' step, the synthetase itself rapidly hydrolyzes on ly the valylated products. For this two-step substrate selection, a '' double-sieve'' mechanism has already been proposed. The present crysta l structures of the synthetase in complexes with L-isoleucine and L-va line demonstrate that the first sieve is on the aminoacylation domain containing the Rossmann fold, whereas the second, editing sieve exists on a globular beta-barrel domain that protrudes from the aminoacylati on domain.