Aminoacyl-tRNA syn the tas es play a cen tral role in main tain ing ac cu racy dur ing the trans la tion of the ge netic code. To achieve this chal leng ing task they have to dis crim i- nate against amino ac ids that are very closely re lated not only in struc ture but also in chem i cal na ture. A 'dou ble-sieve' ed it ing model was pro posed in the late sev en ties to ex plain how two closely re lated amino ac ids may be dis crim i nated. How ever, a clear un der stand ing of this mech a nism re quired struc tural in for ma tion on syn the tas es that are faced with such a prob lem of amino acid dis crim i na tion. The first struc tural ba sis for the editing model came recently from the crystal structure of isoleucyl-tRNA synthetase, a class I synthetase, which has to dis crim i nate against valine. The structure showed the pres ence of two cat a lytic sites in the same en zyme, one for ac ti va tion, a coarse sieve which binds both isoleucine and valine, and an other for ed it ing, a fine sieve which binds only valine and rejects isoleucine. An other struc ture of the en zyme in com plex with tRNA showed that the tRNA is re spon si ble for the translocation of the misactivated amino-acid substrate from the catalytic site to the editing site. These studies were mainly fo cused on class I syn the tas es and the sit u a tion was not clear about how class II enzymes discriminate against similar amino acids. The recent struc tural and en zy matic stud ies on threonyl-tRNA synthetase, a class II en zyme, reveal how this chal leng ing task is achieved by us ing a unique zinc ion in the ac tive site as well as by em ploy ing a sep a rate do main for spe cific ed it ing ac tiv ity. These stud ies led us to pro pose a model which em pha sizes the mir ror sym met ri cal ap proach of the two classes of en zymes and high lights that tRNA is the key player in the evo lu tion of these class of enzymes.
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