RECONSTRUCTION OF QUATERNARY STRUCTURES OF CLASS-II TRANSFER-RNA SYNTHETASES BY RATIONAL MUTAGENENSIS OF A CONSERVED DOMAIN

Class II tRNA synthetases have long been known to have quaternary stru ctures of alpha, alpha 2, alpha(2) beta(2), and alpha(4), depending on the amino acid specificity and the organism from which the synthetase was isolated, Even the quaternary structures of enzymes for the same amino acid show variations in evolution. The basis for these variation s has not been understood. We report here that sequence manipulations of a structural motif (motif 1) characteristic of all class II tRNA sy nthetases can generate most of the evolutionary diversity of quaternar y forms of class II synthetases. Thus, the principles elucidated here for quaternary structure assembly may be general.