Leucyl tRNA Synthetase

Two factors which are involved in an apparent interconversion between the two forms of leucyl‐tRNA synthetase were separated. One of the factors, F 1 , is a protein of 2 molecular weight and catalyzes the transformation of E II , i.e the fully active enzyme, into E I , i.e the transfer‐inactive enzyme. The other factor, F 2 , is a peptide of about 3000 molecular weight which converts E I into an enzyme with catalytic and chromatographic prperties of E II A mild tryptic proteolysis of E II leads to modifications similar to those obseved after treatment of E II with factor F 1 . Moreover, after this proteolysis a peptide can be isolated, which is able to reassociate with E I . The only observed difference between native E II and the enzyme obtained by E I –F association is that the first cannot be dissociated by 8M urea or 6M guanidinium chloride, whereas the second, as well as E I , dissociates into two subunits. Results reported here suggest that E i is a proteolyzed form of native leucyl‐rRNA synthetase from which a small peptide is removed and that the enzyme is constituted from two subunitlike fragments. These fragments are in E I associated by low‐energy bonds which are, in the native enzyme, completed by two peptide bonds at the ends of peptide F 2 . The association of this peptide with the two “subunits” is necessary for the interactions between the site of the enzyme for leucyladenylate and its tRNA receptor‐site and for the transfer activity. Low‐energy bonds are sufficient to keep this association which maintains a conformation and properties close to those of native enyzme.