Abstract
The trypsin-modified methionyl-tRNA synthetase, a monomer of molecular weight 64 × 103, binds one molecule of tRNAMetf with strong affinity. A separation of about 25is observed between the centres of mass of the protein and the tRNA in the 1 : 1 complex. This can be reconciled with data on the contact regions only if a conformational change is assumed for the tRNA upon binding, probably involving the CCA end. The protein moiety of the complex might be slightly more contracted than in the free conformation, but this effect is not clearly outside experimental error. There is no apparent change in the protein structure as a function of MgCl2 concentration or of added small ligands. In conditions of 10 mm-MgCl2 and limiting tRNA, a 2 : 1, enzyme: tRNA complex is formed, the second enzyme binding with considerably lower affinity. The aggregate dissociates in favour of the 1:1 complex upon increasing tRNA concentration. The results are discussed in the context of the mode of action of the dimeric native enzyme.
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