Comparative study of subunits of phenylalanyl-tRNA synthetase from Escherichia coli and Thermus thermophilus.

FPLC separation of alpha- and beta-subunits of phenylalanyl-tRNA synthetases from E. coli MRE-600 and Thermus thermophilus HB8 has been carried out in the presence of urea. Native alpha-subunits of both enzymes were primarily alpha 2-dimers and tended to aggregate. Most E. coli enzyme beta-subunits were monomeric and only a small fraction was represented by beta 2-dimers. All thermophilic beta-subunits were beta 2-dimers. It was shown that monomers and all forms of homologous subunits had no catalytic activity in tRNA(Phe) aminoacylation. For the enzymes and their subunits, titration curves were obtained and isoelectric points were determined. The comparison of the relative surface charges indicated similarity of the surfaces of entire enzymes and the corresponding beta-subunits. Alpha-subunits displayed a distinctly different pH dependence of the surface charge. A spatial model of the oligomeric structure and a putative mechanism for its formation are discussed.