The new form of
valyl-tRNA synthetase (EC 6.1.1.9) that appears immediately after
infection of Escherichia coli with bacteriophage T4 was purified and subjected to mild proteolysis using five different
proteases. The inactivation of aminoacylation activity was both more extensive and rapid than that obtained with
valyl-tRNA synthetase purified from uninfected E. coli. The addition of bulk
tRNA from E. coli B protected the phage-specific form of
valyl-tRNA synthetase from proteolysis, but
ATP and
valine did not exhibit a similar protective effect. The characteristic property of phage-modified
valyl-tRNA synthetase, resistance to denaturation by 4 M
urea, remained unaffected during treatment with
trypsin. This suggested that the phage-specific factor tau, known to be associated with the
synthetase in phage-infected cells, was protected from proteolysis in the
synthetase-tau complex. Comparison by isoelectric focusing of normal
valyl-tRNA synthetase, the phage-specific form of this
enzyme, and phage
enzyme from which tau had been removed, revealed no differences in the isoelectric points of these three molecules. Based on these results a model was drawn for the structural changes occurring in
valyl-tRNA synthetase after association with the phage factor tau.