Relation between Subunit Structure and Temperature-Sensitivity of Mutant Phenylalanyl RNA Synthetases of Escherichia coli

Four independent temperature-sensitive phenylalanyl RNA synthetase mutants of Escherichia coli differ from the wild type in that cell-free extracts contain phenylalanyl RNA synthetase cross-reacting material exclusively of about half the size of the wild type enzyme. It therefore seems probable that dissociation or at least weakening of subunit interactions might be the cause of the thermolability of the mutant enzymes. Further evidence supporting this assumption comes from the fact that the wild type enzyme can be dissociated in vitro by treatment with guanidine or urea under reducing conditions. The resulting dissociation products have the same size as the mutant cross-reacting material. Neither the subunits arising by in vitro dissociation of the wild type enzyme nor those present in mutant extracts show any catalytic activity. Direct demonstration of thermal dissociation of phenylalanyl RNA synthetase was made possible by means of a revertant strain possessing an enzyme with intermediate stability. Heat inactivation of this revertant enzyme is accompanied by a size change of phenylalanyl RNA synthetase cross-reacting material.