Modification of valyl tRNA synthetase by bacteriophage in Escherichia coli

Abstract

Infection of Escherichia coli with T4 bacteriophage is known to bring about the appearance of a new valyl tRNA synthetase activity distinguished by its behavior during hydroxylapatite fractionation, by its sedimentation in sucrose gradients and, in certain temperature-sensitive bacterial mutants, by its increased stability. Studies were made to determine the origin of this new activity. Its formation was accompanied at all times by a proportional decrease in the original valyl tRNA synthetase activity; 20 minutes after infection at 30 °C no original enzyme remained and no further increase in the phage-induced activity occurred. The addition of chloramphenicol at any time completely blocked both the gain of new and the loss of old activity. An increase of 2.6% in the buoyant density of valyl tRNA synthetase could be achieved by growth of the cells in 80% D 2O medium, permitting resolution of heavy and light enzyme by equilibrium centrifugation with cesium chloride. By appropriate transfer of cells between deuterium-labeled and unlabeled media it was possible to demonstrate that the new enzyme activity brought about by phage infection in both mutant and wild cells consists of polypeptide chains synthesized prior to infection. From these results it was concluded that T4 infection causes a chloramphenicol-sensitive conversion of host valyl tRNA synthetase into a new, possibly dimeric form.