Histidine Regulation in Salmonella typhimurium

The histidyl transfer RNA synthetase from Salmonella typhimurium has been purified 800-fold in three steps with a 45% yield. The purified enzyme shows only one band upon electrophoresis in polyacrylamide gels, is antigenically pure by immunodiffusion, and is homogeneous with respect to size as shown by analytical ultracentrifugation. The molecular weight of the enzyme is approximately 100,000 and its sedimentation coefficient s20, is 6.2 S. The enzyme has optimal activity at a temperature of 37° and at pH values between 7.2 and 8.5. The Km values for the formation of histidyl-tRNA were determined as 2.5 x 10-5 m for histidine, 1.4 x 10-4 m for ATP, and 1.1 x 10-7 m for histidine acceptor transfer RNA. Some of the properties of the synthetase in relation to the mechanism of repression of the histidine operon are discussed.