Prolyl Transfer Ribonucleic Acid Synthetase of Escherichia coli

Prolyl transfer ribonucleic acid synthetase of Escherichia coli B exists in vitro as an active dimer or as an inactive monomer, separable by hydroxylapatite chromatography. The monomer neither charges E. coli transfer ribonucleic acid nor catalyzes the proline-dependent adenosine triphosphate-pyrophosphate exchange reaction. At temperatures below 37°, in the absence of glycerol or sucrose, the enzyme is in equilibrium between the active dimer and the inactive monomer. Incubation at temperatures between 37° and 40° or addition of sucrose or glycerol at 0° completely converts the enzyme into the dimer, whereas cooling in the absence of sucrose or glycerol converts the enzyme into the monomer. The dimerization follows second order kinetics. It requires neither divalent cations nor adenosine triphosphate and can occur in various buffers. The dimer is stabilized by adenosine triphosphate or transfer ribonucleic acid. The monomerization follows first order kinetics, the dimer having a half-life of 45 min at pH 6.9 at 0°. In sucrose and glycerol gradient centrifugations the sedimentation velocities of the monomer and dimer are consistent with molecular weights of 47,000 and 94,000, respectively. The enzyme has been purified 250-fold in the presence of 10 to 40% glycerol.