Physico-chemical and Reassociation Properties of E. coli Ribosomal Subunits

Abstract

Large amounts of 30, 50, and 70S ribosomes from Escherichia coli strain Q₁₃ have been fractionated by sucrose density gradient sedimentation with a fixed angle rotor. The purity of each fractionated sample was satisfactory. The molecular weights of 30 and 50S subunits were measuredby means of light scattering and values of 77.5×10⁴ and 148×10⁴ were obtained, respectively. The dimensions of 70S ribosomes and those of the subunits were determinedfrom the values of intrinsic viscosity, radius of gyration, and partial specific volume. Measurements of the volume obtained by the former two methods gave values about 4 times larger than the volume calculated from partial specific volume and molecular weight. This suggests that the ribosomes arehydrated with a large amount of water and that they are swollen in solution. Experiments involving reassociation of the subunits revealed that fractionated subunits lose their ability to form 70S particles. A small RNA component obtained from 70S ribosomes on their dissociation to subunits played an important role in the reassociation of the subunits to 70S ribosomes. The component appeared to bea peptidyl (or aminoacyl)-tRNA. 30S subunit polymerized with itself to form dimer and trimer under someexperimental conditions. However, there was no evidence that 50S subunit forms a dimer.