Surface topography of the Bacillus stearothermophilus ribosome

The surface topography of the intact 70S ribosomes and free 30S and 50S subunits from Bacillus stearothermophilus strain 2184 was investigated by lactoperoxidase-catalyzed iodination. Two-dimensional polyacrylamide gel electrophoresis was employed to separate ribosomal proteins for analysis of their reactivity. Free 50S subunits incorporated about 18% more 125I than did 50S subunits derived from 70S ribosomes, whereas, free 30S subunits and 30S subunits derived from 70S ribosomes incorporated similar amounts of 125I. Iodinated 70S ribosomes and subunits retained 62-78% of the protein synthesis activity of untreated particles and sedimentation profiles showed no gross conformational changes due to iodination. The proteins most reactive to enzymatic iodination were S4, S7, S10 and Sa of the small subunit and L2, L4, L5/9, L6 and L36 of the large subunit. Proteins S2, S3, S7, S13, Sa, L5/9, L10, L11 and L24/25 were labeled substantially more in the free subunits than in the 70S ribosome. Other proteins, including S5, S9, S12, S15/16, S18 and L36 were more extensively iodinated in the 70S ribosomes than in the free subunits. The locations of tyrosine residues in some homologous ribosomal proteins from B. stearothermophilus and E. coli are compared.