AN ACCURACY CENTER IN THE RIBOSOME CONSERVED OVER 2 BILLION YEARS

The accuracy of translation in Escherichia coli is profoundly influenc ed by three interacting ribosomal proteins, S12, S4, and S5. Mutations at lysine-42 of S12, originally isolated as causing resistance to str eptomycin, increase accuracy. Countervailing ''ribosomal ambiguity mut ations'' (ram) in S4 or S5 decrease accuracy. In the eukaryotic riboso me of Saccharomyces cerevisiae, mutations in SUP46 and SUP44, encoding the proteins equivalent to S4 and SS, lead to omnipotent suppression- i.e., to less accurate translation. The evolution of ribosomal protein S12 can be traced, by comparison with archaebacteria and Tetrahymena, to S28 of S. cerevisiae, even though the two proteins share only very limited regions of homology. However, one region that has been conser ved contains a lysine residue whose mutation leads to increased accura cy in E. coli. We have introduced into S28 of yeast the same amino aci d substitutions that led to the original streptomycin-resistant mutati ons in E. coli. We find that they have a profound effect on the accura cy of translation and interact with SUP44 and SUP46, just as predicted from the E. coli model. Thus, the interplay of these three proteins t o provide the optimal level of accuracy of translation has been conser ved during the 2 billion years of evolution that separate E. coli from S. cerevisiae.