Two novel growth cycle-reflecting proteins associated with Escherichia coli ribosomes

The isolation and characterization of two low molecular weight, growth cycle-reflecting proteins which are associated with Escherichia coli ribosomes is described. These proteins which show greatly enhanced stoichiometry in ribosomes derived from post-exponential cells, exhibit molecular weights of 16 600, and 11 700 on sodium dodecyl sulfate (SDS) gel electrophoresis. Both proteins are highly acidic, the larger being the most acidic protein associated with the ribosome. Their amino acid compositions are unique and distinguish them from all other ribosomal proteins. Neither of the proteins showed crossreaction with either anti-L7 or anti-S6 sera although their electrophoretic behavior resembles that of L7 and S6. During the purification we have also isolated small amounts of three low molecular weight proteins showing some immunological homology with L7/L12. The isolated proteins were found to be without effect on the in vitro translation of f2-RNA, indicating that these adaptive modifications of the ribosome do not seriously affect its intrinsic activity.