Structural and immunochemical characterization of a ribosomal protein from gram‐positive Micrococcus luteus which is functionally homologous to Escherichia coli ribosomal protein S1

Ribosomes from gram‐positive Micrococcus luteus contain an acidic protein (ML‐S1). ML‐S1 has been purified by chromatography of ribosomes on a poly(U)‐Sepharose column and the purified protein has a mobility in sodium dodecyl sulphate/polyacrylamide gels similar to that of ribosomal protein S1 of Escherichia coli (apparent M r 72000). Protein ML‐S1 reacted with E. coli anti‐S1 serum with an immunological partial‐identity reaction. ML‐S1 also reacted with antibodies raised against two structural domains of E. coli S1 (the N‐terminal ribosome‐binding domain and central and C‐terminal nucleic‐acid‐binding domain). Weak reaction with antiserum to the nucleic‐acid‐binding domain of E. coli S1 was observed. ML‐S1 was digested with trypsin under mild and exhaustive conditions. Mild digestion resulted in the production of a trypsin‐resistant core (ML‐S1F1) like E. coli S1. The fragment pattern obtained after exhaustive digestion differed appreciably from that obtained with E. coli S1. ML‐S1 bound to poly(U) as strongly as E. coli S1 and also showed appreciable binding to denatured DNA. Addition of ML‐S1 to S1‐depleted ribosomes from E. coli and M. luteus markedly stimulated the poly(U)‐directed polyphenylalanine synthesis. Phage MS2‐RNA‐dependent translation was also found to be stimulated by ML‐S1 although to a much lesser extent than the stimulation by E. coli S1. At a molar excess of ML‐S1 to ribosomes the protein showed a similar inhibitory effect to E. coli S1 on polypeptide synthesis. Our data indicate that ML‐S1 retained the structural domains important for its function despite certain structural differences from E. coli S1.