Structural and immunochemical characterization of a ribosomal protein from gram-positive <em>Micrococcus luteus</em> which is functionally homologous to <em>Escherichia coli</em> 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 SI of Escherichia coil (apparent M1 72000). Protein ML-S1 reacted with E. coil anti-Si serum with an immunological partial-identity reaction. ML-S1 also reacted with antibodies raised against two structural domains of E. coii 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. coil SI 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-SI Fl) like E. coil S1. The fragment pattern obtained after exhaustive digestion differed appreciably from that obtained with E. coil S1. ML-S1 bound to poly(U) as strongly as E. coli S1 and also showed appreciable binding to denatured DNA. Addition of ML-Sl to SI-depleted ribosomes from E. coil 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. coil S1. At a molar excess of ML-S1 to ribosomes the protein showed a similar inhibitory effect to E. coil S1 on polypeptide synthesis. Our data indicate that ML-S1 retained the structural domains important for its function despite certain structural differences from E. coil S1. [ABSTRACT FROM AUTHOR]