At a high concentration of MgC12 (30 mM) and a low concentration of proteins from the 50-S subunit (0.2 mgiml), only three proteins, L15, L18 and L25, bind to 5-S RNA in significant amounts. On the other hand, in a buffer containing only 1 mM Mg C12, but otherwise at the same ionic strength (0.2 M), or at a protein concentration about 1 .5 mg/ml, a large, stable complex can form between immobilized 5-S RNA and 50-S ribosomal proteins. This complex contains proteins L2, L3, L5, L15, L16, L17, L18, L21, L22, L25, L33 and L34, and it possesses properties relevant to the function of the 50-S subunit: it has a binding site for deacylated tRNA, with a dissociation constant of 4.5 x M. The complex formed with 5-S RNA immobilized on an affinity column interacts also with 30-S subunits. The 5-S RNA-protein complex is interpreted as a sub-ribosomal domain which includes a considerable fraction of the peptidyl transferase center of the Escherichiu coli ribosome.

Escherichia coli ribosomal protein L2 interacts with fMet-tRNApt and NacPhe-tRNAPhe in solution, protecting their 3'-ends from enzymatic degradation. At the same time L2 enhances the rate of spontaneous hydrolysis of the ester bonds between terminal riboses and amino acyl moieties of these two peptidyl-tRNA analogues. L2 has, however, only a slight effect on the rate of spontaneous deacylation of aminoacyltRNAs. We suggest that the role of L2 is in the fixation of the aminoacyl stem of tRNA to the ribosome at its P-site, and speculate that this protein is directly involved in the peptidyl transferase (PT) reaction.Peptidyl transferase Protein L2 tRNA-protein complex

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The immobilized tRNA-50 S ribosomal subunit protein (TP50) complex binds the smaller ribosomal subunit. We constructed tRNA . TP50.5 S [32P] RNA and tRNA * TP50. t [32P] RNA complexes and investigated the accessibility of the 32P-labelled tRNAs to ribonuclease Tr. It was found that in this complex both 5 S RNA and tRNA are attacked by Tr RNase. In sharp contrast, the addition of 30 S subunit protects 5 S RNA as well as tRNA from degradation. We suggest that 5 S RNA-TP50 complex is exposed to the ribosomal interface and is involved in subunit interaction.E. coli ribosome S RNA-Protein complex