Rate determining step in the reconstitution of Escherichia coli 30S ribosomal subunits

Immunochemical accessibility of ribosomal protein S4 in the 30 S ribosome. The interaction of S4 with S5 and S12

The reactivity of protein S4-specific antibody preparations with 30 S ribosomal subunits and intermediates of in vitro subunit reconstitution has been characterized using a quantitative antibody binding assay. Anti-S4 antibody preparations did not react with native 30 S ribosomal subunits; however, they did react with various subunit assembly intermediates that lacked proteins S5 and S12. The inclusion of proteins S5 and S12 in reconstituted particles resulted in a large decrease in anti-S4 reactivity, and it was concluded that proteins S5 and S12 are primarily responsible for the masking of S4 antigenic determinants in the 30 S subunit. The effect of S5 and S12 on S4 accessibility is consistent with data from a variety of other approaches, suggesting that these proteins form a structural and functional domain in the small ribosomal subunit.

Ribosomal protein S4 is an internal protein: Localization by immunoelectron microscopy on protein-deficient subribosomal particles

The location of protein S4 in the small ribosomal subunit has been identified by immunoelectron microscopy. Although intact small subunits are not reactive with antibodies directed against protein S4, subribosomal particles reconstituted without proteins S5 and S12 are reactive. By using these "incomplete" subparticles, we have mapped the position of S4. It is located at a single site on the exterior (cytoplasmic) side of the subunit, at the partition that separates the one-third, or head, from two-thirds, or base, of the subunit. In this location, protein S4 is "beneath" proteins S5 and S12. All three proteins are members of a complex on, or near, the external surface of the small ribosomal subunit that plays an important role in regulation of translational fidelity.

Identification of a site of psoralen crosslinking in E. coli 16S ribosomal RNA

We have developed a 2-dimensional gel method for identification of RNA sequences crosslinked by the intercalative drug 4'-hydroxymethyl-4,5',8-trimethylpsoralen (HMT). This method is being used to localize such sites in E. coli ribosomal RNA. We report here the identification of a site for HMT crosslinking within positions 434 and 497 of 16S rRNA. We suggest a likely site for HMT intercalation, in which residues U548 and U473 become crosslinked via the drug.

Physical characteristics of the reconstitution intermediates (RI30 and RI30*) from the 30S ribosomal subunit of Escherichia coli

The isolated reconstitution intermediates (RI30 and RI30*) from the 30S ribosomal subunit of Escherichia coli were found to contain ten proteins. The sedimentation coefficients, diffusion coefficients, density increments, extinction coefficients, and molecular weights were determined for the reconstitution particles and compared with those obtained from the 16S rRNA under identical buffer conditions. The results show that the binding of the proteins on the 16S rRNA at 4 degrees C does not markedly affect the folding of the RNA molecule. However, upon heating the RI30 particle at 40 degrees C to form the RI30* particle, significant folding of the RNA took place, giving a structure considerably more compact than that of the 16S rRNA or the RI30 particle.

The accessibility of antigenic determinants of ribosomal protein S4 in situ

Antibodies to Escherichia coli ribosomal protein S4 react with S4 in subribosomal particles, eg, the complex of 16S RNA with S4, S7, S8, S15, S16, S17, and S19 and the RI reconstitution intermediate, but they do not react with intact 30S subunits. Antibodies were isolated by three different methods from antisera obtained during the immunization of eight rabbits. Some of these antibody preparations, which contained contaminant antibodies directed against other ribosomal proteins, reacted with subunits, but this reaction was not affected by removal of the anti-S4 antibody population. Other antibody preparations did not react with subunits. It is concluded that the antigenic determinants of S4 are accessible in some protein deficient subribosomal particles but not in intact 30S subunits.

Conformational changes in 16S ribosomal RNA induced by 30S ribosomal subunit proteins from Escherichia coli

Laser light scattering has been used to evaluate conformational differences between free 16S RNA and several specific protein-16S RNA complexes. Proteins that interact strongly with the 16S RNA early in subunit assembly stabilize the RNA chain against unfolding in 1 mM Mg2+ and actually promote the formation of a more compact teriary structure in 20 mM Mg2+. A vital function of these proteins may therfore consist in altering the configuration of the RNA so that further assembly reactions can take place.

Studies on the ability of partially iodinated 16S RNA to participate in 30S ribosome assembly

Deproteinated 16S RNA was iodinated at pH 5.0 in an aqueous solution containing TlCl3 plus KI for 1-5 hours at 42 degrees C. Under these conditions 33 moles of iodine are incorporated per mole of RNA. As judged by sucrose gradient sedimentation, the iodinated RNA does not exhibit any large alteration in conformation as compared to unmodified 16S. The iodinated RNA was examined for its ability to reconstitute with total 30S proteins. Sedimentation velocity analysis reveals that the reconstituted subunit has a sedimentation constant of approximately 20S. In addition, protein analysis of particles reconstituted with 16S RNA iodinated for 5 hours indicates that proteins S2, S10, S13, S14, S15, S17, S18, S19, and S21 are no longer able to participate in the 30S assembly process and that proteins S6, S16 and S20 are present in reduced amounts. The ramifications of these results concerning protein-RNA and RNA-RNA interactions occurring in ribosome assembly are discussed.

The protein topography of the E. coli 30S ribosomal subunit: a preliminary model E. coli/30S subunit/ribosome/spatial arrangement of proteins

A three dimensional model is presented which shows the apatial arrangement of 20 of the 21 proteins of the 30S ribosomal subunit of Escherichia coli. The model fulfills several purposes: (a) It summarizes currently available structural and functional data on ribosomal proteins; (b) It suggests an interesting correlation between stoichiometry and function. Functional proteins are both clustered and fractional; (c) It can be evaluated in relationships or point out critical experiments by which it can be tested.