On the role of protein S4 N-terminal residues 1 through 30 in 30S ribosome function.

30S ribosomal protein S4 contains a single cysteine residue at position 31. We have selectively cleaved the peptide bond adjacent to this residue using the reagent 2-nitro-5-thiocyanobenzoic acid. The two resultant fragments were purified. The smaller S4-fragment (1-30) was found to be incapable of interacting with 16S RNA directly. This fragment also is not incorporated into a particle reconstituted from 16S RNA and 20 purified proteins with S4 missing. In contrast, the large S4-fragment (31-203) appears to be fully functional in ribosome assembly. Replacement of S4 with this fragment in the reconstitution reaction leads to a complete 30S ribosome containing all 30S proteins. This particle has a full capacity to bind poly U but has lost all activity for poly U directed phe-tRNA binding. We therefore propose that the N-terminus of protein S4 is not critical for ribosome assembly but is essential for tRNA binding.     

Studies on the role of amino acid residues 31 through 46 of ribosomal protein S4 in the mechanism of 30 S ribosome assembly.

We have described previously the isolation of a large fragment of 30 S ribosomal protein S4 (Changchien &; Craven, 1976). This S4-fragment is produced by the digestion of the S4–16S RNA complex with trypsin and it retains a full capacity to associate specifically with 16S RNA. It was also demonstrated that the S4-fragment has approximately 46 amino acid residues missing from the N-terminus and an intact C-terminus (also shown by Newberry et al., 1977). Preliminary experiments with this S4-fragment indicated that it could not fully replace the intact protein S4 in the process of 30 S ribosome assembly in vitro.We have also recently reported (Changchien et al., 1978) the preparation of a new fragment of protein S4 which has only 30 amino acid residues cleaved from the N-terminus. This was achieved by the use of the reagent 2-nitro-5-thiocyanobenzoic acid which selectively modifies the cysteine residue at position 31 followed by a cleavage of the adjacent peptide bond.We have now fully characterized the capacity of these two fragments, S4-fragment (47–203) and S4-fragment(31–203), to participate in the 30 S ribosome assembly process in vitro. Using 2-dimensional polyacrylamide gel electrophoresis, we find that when S4-fragment(47–203) is a component of the in vitro assembly reaction, proteins S1, S2, S10, S18 and S21 fail to become incorporated into the final particle. In contrast, S4-fragment(31–203) appears to participate in the reconstitution reaction without impairment allowing the complete incorporation of all 20 proteins of the 30 S subunit. The resultant particle, containing the S4-fragment (31–203), is fully active in the binding of poly(U), but is completely inactive for non-enzymatic poly(U)-directed binding of Phe-tRNA (Changchien et al., 1978). These results suggest that residues 1 through 30 of protein S4 are not involved in the assembly of the 30 S ribosome, but are required for the proper construction of the tRNA binding site. In addition residues 31 through 46 must be somehow critically important for the assembly of proteins S1, S2, S10, S18 and S21. We present evidence to show that the absence of residues 31 through 46 of protein S4 prevents a conformational change in the structure of 16 S RNA which normally accompanies the RI to RI transition and that this results in the inability of these proteins to participate in the assembly process.     

Functional role of ribosomal signatures

Although structure and sequence signatures in ribosomal RNA and proteins are defining characteristics of the three domains of life and instrumental in constructing the modern phylogeny, little is known about their functional roles in the ribosome. In this work, the largest coevolving RNA/protein signatures in the bacterial 30S ribosome are investigated both experimentally and computationally through all-atom molecular-dynamics simulations. The complex includes the N-terminal fragment of the ribosomal protein S4, which is a primary binding protein that initiates 30S small subunit assembly from the 5′ domain, and helix 16 (h16), which is part of the five-way junction in 16S rRNA. Our results show that the S4 N-terminus signature is intrinsically disordered in solution, whereas h16 is relatively stable by itself. The dynamic disordered property of the protein is exploited to couple the folding and binding process to the five-way junction, and the results provide insight into the mechanism for the early and fast binding of S4 in the assembly of the ribosomal small subunit.     

Molecular interactions of ribosomal components

Summary Five of the 30S ribosomal proteins from E. coli were tested for their ability to bind to 16S ribosomal RNA. Only one of these, S15, can form a complex with the RNA. Quantitative measurements as well as competition experiments show that the RNA binding site for the attachment of S15 is specific for this protein.These experiments complete our analysis of all 21 of the 30S ribosomal proteins. Five of these have now been shown to form a site-specific complex with 16S RNA. These are S4, S7, S8, S15 and S20. The relationship of these data to the assembly and structure of the ribosome are discussed.     

A cyanogen bromide fragment of S4 that specifically rebinds 16S RNA.

Escherichia coli ribosomal protein S4 was subjected to cyanogen bromide cleavage and was found to generate a complete cleavage product capable of rebinding 16S rRNA. This fragment, consisting of residues 1-103, was found to bind with an apparent association constant of 11 microM-1. This fragment was used in place of S4 in an in vitro reconstitution experiment. Although the particles formed had a protein composition not significantly different from reconstituted 30S ribosomal subunits, their sedimentation behavior was more like that of particles reconstituted without S4. These results indicate to us that, although residues 104-203 of S4 are involved in the assembly of the 30S ribosome, they are not necessary for the binding of S4 to 16S RNA. Taken with previous results, the domain of S4 involved in specific binding of 16S RNA can be confined to residues 47-103.     

Nonbridging phosphate oxygens in 16S rRNA important for 30S subunit assembly and association with the 50S ribosomal subunit

Ribosomes are composed of RNA and protein molecules that associate together to form a supramolecular machine responsible for protein biosynthesis. Detailed information about the structure of the ribosome has come from the recent X-ray crystal structures of the ribosome and the ribosomal subunits. However, the molecular interactions between the rRNAs and the r-proteins that occur during the intermediate steps of ribosome assembly are poorly understood. Here we describe a modification-interference approach to identify nonbridging phosphate oxygens within 16S rRNA that are important for the in vitro assembly of the Escherichia coli 30S small ribosomal subunit and for its association with the 50S large ribosomal subunit. The 30S small subunit was reconstituted from phosphorothioate-substituted 16S rRNA and small subunit proteins. Active 30S subunits were selected by their ability to bind to the 50S large subunit and form 70S ribosomes. Analysis of the selected population shows that phosphate oxygens at specific positions in the 16S rRNA are important for either subunit assembly or for binding to the 50S subunit. The X-ray crystallographic structures of the 30S subunit suggest that some of these phosphate oxygens participate in r-protein binding, coordination of metal ions, or for the formation of intersubunit bridges in the mature 30S subunit. Interestingly, however, several of the phosphate oxygens identified in this study do not participate in any interaction in the mature 30S subunit, suggesting that they play a role in the early steps of the 30S subunit assembly.     

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.     

A computational investigation on the connection between dynamics properties of ribosomal proteins and ribosome assembly

Assembly of the ribosome from its protein and RNA constituents has been studied extensively over the past 50 years, and experimental evidence suggests that prokaryotic ribosomal proteins undergo conformational changes during assembly. However, to date, no studies have attempted to elucidate these conformational changes. The present work utilizes computational methods to analyze protein dynamics and to investigate the linkage between dynamics and binding of these proteins during the assembly of the ribosome. Ribosomal proteins are known to be positively charged and we find the percentage of positive residues in r-proteins to be about twice that of the average protein: Lys+Arg is 18.7% for E. coli and 21.2% for T. thermophilus. Also, positive residues constitute a large proportion of RNA contacting residues: 39% for E. coli and 46% for T. thermophilus. This affirms the known importance of charge-charge interactions in the assembly of the ribosome. We studied the dynamics of three primary proteins from E. coli and T. thermophilus 30S subunits that bind early in the assembly (S15, S17, and S20) with atomic molecular dynamic simulations, followed by a study of all r-proteins using elastic network models. Molecular dynamics simulations show that solvent-exposed proteins (S15 and S17) tend to adopt more stable solution conformations than an RNA-embedded protein (S20). We also find protein residues that contact the 16S rRNA are generally more mobile in comparison with the other residues. This is because there is a larger proportion of contacting residues located in flexible loop regions. By the use of elastic network models, which are computationally more efficient, we show that this trend holds for most of the 30S r-proteins.     

RNA-protein interactions in the ribosome. II. Binding of ribosomal proteins to isolated fragments of the 16 S RNA

Specific fragments of the 16 S ribosomal RNA of Escherichia coli have been isolated and tested for their ability to interact with proteins of the 30 S ribosomal subunit. The 12 S RNA, a 900-nucleotide fragment derived from the 5′-terminal portion of the 16 S RNA, was shown to form specific complexes with proteins S4, S8, S15, and S20. The stoichiometry of binding at saturation was determined in each case. Interaction between the 12 S RNA and protein fraction $\text{S}\text{16}\text{S}\text{17}$ was detected in the presence of S4, S8, S15 and S20; only these proteins were able to bind to this fragment, even when all 21 proteins of the 30 S subunit were added to the reaction mixture. Protein S4 also interacted specifically with the 9 S RNA, a fragment of 500 nucleotides that corresponds to the 5′-terminal third of the 16 S RNA, and protein S15 bound independently to the 4 S RNA, a fragment containing 140 nucleotides situated toward the middle of the RNA molecule. None of the proteins interacted with the 600-nucleotide 8 S fragment that arose from the 3′-end of the 16 S RNA.When the 16 S RNA was incubated with an unfractionated mixture of 30 S subunit proteins at 0 °C, 10 to 12 of the proteins interacted with the ribosomal RNA to form the reconstitution intermediate (RI) particle. Limited hydrolysis of this particle with T₁ ribonuclease yielded 14 S and 8 S subparticles whose RNA components were indistinguishable from the 12 S and 8 S RNAs isolated from digests of free 16 S RNA. The 14 S subparticle contained proteins S6 and S18 in addition to the RNA-binding proteins S4, S8, S15, S20 and $\text{S}\text{16}\text{S}\text{17}$. The 8 S subparticle contained proteins S7, S9, S13 and S19. These findings serve to localize the sites at which proteins incapable of independent interaction with 16 S RNA are fixed during the early stages of 30 S subunit assembly.     

Circularly permuted GTPase YqeH binds 30S ribosomal subunit: Implications for its role in ribosome assembly

YqeH, a circularly permuted GTPase, is conserved among bacteria and eukaryotes including humans. It was shown to be essential for the assembly of small ribosomal (30S) subunit in bacteria. However, whether YqeH interacts with 30S ribosome and how it may participate in 30S assembly are not known. Here, using co-sedimentation experiments, we report that YqeH co-associates with 30S ribosome in the GTP-bound form. In order to probe whether YqeH functions as RNA chaperone in 30S assembly, we assayed for strand dissociation and annealing activity. While YqeH does not exhibit these activities, it binds a non-specific single and double-stranded RNA, which unlike the 30S binding is independent of GTP/GDP binding and does not affect intrinsic GTP hydrolysis rates. Further, S5, a ribosomal protein which participates during the initial stages of 30S assembly, was found to promote GTP hydrolysis and RNA binding activities of YqeH.     

An RNA core in the 30S ribosomal subunit of Escherichia coli and its structural and functional significance.

1. Evidence is presented for the occurrence of a very stable RNA core (S4-RNA) in "native" 16S RNA that is also present in the 30S subunit of Escherichia coli. A model giving the approximate location of this RNA core in the 30S subunit is presented. 2. It is proposed (a) that this S4-RNA acts as a nucleus for the assembly of the 30S subunit, and (b) that a small class of "linkage" proteins, including S4, further facilitate the assembly of the proteins to the RNA, thereby explaining some of the "cooperative" effects that are observed during in vitro assembly. 3. Evidence for the importance of the RNA core in the functioning of the ribosome is discussed.     

Efficient reconstitution of functional Escherichia coli 30S ribosomal subunits from a complete set of recombinant small subunit ribosomal proteins.

Previous studies have shown that the 30S ribosomal subunit of Escherichia coli can be reconstituted in vitro from individually purified ribosomal proteins and 16S ribosomal RNA, which were isolated from natural 30S subunits. We have developed a 30S subunit reconstitution system that uses only recombinant ribosomal protein components. The genes encoding E. coli ribosomal proteins S2-S21 were cloned, and all twenty of the individual proteins were overexpressed and purified. Reconstitution, following standard procedures, using the complete set of recombinant proteins and purified 16S ribosomal RNA is highly inefficient. Efficient reconstitution of 30S subunits using these components requires sequential addition of proteins, following either the 30S subunit assembly map (Mizushima & Nomura, 1970, Nature 226:1214-1218; Held et al., 1974, J Biol Chem 249:3103-3111) or following the order of protein assembly predicted from in vitro assembly kinetics (Powers et al., 1993, J MoI Biol 232:362-374). In the first procedure, the proteins were divided into three groups, Group I (S4, S7, S8, S15, S17, and S20), Group II (S5, S6, S9, Sll, S12, S13, S16, S18, and S19), and Group III (S2, S3, S10, S14, and S21), which were sequentially added to 16S rRNA with a 20 min incubation at 42 degrees C following the addition of each group. In the second procedure, the proteins were divided into Group I (S4, S6, S11, S15, S16, S17, S18, and S20), Group II (S7, S8, S9, S13, and S19), Group II' (S5 and S12) and Group III (S2, S3, S10, S14, and S21). Similarly efficient reconstitution is observed whether the proteins are grouped according to the assembly map or according to the results of in vitro 30S subunit assembly kinetics. Although reconstitution of 30S subunits using the recombinant proteins is slightly less efficient than reconstitution using a mixture of total proteins isolated from 30S subunits, it is much more efficient than reconstitution using proteins that were individually isolated from ribosomes. Particles reconstituted from the recombinant proteins sediment at 30S in sucrose gradients, bind tRNA in a template-dependent manner, and associate with 50S subunits to form 70S ribosomes that are active in poly(U)-directed polyphenylalanine synthesis. Both the protein composition and the dimethyl sulfate modification pattern of 16S ribosomal RNA are similar for 30S subunits reconstituted with either recombinant proteins or proteins isolated as a mixture from ribosomal subunits as well as for natural 30S subunits.     

Ribosomal protein-nucleic acid interactions. I. Isolation of a polypeptide fragment from 30S protein S8 which binds to 16S rRNA.

Within the bacterial ribosome a large number of specific protein and rRNA interactions appear to be required for assembly of the particle and its subsequent function in protein synthesis. In this communication it is shown that it is possible to isolate cyanogen bromide digestion products from ribosomal 30S protein S8 which will interact stoichiometrically with 16S rRNA. In addition to this a small binding polypeptide was generated from S8-16S rRNA complexes which were treated with proteinase K. The digestion of the complex yields a "protected" fragment of protein S8 which binds to 16S-rRNA. The isolated fragment will reassociate with 16S rRNA. It is not displaced by other 30S ribosomal proteins and blocks the binding of intact S8 to 16S rRNA. The size the possible structure of the S8 protein binding site are discussed and compared with the binding of cyanogen bromide digestion products which bind to 16S rRNA.     

Molecular interactions of ribosomal components. IV: Cooperative interactions during assembly in vitro

Cooperative interactions between different 30S ribosomal proteins during assembly in vitro are described. The site specific binding of S7 to 16S RNA is enhanced by S20; that of S16 requires S4 and S20; and S7 is required for the maximum binding of S9, S13 and S19. Some of these interactions are reflected in the protein neighborhoods of the functional ribosome, but this may not be a general rule. Finally, we suggest that the assembly cooperativety observed may not be a consequence of direct-protein interactions.     

Molecular interactions of ribosomal components

Summary The site-specific complex formed between 16S RNA and the 30S ribosomal protein S4 from Escherichia coli has been degraded with pancreatic ribonuclease. We have recovered the nuclease-resistant RNA from this complex; we call it S4aR. S4aR will bind to S4, but it will not bind to the other 30S proteins that can form site-specific complexes with 16S RNA. The data presented here as well as elsewhere (Schaup et al., 1971b) show that S4aR has a mass of about 150000 daltons and that it is made up of several separate RNA fragments, each of which enters the complex with S4. We conclude that S4 interacts with several separate binding sites on the RNA and that these probably contain a great deal of double stranded structure.     

Molecular interactions of ribosomal components

Summary The formation of a complex between individual 30S ribosomal proteins and 16S ribosomal RNA was studied by three techniques: zone centrifugation, molecular-sieve chromatography and electrophoresis in polyacrylamide gels. Five 30S proteins form a stable complex with the RNA under the conditions used to assemble ribosomes. Specific and nonspecific complex formation can be distinguished by an analysis of the concentration-dependence for complex formation. Similarly, competition experiments between heterologous proteins that bind to RNA can also be used to establish the uniquness of the RNA binding sites for ribosomal proteins. The data show that four of the five proteins bind to unique sites on the RNA. The fifth protein binds nonspecifically to the RNA. In addition, cooperative interactions between several proteins were observed; these enhance the interaction of proteins with the 16S RNA. A partial assembly sequence for the 30S ribosomal subunit is presented.     

A structural model for the assembly of the 30S subunit of the ribosome

The order in which proteins bind to 16S rRNA, the assembly map, was determined by Nomura and co-workers in the early 1970s. The assembly map shows the dependencies of binding of successive proteins but fails to address the relationship of these dependencies to the three-dimensional folding of the ribosome. Here, using molecular mechanics techniques, we rationalize the order of protein binding in terms of ribosomal folding. We determined the specific contacts between the ribosomal proteins and 16S rRNA from a crystal structure of the 30S subunit (1FJG). We then used these contacts as restraints in a rigid body Monte-Carlo simulation with reduced-representation models of the RNA and proteins. Proteins were added sequentially to the RNA in the order that they appear in the assembly map. Our results show that proteins nucleate the folding of the head, platform, and body domains, but they do not strongly restrict the orientations of the domains relative to one another. We also examined the contributions of individual proteins to the formation of binding sites for sequential proteins in the assembly process. Binding sites for the primary binding proteins are generally more ordered in the naked RNA than those for other proteins. Furthermore, we examined one pathway in the assembly map and found that the addition of early binding proteins helps to organize the RNA around the binding sites of proteins that bind later. It appears that the order of assembly depends on the degree of pre-organization of each protein's binding site at a given stage of assembly, and the impact that the binding of each protein has on the organization of the remaining unoccupied binding sites.     

A method of preparing Escherichia coli 16 S RNA possessing previously unobserved 30 S ribosomal protein binding sites

A method of preparing 16 S RNA has been developed which yields RNA capable of binding specifically at least 12, and possibly 13, 30 S ribosomal proteins. This RNA, prepared by precipitation from 30 S subunits using a mixture of acetic acid and urea, is able to form stable complexes with proteins S3, S5, S9, S12, S13, S18 and possibly S11. In addition, this RNA has not been impaired in its capacity to interact with proteins S4, S7, S8, S15, S17 and S20, which are proteins that most other workers have shown to bind RNA prepared by the traditional phenol extraction procedure (Held et al., 1974; Garrett et al., 1971; Schaup et al., 1970,1971).We have applied several criteria of specificity to the binding of proteins to 16 S RNA prepared by the acetic acid-urea method. First, the new set of proteins interacts only with acetic acid-urea 16 S RNA and not with 16 S RNA prepared by the phenol method or with 23 S RNA prepared by the acetic acid-urea procedure. Second, 50 S ribosomal proteins do not interact with acetic acidurea 16 S RNA but do bind to 23 S RNA. Third, in the case of protein S9, we have shown that the bound protein co-sediments with acetic acid-urea 16 S RNA in a sucrose gradient. Additionally, a saturation binding experiment showed that approximately one mole of protein S9 binds acetic acid-urea 16 S RNA at saturation. Thus, we conclude that the method employed for the preparation of 16 S RNA greatly influences the ability of the RNA to form specific protein complexes. The significance of these results is discussed with regard to the in vitro assembly sequence.     

Structural preordering in the N-terminal region of ribosomal protein S4 revealed by heteronuclear NMR spectroscopy

Protein S4, a component of the 30S subunit of the prokaryotic ribosome, is one of the first proteins to interact with rRNA in the process of ribosome assembly and is known to be involved in the regulation of this process. While the structure of the C-terminal 158 residues of Bacillus stearothermophilus S4 has been solved by both X-ray crystallography and NMR, that of the N-terminal 41 residues is unknown. Evidence suggests that the N-terminus is necessary both for the assembly of functional ribosomes and for full binding to 16S RNA, and so we present NMR data collected on the full-length protein (200 aa). Our data indicate that the addition of the N-terminal residues does not significantly change the structure of the C-terminal 158 residues. The data further indicate that the N-terminus is highly flexible in solution, without discernible secondary structure. Nevertheless, structure calculations based on nuclear Overhauser effect spectroscopic data combined with (15)N relaxation data revealed that two short segments in the N-terminus, S(12)RRL(15) and P(30)YPP(33), adopt transiently ordered states in solution. The major conformation of S(12)RRL(15) appears to orient the arginine side chains outward toward the solvent in a parallel fashion, while that of P(30)YPP(33) forms a nascent turn of a polyproline II helix. These segments contain residues that are highly conserved across many prokaryotic species, and thus they are reasonable candidates respectively for sites of interaction with RNA and other ribosomal proteins within the intact ribosome.