Biochemical characterization of the ribosomal decoding site

Prior to the emergence of crystal structures of the ribosome, different ribosomal functions were identified with specific regions of ribosomal RNA by biochemical and genetic approaches. In particular, three universally conserved bases of 16S rRNA, G530, A1492 and A1493, were implicated in the interaction of the incoming aminoacyl-tRNA with the 30S subunit and mRNA. The conserved region surrounding A1492 and A1493 was called the "decoding site", based on the results of chemical probing experiments and antibiotic resistance mutations. Crystallographic studies from the Ramakrishnan laboratory have now shown that G530 loop, A1492 and A1493 undergo localized conformational changes to form an RNA structure that positions these three bases to inspect the accuracy of the codon-anticodon match with high stereochemical precision, using A-minor interactions. Some results from the pre-X-ray era may provide clues to further aspects of the decoding process.     

A genetic model to investigate drug-target interactions at the ribosomal decoding site

Recent advances in X-ray crystallography have greatly contributed to the understanding of the structural interactions between aminoglycosides and the ribosomal decoding site. Efforts to genetically probe the functional relevance of proposed drug-nucleotide contacts have in part been hampered by the presence of multiple rRNA operons in most bacteria. A derivative of the Gram-positive Mycobacterium smegmatis was rendered single rRNA operon allelic by means of gene inactivation techniques. In this system, genetic manipulation of the single chromosomal rRNA operon results in cells carrying homogeneous populations of mutant ribosomes. An exhaustive mutagenesis study of the ribosomal A site has been performed to define the importance of individual drug-nucleotide contacts. Mutational alterations in the M. smegmatis decoding site are discussed here, comparing the results with those obtained in other organisms. Implications for the selectivity of antimicrobial agents and for the fitness cost of resistance mutations are addressed.     

Crystal structure of paromomycin docked into the eubacterial ribosomal decoding A site

BACKGROUND: Aminoglycoside antibiotics interfere with translation in both gram-positive and gram-negative bacteria by binding to the tRNA decoding A site of the 16S ribosomal RNA. RESULTS: Crystals of complexes between oligoribonucleotides incorporating the sequence of the ribosomal A site of Escherichia coli and the aminoglycoside paromomycin have been solved at 2.5 A resolution. Each RNA fragment contains two A sites inserted between Watson-Crick pairs. The paromomycin molecules interact in an enlarged deep groove created by two bulging and one unpaired adenines. In both sites, hydroxyl and ammonium side chains of the antibiotic form 13 direct hydrogen bonds to bases and backbone atoms of the A site. In the best-defined site, 8 water molecules mediate 12 other hydrogen bonds between the RNA and the antibiotics. Ring I of paromomycin stacks over base G1491 and forms pseudo-Watson-Crick contacts with A1408. Both the hydroxyl group and one ammonium group of ring II form direct and water-mediated hydrogen bonds to the U1495oU1406 pair. The bulging conformation of the two adenines A1492 and A1493 is stabilized by hydrogen bonds between phosphate oxygens and atoms of rings I and II. The hydrophilic sites of the bulging A1492 and A1493 contact the shallow groove of G=C pairs in a symmetrical complex. CONCLUSIONS: Water molecules participate in the binding specificity by exploiting the antibiotic hydration shell and the typical RNA water hydration patterns. The observed contacts rationalize the protection, mutation, and resistance data. The crystal packing mimics the intermolecular contacts induced by aminoglycoside binding in the ribosome.     

Codon-anticodon interaction at the ribosomal P site improves the accuracy of the decoding process

The competing incorporation of [14C]Phe (cognate) and [3H]Leu (miscognate) into polypeptide material was measured. Kinetic analysis revealed that at the very beginning of protein synthesis the error fraction was 3 to 10 times larger than the "system-inherent" error, which is the error fraction found on the synthesized proteins. However, if a cognate tRNA, namely either deacylated tRNAPhe or AcPhe-tRNAPhe were prebound to the ribosomal P site, no enhanced error fraction was observed at the beginning. Furthermore, the initial rate of protein synthesis was increased by a factor of 10 to 50, regardless as to whether the ribosomes were precharged with tRNAPhe or AcPhe-tRNAPhe. In contrast, preincubation of ribosomes with non-cognate tRNA such as deacylated tRNALys had no effect. We conclude that codon-anticodon interaction at the P site facilitates the precise exposure of the adjacent codon at the ribosomal A site, thus contributing to the velocity and precision of the decoding process.     

Tetracycline can inhibit tRNA binding to the ribosomal P site as well as to the A site

A and P sites of Escherichia coli ribosomes were titrated with AcPhe-tRNAPhe, in the absence or presence of tetracycline. The P-site location of the bound AcPhe-tRNA was assessed by means of a quantitative puromycin reaction. The results demonstrate that, in agreement with the generally held view, tetracycline exclusively inhibits the A-site binding, if the statistical number of bound acyl-tRNA molecules per ribosome does not exceed about 0.5. However, above this value the P site becomes sensitive to tetracycline as well. It follows that the tightly coupled 70S ribosomes used in functional studies appear to be functionally heterogeneous, i.e. those P sites which cannot be affected by tetracycline are preferentially occupied by AcPhe-tRNA, whereas higher concentrations of this tRNA species are required to fill tetracycline-sensitive P sites. Furthermore, the results imply that under tRNA saturation conditions the tetracycline inhibition cannot be used as an indicator for the site location of bound tRNA.     

The ribosomal binding site for peptidyl-transfer-ribonucleic acid

The properties of a site, on Escherichia coli ribosomes, which binds peptidyl-s-RNA (where s-RNA refers to ;soluble' or transfer RNA) have been investigated. The binding is stable both in low Mg(2+) concentration (0.1mm), and in high Mg(2+) concentration (10mm) in the absence or presence of potassium chloride (86mm). Puromycin has been used to break the bond between the s-RNA and the polypeptide, and in the absence of further protein synthesis this technique exposes free s-RNA molecules on the ribosomes. The s-RNA exposed remains bound in high Mg(2+) concentration, but the binding is unstable in high Mg(2+) concentration with potassium chloride and the s-RNA can be freed completely from the ribosomes by lowering the Mg(2+) concentration. It can also be displaced by s-RNA in the medium. It is suggested that this ribosomal binding site for peptidyl-s-RNA is the site for peptide bond formation. Further, it is proposed that it is the same site that can be demonstrated on ribosomes not engaged in protein synthesis and that, in high Mg(2+) concentration, will bind s-RNA molecules charged or uncharged with amino acids.     

Complementary binding of oligonucleotides with 16S RNA and ribosomal ribonucleoproteins

The accessibility of single-stranded sequences of 16S RNA in free state and in ribonucleoprotein particles (RNP) to complementary binding with isoplith fractions of oligonucleotides was studied. RNP had different protein composition and corresponded to intermediate stages of E. coli 30S subunit assembly in vitro. Gel-filtration was used to detect the most strong binding. It was found that S4 essentially inhibited the hexamer binding to RNA. Core proteins bound to 16S RNA strongly increased the shielding of single-stranded regions while split proteins insignificantly changed the hexamer binding. Nevertheless evidence is presented that split proteins might also interact directly with 16S RNA in the 30S subunit.     

From stand-by to decoding site. Adjustment of the mRNA on the 30S ribosomal subunit under the influence of the initiation factors.

The hypothesis of an adjustment of the mRNA in its ribosomal channel under the influence of the initiation factors has been tested by site-directed crosslinking experiments. Complexes containing 30S subunits with bound mRNA having 4-thio-uracil at specific positions were prepared in the presence or absence of initiation factors and/or fMet-tRNA and subjected to UV irradiation to obtain specific crosslinks of the radioactively labeled mRNA with bases of the 16S rRNA and with ribosomal proteins. The subsequent identification of the specific sites of both mRNA and rRNA and individual ribosomal proteins involved in the crosslinking, obtained under different conditions of complex formation, provide direct evidence for the occurrence of a partial relocation of the mRNA on the 30S ribosomal subunits under the influence of the factors. The nature of this mRNA relocation is compatible with our previous proposal of a shift of the template from an initial ribosomal "stand-by site" to a second site closer to that occupied when the initiation triplet of the mRNA is decoded in the P-site.     

Ribosomal binding to the internal ribosomal entry site of classical swine fever virus

Most eukaryotic mRNAs require the cap-binding complex elF4F for efficient initiation of translation, which occurs as a result of ribosomal scanning from the capped 5' end of the mRNA to the initiation codon. A few cellular and viral mRNAs are translated by a cap and end-independent mechanism known as internal ribosomal entry. The internal ribosome entry site (IRES) of classical swine fever virus (CSFV) is approximately 330 nt long, highly structured, and mediates internal initiation of translation with no requirement for elF4F by recruiting a ribosomal 43S preinitiation complex directly to the initiation codon. The key interaction in this process is the direct binding of ribosomal 40S subunits to the IRES to form a stable binary complex in which the initiation codon is positioned precisely in the ribosomal P site. Here, we report the results of analyses done using enzymatic footprinting and mutagenesis of the IRES to identify structural components in it responsible for precise binding of the ribosome. Residues flanking the initiation codon and extending from nt 363-391, a distance equivalent to the length of the 40S subunit mRNA-binding cleft, were strongly protected from RNase cleavage, as were nucleotides in the adjacent pseudoknot and in the more distal subdomain IIId1. Ribosomal binding and IRES-mediated initiation were abrogated by disruption of helix 1b of the pseudoknot and very severely reduced by mutation of the protected residues in IIId1 and by disruption of domain IIIa. These observations are consistent with a model for IRES function in which binding of the region flanking the initiation codon to the decoding region of the ribosome is determined by multiple additional interactions between the 40S subunit and the IRES.     

Crystal structures of complexes between aminoglycosides and decoding A site oligonucleotides: role of the number of rings and positive charges in the specific binding leading to miscoding

The crystal structures of six complexes between aminoglycoside antibiotics (neamine, gentamicin C1A, kanamycin A, ribostamycin, lividomycin A and neomycin B) and oligonucleotides containing the decoding A site of bacterial ribosomes are reported at resolutions between 2.2 and 3.0 Å. Although the number of contacts between the RNA and the aminoglycosides varies between 20 and 31, up to eight direct hydrogen bonds between rings I and II of the neamine moiety are conserved in the observed complexes. The puckered sugar ring I is inserted into the A site helix by stacking against G1491 and forms a pseudo base pair with two H-bonds to the Watson–Crick sites of the universally conserved A1408. This central interaction helps to maintain A1492 and A1493 in a bulged-out conformation. All these structures of the minimal A site RNA complexed to various aminoglycosides display crystal packings with intermolecular contacts between the bulging A1492 and A1493 and the shallow/minor groove of Watson–Crick pairs in a neighbouring helix. In one crystal, one empty A site is observed. In two crystals, two aminoglycosides are bound to the same A site with one bound specifically and the other bound in various ways in the deep/major groove at the edge of the A sites.     

Mutational analysis of the donor substrate binding site of the ribosomal peptidyltransferase center.

Previous experiments have shown that the top of helix 90 of 23S rRNA is highly important for the ribosomal peptidyltransferase activity and might be part of the donor (P) site. Developing on these studies, mutations in the 23S rRNA at the highly conserved positions G2505, G2582, and G2583 were investigated. None of the mutations affected assembly, subunit association, or the capacity of tRNA binding to A and P sites. A "selective transpeptidation assay" revealed that the mutations specifically impaired peptide bond formation. Results with a modified "fragment" assay using the minimal donor substrate pA-fMet are consistent with a model where the nucleotides psiGG2582 form a binding pocket for C75 of the tRNA.     

GTP hydrolysis during methionyl-tRNAf binding to 40 S ribosomal subunits and the site of edeine inhibition.

Three lines of evidence are presented indicating that GTP hydrolysis associated with eukaryotic peptide initiation occurs in the absence of 60 S subunits when methionyl-tRNAf is bound to 40 S ribosomal subunits. An enzyme fraction required for binding of methionyl-tRNAf to 40 S subunits and peptide initiation, tentatively equated with eIF-(4 + 5), has GTPase activity and appears to be responsible for hydrolysis of GTP in the methionyl-tRNAf.eIF-2.GTP complex. Direct analysis of the methionyl-tRNAf.40 S complex formed with with eIF-2 and [8-3H] guanine, [gamma-32P]GTP reveals bound guanine but not gamma-phosphate. Edeine, a peptide antibiotic containing spermidine and beta-tyrosine residues at its COOH terminus and NH2 terminus, respectively, blocks peptide initiation and interferes with binding of methionyl-tRNAf to 40 S ribosomal subunits. Inhibition of binding is observed when the eIF-2-mediated binding reaction is carried out with GTP but not with guanosine 5'-(beta,gamma-methylene)triphosphate or guanosine 5'-(beta,gamma-imido)triphosphate. Edeine was labeled by iodination and shown to bind with high affinity to 40 S but not to 60 S ribosomal subunits. It is suggested that edeine blocks a specific site on the 40 S ribosomal subunit to which a segment of the methionyl-tRNAf molecule is bound during the course of the initiation reaction sequence.     

Two conformational states in the crystal structure of the Homo sapiens cytoplasmic ribosomal decoding A site

The decoding A site of the small ribosomal subunit is an RNA molecular switch, which monitors codon–anticodon interactions to guarantee translation fidelity. We have solved the crystal structure of an RNA fragment containing two Homo sapiens cytoplasmic A sites. Each of the two A sites presents a different conformational state. In one state, adenines A1492 and A1493 are fully bulged-out with C1409 forming a wobble-like pair to A1491. In the second state, adenines A1492 and A1493 form non-Watson–Crick pairs with C1409 and G1408, respectively while A1491 bulges out. The first state of the eukaryotic A site is, thus, basically the same as in the bacterial A site with bulging A1492 and A1493. It is the state used for recognition of the codon/anticodon complex. On the contrary, the second state of the H.sapiens cytoplasmic A site is drastically different from any of those observed for the bacterial A site without bulging A1492 and A1493.     

Enhancement of ribosomal frameshifting by oligonucleotides targeted to the HIV gag-pol region.

The pol gene of all retroviruses is expressed as a gag-pol fusion protein which is proteolytically processed to produce all viral enzymes. In the human immunodeficiency virus (HIV), the gag and pol genes overlap by 241 nucleotides with pol in the -1 phase with respect to gag. The gag-pol fusion is produced via a -1 ribosomal frameshifting event that brings the overlapping, out-of-phase gag and pol genes into translational phase. Frameshifting occurs at a so called 'shift site' 8-10 nucleotides upstream of a hairpin loop which may play a role in the regulation of frameshifting. We have fused this region of HIV-1 to the 5' end of the firefly luciferase reporter gene in order to quantitatively measure ribosomal frameshifting both in cells and by in vitro translation. A series of 2'-O-methyl oligonucleotides was designed to specifically bind the sequences which flank the gag-pol hairpin. Ribosomal frameshifting is enhanced up to 6 fold by those oligonucleotides which bind the area just 3 to the stem. Oligonucleotides which bind 5' to the stem have no effect on frameshift efficiency. In addition, we have constructed a series of fusion genes which mimic the effect of the bound oligonucleotides with intramolecular hairpins. The results suggest that increasing RNA secondary structure downstream of the shift site increases the frequency of ribosomal frameshifting, and that this effect can be mimicked by antisense oligonucleotides.     

The binding site for ribosomal protein L2 within 23S ribosomal RNA of Escherichia coli.

Ribosomal protein L2 from Escherichia coli binds to and protects from nuclease digestion a substantial portion of 'domain IV' of 23S rRNA. In particular, oligonucleotides derived from the sequence 1757-1935 were isolated and shown to rebind specifically to protein L2 in vitro. Other L2-protected oligonucleotides, also derived from domain IV (i.e. from residues 1955-2010) did not rebind to protein L2 in vitro nor did others derived from domain I. Given that protein L2 is widely believed to be located in the peptidyl transferase centre of the 50S ribosomal subunit, these data suggest that domain IV of 23S rRNA is also present in that active site of the ribosomal enzyme.