Study of the photoaffinity modification of Escherichia coli ribosomes near the donor tRNA-binding center

Affinity labelling of E. coli ribosomes near the donor tRNA-binding (P) site was studied with the use of photoreactive derivatives of tRNAPhe bearing arylazidogroups on N7 atoms of guanine residues (azido-tRNA). UV-irradiation of complexes 70S ribosome.poly(U).azido- tRNA(P-site) and 70S ribosome.poly(U).azido-tRNA(P-site).Phe- tRNAPhe(A-site) resulted in covalent attachment of azido-tRNA to ribosomes, both subunits being labelled. In both cases modification extent of 30S subunit was two-fold than that of the 50S one. It was shown that when the A-site was free the azido-tRNA located in P-site labelled proteins S9, S11, S12, S13, S21 and L14, L27, L31. Azido-tRNA located in P-site when the A-site was occupied with Phe-tRNAPhe labelled proteins S11, S12, S13, S14, S19, L32/L33 and possibly L23, L25. From the comparison of the sets of proteins labelled when A-site was free or occupied a conclusion was drawn that aminoacyl-tRNA located in ribosomal A-site affects the arrangement of deacylated tRNA in P-site. Data obtained allow to propose that proteins S5, S19, S20 and L24, L33 interact with guanine residues important for the tRNA tertiary structure formation.     

Transit of tRNA through the Escherichia coli ribosome: cross-linking of the 3' end of tRNA to ribosomal proteins at the P and E sites

Photoreactive derivatives of yeast tRNA(Phe) containing 2-azidoadenosine at their 3' termini were used to trace the movement of tRNA across the 50S subunit during its transit from the P site to the E site of the 70S ribosome. When bound to the P site of poly(U)-programmed ribosomes, deacylated tRNA(Phe), Phe-tRNA(Phe) and N-acetyl-Phe-tRNA(Phe) probes labeled protein L27 and two main sites within domain V of the 23S RNA. In contrast, deacylated tRNA(Phe) bound to the E site in the presence of poly(U) labeled protein L33 and a single site within domain V of the 23S rRNA. In the absence of poly(U), the deacylated tRNA(Phe) probe also labeled protein L1. Cross-linking experiments with vacant 70S ribosomes revealed that deacylated tRNA enters the P site through the E site, progressively labeling proteins L1, L33 and, finally, L27. In the course of this process, tRNA passes through the intermediate P/E binding state. These findings suggest that the transit of tRNA from the P site to the E site involves the same interactions, but in reverse order. Moreover, our results indicate that the final release of deacylated tRNA from the ribosome is mediated by the F site, for which protein L1 serves as a marker. The results also show that the precise placement of the acceptor end of tRNA on the 50S subunit at the P and E sites is influenced in subtle ways both by the presence of aminoacyl or peptidyl moieties and, more surprisingly, by the environment of the anticodon on the 30S subunit.     

Polynucleotide-protein interactions in the translation system. Identification of proteins interacting with tRNA in the A- and P-sites of E. coli ribosomes.

Ultraviolet irradiation (lambda = 254 nm) of ternary complexes of E. coli 70 S ribosomes with poly(U) and either Phe-tRNAPhe (in the A-site) or NAcPhe-tRNAPhe (in the P-site) effectively induces covalent linking of tRNA with a limited number of ribosomal proteins. The data obtained indicate that in both sites tRNA is in contact with proteins of both 30 S and 50 S subunits (S5, S7, S9, S10, L2, L6 and L16 proteins in the A-site and S7, S9, S11, L2, L4, L7/L12 and L27 proteins in the P-site). Similar sets of proteins are in contact with total aminoacyl-tRNA and N-acetylaminoacyl-tRNA. However, here no contacts of tRNA in the P-site with the S7 and L25/S17 proteins were revealed, whereas in the A-site total aminoacyl-tRNA contacts L7/L12. Proteins S9, L2 and, probably, S7 and L7/L12 are common to both sites.     

Photoaffinity modification of Escherichia coli ribosomes by fMet-tRNAf Met derivatives in the 70S initiation complex

Photoaffinity labeling of E. coli ribosomes within the 70S initiation complex was studied by using photoreactive derivatives of fMet-tRNAfMet bearing arylazidogroups scattered statistically over guanosine residues. It is shown that fMet-azido-tRNAfMet-II bearing 2 moles of the reagent residues per mole of tRNA (modified in the conditions of stability of tRNA tertiary structure) is fully active in aminoacylation and in the factor-dependent binding with ribosomes to form the 70S initiation complex. Functional activity of fMet-azido-tRNAfMet-I bearing also 2 moles of the reagent residues per mole of tRNA (but modified in conditions of lability of tRNA tertiary structure) decreases up to approximately 45% in aminoacylation and up to 70% in IF-2 X GTP-dependent binding to the ribosomes. Irradiation of complexes 70S ribosome-MS2-RNA-fMet-azido-tRNAfMet results in covalent linking of the tRNA derivative to the ribosomes. Both subunits are labeled, the 30S to a larger extent than 50S. It is shown that fMet-azido-tRNAfMet-II labels proteins S1, S7, S9, L27 whereas fMet-azido-tRNAfMet-1--proteins S1, S3, S5, S9, S14, L1, L2, L7/L12.     

Photoaffinity modification of Escherichia coli ribosomes near the tRNA-binding centers by tRNAPhe derivatives carrying arylazido groups on guanosine residues

Photoreactive derivatives of tRNAPhe (E. coli) bearing arylazido groups scattered statistically over guanosine residues (azido-tRNA) were applied for affinity labelling of E. coli ribosomes in elongation factor-dependent or factor-free model systems mimicking different steps of elongation. It is shown that UV-irradiation of the corresponding complexes of ribosomes with tRNA derivatives results in labelling of both subunits, the 30S one is labelled preferentially. In all experiments only ribosomal proteins were labelled. Comparison of the sets of proteins labelled by tRNA derivatives in different states at P-site allowed us to draw important conclusions concerning the influence of peptidyl moiety and of the occupancy of the A-site with aminoacyl- or peptidyl-tRNA on the arrangement of tRNA located at the P-site. Three of the 30S proteins--S11, S13 S19--are labelled with tRNA derivatives located at P-site in all states.     

Structural arrangement of the decoding site of Escherichia coli ribosomes as revealed from the data on affinity labelling of ribosomes by analogs of mRNA--derivatives of oligoribonucleotides

Using derivatives of oligoribonucleotides bearing an active group at the 5'- or 3'-end, the affinity modification of Escherichia coli ribosomes has been investigated in model complexes imitating various steps of initiation and elongation with a different extent of approximation to the real protein-synthesizing system. The protein environment of the ribosome decoding site is determined. The S3, S4, S9, L2, L7/L12 proteins belong to the 5'-region of the decoding site, and the S5, S7, S9, L1, L16 proteins to the 3'-region. In the process of translation the template moves along the external side of the 30 S subunit, from the L1 ridge to the L7/L12 stalk. The structural arrangement of the decoding site or its nearest environment depends on the functional state of ribosomes in the process of translation.     

Modifications of ribosomes from rat liver with alkylating derivatives of tRNA]

Alkylating analogs of peptidyl-tRNA: N-chloroambucilyl-14C-phenylanalyl-tRNA (1), N-iodoacetyl-14C-phenylalanyl-tRNA (2) and N-bromo-acetyl-14C-phenlalanyl-tRNA (3) were applied for the modification of the peptidyl-transferase center of the 80S ribosomes from rat liver. These analogs, being in the teronary complex poly-U: ribosome : tRNA analog, modified ribosomal proteins and ribosomal RNA. The modification is directed to large ribosomal subunit. It is found, that (1) modifies ribosomal proteins L5, L25, L31 and L32 and (2) modifies ribosomal proteins L4, L6, L10+L11, L13 and L30.     

The ribosomal neighbourhood of the central fold of tRNA: cross-links from position 47 of tRNA located at the A, P or E site.

The naturally occurring nucleotide 3-(3-amino-3-carboxy-propyl)uridine (acp3U) at position 47 of tRNA(Phe) from Escherichia coli was modified with a diazirine derivative and bound to ribosomes in the presence of suitable mRNA analogues under conditions specific for the ribosomal A, P or E sites. After photo-activation at 350 nm the cross-links to ribosomal proteins and RNA were identified by our standard procedures. In the 30S subunit protein S19 (and weakly S9 and S13) was the target of cross-linking from tRNA at the A site, S7, S9 and S13 from the P site and S7 from the E site. Similarly, in the 50S subunit L16 and L27 were cross-linked from the A site, L1, L5, L16, L27 and L33 from the P site and L1 and L33 from the E site. Corresponding cross-links to rRNA were localized by RNase H digestion to the following areas: in 16S rRNA between positions 687 and 727 from the P and E sites, positions 1318 and 1350 (P site) and 1350 and 1387 (E site); in the 23S rRNA between positions 865 and 910 from the A site, 1845 and 1892 (P site), 1892 and 1945 (A site), 2282 and 2358 (P site), 2242 and 2461 (P and E sites), 2461 and 2488 (A site), 2488 and 2539 (all three sites) and 2572 and 2603 (A and P sites). In most (but not all) cases, more precise localizations of the cross-link sites could be made by primer extension analysis.     

Affinity modification of Escherichia coli ribosomes with 2',3'-O-[4-(N-2-chloroethyl)-N-methylamino]-benzylidene derivative of AUGU3 in the 70S initiation complexes

Affinity labelling of the Escherichia coli ribosomes with the 2',3'-O-[4-(N-(2-chloroethyl)-N-methylamino]benzylidene derivative of AUGU3(AUGU3[14C]CHRCl) has been studied within 70S initiation complexes ribosome.AUGU3[14C]CHRCl.fMet-tRNA(Metf) and binary complex ribosome.AUGU3[14C]CHRCl. Various ways of the 70S initiation complex formation resulted in differently labelled products. Proteins S5, S7, S9, L1, L16 were thus identified as cross-linked with AUGU3[14C]CHRCl within an initiation complex obtained in the presence of initiation factors IF-1, IF-2, IF-3, whereas only proteins S5 and S7 were cross-linked within the complex obtained with the sole factor IF-2. Proteins S1, S3, L1 and L33 were labelled within the initiation complex obtained nonenzymatically but only protein S1 within the binary complex. In all complexes formed with use of initiation factors labelling of IF-2 factor was invariably observed.     

The effect of GTP hydrolysis and transpeptidation on the arrangement of aminoacyl-tRNA at the A-site of Escherichia coli 70 S ribosomes

From the affinity labelling of 70 S ribosomes with a photoreactive derivative of Phe-tRNAPhe bearing an arylazido group on guanine residues, it has been found that different sets of ribosomal proteins are labelled in the course of three successive steps of EF-Tu-dependent binding of aminoacyl-tRNA derivative at the A-site. Proteins S5, S7, S8, S16, S17, L9, L14, L15 and L24 were labelled before GTP hydrolysis; proteins S5, S7, S9, S11, S14, S18, S19, S21, L9, L21 and L29--after GTP hydrolysis; proteins S2, S5, S7, S21, L11 and L23--after GTP hydrolysis and transpeptidation.     

Number of tRNA binding sites on 80 S ribosomes and their subunits

The ability of rabbit liver ribosomes and their subunits to form complexes with different forms of tRNAPhe (aminoacyl-, peptidyl- and deacylated) was studied using the nitrocellulose membrane filtration technique. The 80 S ribosomes were shown to have two binding sites for aminoacyl- or peptidyl-tRNA and three binding sites for deacylated tRNA. The number of tRNA binding sites on 80 S ribosomes or 40 S subunits is constant at different Mg2+ concentrations (5-20 mM). Double reciprocal or Scatchard plot analysis indicates that the binding of Ac-Phe-tRNAPhe to the ribosomal sites is a cooperative process. The third site on the 80 S ribosome is formed by its 60 S subunit, which was shown to have one codon-independent binding site specific for deacylated tRNA.     

Characterisation of the ribosomal binding site for eukaryotic elongation factor 2 by chemical cross-linking

Ribosomal complexes containing elongation factor 2 (EF-2) were formed by incubation of 80 S ribosomes in the presence of EF-2 and the non-hydrolysable GTP analogue GuoPP[CH2]P. The factor was covalently coupled to the ribosomal proteins located at the factor binding site, by treatment with bifunctional reagents. After isolation of the covalent EF-2.ribosomal protein complexes, the proteins were labelled with 125I and the introduced covalent links cleaved. The ribosomal proteins were identified by electrophoresis in two independent two-dimensional gel systems, followed by autoradiography. After cross-linking with bis(hydroxysuccinimidyl) tartrate (4 A between the reactive groups), protein S3/S3a, S7 and S11 were found as the major ribosomal proteins covalently linked to EF-2. The longer reagent, dimethyl 3,8-diaza-4,7-dioxo-5,6-dihydroxydecanbisimidate (11 A between the reactive groups), covalently coupled proteins S7, S11, L5, L13, L21, L23, L26, L27a and L32 to EF-2. After cross-linking with dimethyl suberimidate (9 A between the reactive groups) proteins S3/3a, S7, S11, L5, L8, L13, L21, L23, L26, L27a, L31 and L32 were identified as belonging to the EF-2-binding site. The results indicate that the ribosomal domain interacting with EF-2 is located on both the small and the large ribosomal subunit close to the subunit interface.     

Photochemical cross-linking of tRNA1Arg to the 30S ribosomal subunit using aryl azide reagents attached to the anticodon loop

The 2-thiocytidine residue at position 32 of tRNA1Arg from Escherichia coli was modified specifically with three photoaffinity reagents of different lengths, and the corresponding N-acetylarginyl-tRNA1Arg derivatives were cross-linked to the P site of E. coli 70S ribosomes by irradiation. Covalent attachment was dependent upon the presence of a polynucleotide template and exposure to light of the appropriate wavelength. From 4% to 6% of the noncovalently bound tRNA became cross-linked to the ribosome as a result of photolysis, and attachment to the P site was confirmed by the reactivity of arginine in the covalent complexes toward puromycin. Analysis of the irradiated ribosomes by sucrose-gradient sedimentation at low Mg2+ concentration revealed that the tRNA was associated exclusively with the 30S subunit in all cases. Two of the N-acetylarginyl-tRNA1Arg derivatives were attached primarily to ribosomal proteins whereas the third was cross-linked mainly to 16S RNA. Partial RNase digestion of the latter complex demonstrated that the tRNA had become attached to the 3' third of the rRNA molecule. In addition, the tRNA-rRNA bond was shown to be susceptible to cleavage by hydroxylamine and mercaptoethanol.     

The mechanism of covalent reaction of bromoacetyl-phenylalanyl-transfer RNA with the peptidyl-transfer RNA binding site of the Escherichia coli ribosome

Results are presented to prove that bromoacetyl-phenylalanyl-transfer RNA reacts covalently with 50 S ribosomal proteins L2 and L27 while it is bound correctly to the peptidyl site on the 70 S ribosome. Attachment of the BrAcPhe moiety to tRNA causes a 100-fold enhancement of its reactivity with ribosomes. This reactivity closely parallels binding of tRNA whether measured by poly(U) stimulation or competition with deacylated tRNA. BrAcPhe-tRNA can bind correctly to the P site as judged by puromycin releasibility and lack of tetracycline inhibition. Little significant reaction of BrAcPhe-tRNA with L2 and L27 occurs during procedures used to purify and analyze ribosomal proteins. If ribosomes are first incubated with BrAcPhe-tRNA and subsequently treated with puromycin before analysis, little inhibition of the covalent reaction with L2 and L27 is observed. In contrast, a few minor reaction products are markedly suppressed. Covalently attached BrAcPhe-tRNA is still capable of accepting an amino acid from Phe-tRNA or puromycin. The products from this reaction are found attached to proteins L2 and L27 and to a lesser extent to L15 and L16. This shows that true affinity labeling of proteins in the peptidyl binding site has been accomplished.Some covalent reaction of BrAcPhe-tRNA with the 30 S protein S18 is also observed. This reaction is not poly(U)-dependent, however, and S18-reacted BrAcPhe-tRNA is not capable of peptide bond formation with Phe-tRNA. It seems likely that reaction with S18 results from a non-functional interaction of the affinity label with the ribosome.     

Determination of tRNA nucleotide residues directly interacting with proteins in the post- and pretranslocated ribosomal complexes

Nucleotide residues of E. coli tRNA interacting directly with proteins in pre- and posttranslocated ribosomal complexes have been identified by analysis of photo-induced tRNA-protein cross-links. A9, G18, A26 and U59 residues of NAcPhePhe-tRNA, located in the Ab-site (pretranslocated complex) have been cross-linked with proteins S10, L27, S7 and L2 respectively. In deacylated tRNA, located in the Pb-site, residues C17, G44, C56 and U60 have been cross-linked with proteins L2, L5, L27 and S9 respectively. The G44-L5 cross-link disappeared after translocation (NAc-PhePhe-tRNA located in the Pt-site).     

Conformational changes in ribosomes upon interaction with elongation factors

Conformational changes in the ribosomes upon interaction with EF-Tu were studied by limited proteolysis with a set of proteases. The main results are: (1) The cleavage rate of S1 protein strongly depends on the cooperative effect of poly(U) and tRNA: (2) The conformation of L7/L12 proteins is modulated by interaction of elongation factors with the ribosome and depends on hydrolysis of GTP; (3) The sensitivity of some ribosomal proteins (S6, S7, S18, S19, L9, L16, L19, and L27) to proteases changes upon binding of EF-Tu and depends on the ribosome functional state in accordance with GTP hydrolysis. Most of these proteins are located far from the factor-binding center of the ribosome. The possible mechanism of conformational changes is discussed.     

Interaction of selectively spin-labeled 70S ribosomes with tRNAPhe. An electron paramagnetic resonance study

70S ribosomes from Escherichia coli, selectively spin labeled on the SH groups of proteins S18, S12, S21, S17, and L27, were used to study the formation of the tertiary complex ribosome-poly(U)-tRNAPhe. Most of these ribosomal proteins are located in the region of binding of tRNA. The electron paramagnetic resonance observable structural change suggests a loosening of the ribosome structure upon binding of the tRNA molecule.     

70-S ribosomes of Escherichia coli have an additional site for deacylated tRNA binding

Escherichia coli 70-S ribosomes contain a third site for tRNA binding, additional to the A and P sites. This conclusion is based on several findings. Direct measurements showed that in the presence of poly(U), when both A and P sites are occupied by Ac[14C]Phe-tRNAPhe, ribosomes are capable of binding additionally deacylated non-cognate [3H]tRNA. If ribosomes in the preparation are active enough, the total binding of labeled ligands amounted to 2.5 mol/mol ribosomes. In the absence of poly(U), when the A site can not bind, the P site and the 'additional' site can be filled simultaneously with Ac[14C]Phe-tRNAPhe and deacylated [3H]tRNA, or with [3H]tRNA alone; the total binding exceeds in this case 1.5 mol/mol ribosomes. The binding at the 'additional' site is not sensitive to the template. [3H]tRNA bound there is able to exchange rapidly for unlabeled tRNA in solution. Deacylated tRNA is preferred to the aminoacylated one. The binding of AcPhe-tRNAPhe was not observed there at all. The 3'-end adenosine is essential for the affinity. The function of the 'additional' site is not known, but its existence has to be considered when tRNA . ribosome complexes are studied.     

Protein environment of the sense codon of the template in the A site of the human ribosome as inferred from crosslinking to oligoribonucleotide derivatives

The protein environment of each nucleotide of the template codon located in the A site of the human ribosome was studied with UUCUCAA and UUUGUU derivatives containing a Phe codon (UUC and UUU, respectively) and a perfluoroarylazido group at U4, U5, or U6. The analogs were positioned in the ribosome with the use of tRNA(Phe), which is cognate to the UUC or UUU codon and directs it to the P site, bringing a modified codon in the A site with a modified nucleotide occupying position +4, +5, or +6 relative to the first nucleotide of the P-site codon. On irradiation of ribosome complexes with tRNA(Phe) and mRNA analogs with mild UV light, the analogs crosslinked predominantly to the 40S subunit, modifying the proteins to a greater extent than the rRNA. The 18S rRNA nucleotides crosslinking to the analogs were identified previously. Of the small-subunit proteins, S3 and S15 were the major targets of modification in all cases. The former was modified both in ternary complexes and in the absence of tRNA, and the latter, only in ternary complexes. The extent of crosslinking of mRNA analogs to S15 decreased when the modified nucleotide was shifted from position +4 to position +6. The results were collated with the data on ribosomal proteins located at the decoding site of the 70S ribosome, and conclusion was made that the protein environment of the A-site codon strikingly differs between bacterial and eukaryotic ribosomes.