BACKGROUND:The bacterial ribosome is a primary target of several classes of antibiotics. Investigation of the structure of the ribosomal subunits in complex with different antibiotics can reveal the mode of inhibition of ribosomal protein synthesis. Analysis of the interactions between antibiotics and the ribosome permits investigation of the specific effect of modifications leading to antimicrobial resistances. Streptogramins are unique among the ribosome-targeting antibiotics because they consist of two components, streptogramins A and B, which act synergistically. Each compound alone exhibits a weak bacteriostatic activity, whereas the combination can act bactericidal. The streptogramins A display a prolonged activity that even persists after removal of the drug. However, the mode of activity of the streptogramins has not yet been fully elucidated, despite a plethora of biochemical and structural data.RESULTS:The investigation of the crystal structure of the 50S ribosomal subunit from Deinococcus radiodurans in complex with the clinically relevant streptogramins quinupristin and dalfopristin reveals their unique inhibitory mechanism. Quinupristin, a streptogramin B compound, binds in the ribosomal exit tunnel in a similar manner and position as the macrolides, suggesting a similar inhibitory mechanism, namely blockage of the ribosomal tunnel. Dalfopristin, the corresponding streptogramin A compound, binds close to quinupristin directly within the peptidyl transferase centre affecting both A- and P-site occupation by tRNA molecules.CONCLUSIONS:The crystal structure indicates that the synergistic effect derives from direct interaction between both compounds and shared contacts with a single nucleotide, A2062. Upon binding of the streptogramins, the peptidyl transferase centre undergoes a significant conformational transition, which leads to a stable, non-productive orientation of the universally conserved U2585. Mutations of this rRNA base are known to yield dominant lethal phenotypes. It seems, therefore, plausible to conclude that the conformational change within the peptidyl transferase centre is mainly responsible for the bactericidal activity of the streptogramins and the post-antibiotic inhibition of protein synthesis.

The linkage between internal ribosomal symmetry and transfer RNA (tRNA) positioning confirmed positional catalysis of amino‐acid polymerization. Peptide bonds are formed concurrently with tRNA‐3′end rotatory motion, in conjunction with the overall messenger RNA (mRNA)/tRNA translocation. Accurate substrate alignment, mandatory for the processivity of protein biosynthesis, is governed by remote interactions. Inherent flexibility of a conserved nucleotide, anchoring the rotatory motion, facilitates chirality discrimination and antibiotics synergism. Potential tRNA interactions explain the universality of the tRNA CCA‐end and P‐site preference of initial tRNA. The interactions of protein L2 tail with the symmetry‐related region periphery explain its conservation and its contributions to nascent chain elongation.

High-resolution structures of both ribosomal subunits revealed that most stages of protein biosynthesis, including decoding of genetic information, are navigated and controlled by the elaborate ribosomal architectural-design. Remote interactions govern accurate substrate alignment within a flexible active-site pocket [peptidyl transferase center (PTC)], and spatial considerations, due mainly to a universal mobile nucleotide, U2585, ensure proper chirality by interfering with D-amino-acids incorporation. tRNA translocation involves two correlated motions: overall mRNA/tRNA (messenger and transfer RNA) shift, and a rotation of the tRNA single-stranded aminoacylated-3' end around the bond connecting it with the tRNA helical-regions. This bond coincides with an axis passing through a sizable symmetry-related region, identified around the PTC in all large-subunit crystal structures. Propelled by a bulged universal nucleotide, A2602, positioned at the two-fold symmetry axis, and guided by a ribosomal-RNA scaffold along an exact pattern, the rotatory motion results in stereochemistry optimal for peptide-bond formation and in geometry ensuring nascent proteins entrance into their exit tunnel. Hence, confirming that ribosomes contribute positional rather than chemical catalysis, and that peptide bond formation is concurrent with A- to P-site tRNA passage. Connecting between the PTC, the decoding center, the tRNA entrance and exit points, the symmetry-related region can transfer intra-ribosomal signals between remote functional locations, guaranteeing smooth processivity of amino acids polymerization. Ribosomal proteins are involved in accurate substrate placement (L16), discrimination and signal transmission (L22) and protein biosynthesis regulation (CTC). Residing on the exit tunnel walls near its entrance, and stretching to its opening, protein-L22 can mediate ribosome response to cellular regulatory signals, since it can swing across the tunnel, causing gating and elongation arrest. Each of the protein CTC domains has a defined task. The N-terminal domain stabilizes the intersubunit-bridge confining the A-site-tRNA entrance. The middle domain protects the bridge conformation at elevated temperatures. The C-terminal domain can undergo substantial conformational rearrangements upon substrate binding, indicating CTC participation in biosynthesis-control under stressful conditions. Copyright (C) 2004 John Wiley Sons, Ltd.

Crystal structures of tRNA mimics complexed with the large ribosomal subunit of Deinococcus radiodurans indicate that remote interactions determine the precise orientation of tRNA in the peptidyl-transferase center (PTC). The PTC tolerates various orientations of puromycin derivatives and its flexibility allows the conformational rearrangements required for peptide-bond formation. Sparsomycin binds to A2602 and alters the PTC conformation. H69, the intersubunit-bridge connecting the PTC and decoding site, may also participate in tRNA placement and translocation. A spiral rotation of the 3' end of the A-site tRNA around a 2-fold axis of symmetry identified within the PTC suggests a unified ribosomal machinery for peptide-bond formation, A-to-P-site translocation, and entrance of nascent proteins into the exit tunnel. Similar 2-fold related regions, detected in all known structures of large ribosomal subunits, indicate the universality of this mechanism.

The azalide azithromycin and the ketolide ABT-773, which were derived by chemical modifications of erythromycin, exhibit elevated activity against a number of penicillin- and macrolide-resistant pathogenic bacteria. Analysis of the crystal structures of the large ribosomal subunit from Deinococcus radiodurans complexed with azithromycin or ABT-773 indicates that, despite differences in the number and nature of their contacts with the ribosome, both compounds exert their antimicrobial activity by blocking the protein exit tunnel. In contrast to all macrolides studied so far, two molecules of azithromycin bind simultaneously to the tunnel. The additional molecule also interacts with two proteins, L4 and L22, implicated in macrolide resistance. These studies illuminated and rationalized the enhanced activity of the drugs against specific macrolide-resistant bacteria.

Despite the appearance of bacterial strains resistant to all clinical antibiotics, including vancomycin (the `last resort'), development of new antimicrobial agents has slowed during recent decades. To aid design of new antibiotics, we must develop a detailed understanding of the mechanisms of antibiotic action and antibiotic resistance. Several classes of antibiotics target the ribosome and ribosomal factors, and recent structural studies of the ribosome (Ban et al., 2000; Harms et al., 2001; Nissen et al., 2000; Schlünzen et al., 2000; Wimberly et al., 2000; Yusupov et al., 2001) and complexes of ribosomes with inhibitors(Brodersen et al., 2000; Pioletti et al., 2001; Schlünzen et al., 2001; Hansen et al., 2002; Bashan et al., 2003; Schlünzen et al., 2003)are now revealing the mechanisms underlying their inhibitory activity.FIG1Responsibility for the various steps of polypeptide synthesis is divided among the two ribosomal subunits (30S and 50S). The 30S subunit ensures fidelity of decoding by establishing accurate codon-anticodon interactions. The 50S subunit catalyses peptide bond formation and elongation of the nascent protein, further protecting the nascent chain by channelling it through the ribosomal exit tunnel, which links the peptidyl transferase centre to the bottom of the 50S subunit.Prokaryotic protein synthesis starts with the formation of an initiation complex comprising the mRNA, initiator tRNA (fMet-tRNAfMet), three initiation factors (IFs) and the 30S subunit. IF3 binding prevents association of the two ribosomal subunits, verifies codon-anticodon complementarity and appears to regulate positioning of the mRNA. IF1 blocks the acceptor site(A-site) to prevent premature binding of the A-site tRNA. After binding of fMet-tRNAfMet, IF3 is released; this triggers hydrolysis of IF2-bound GTP, and IF2 and IF1 are released (the exact sequence of events is not known). The 50S subunit then joins the 30S initiation complex, forming the 70S ribosome.Edeine and pactamycin are universal antibiotics that affect protein biosynthesis in all organisms. Both hamper the formation of the initiation complex by displacing the mRNA (Brodersen et al., 2000; Pioletti et al.,2001). Edeine also affects the positioning of peptidyl site(P-site) tRNA and IF3 function. Evernimicin interacts with the tips of the 23S rRNA helices 89 and 91, which bind IF2, and may prevent formation of the 70S initiation complex (Belova et al.,2001).The 30S subunit possesses a mechanism for discriminating cognate from non-cognate aminoacyl tRNAs (aa-tRNAs) that ensures the correct amino acid has been delivered by elongation factor Tu (EF-Tu). The anticodon of the aa-tRNA in the ternary complex interacts with the mRNA codon, but the acceptor stem of the tRNA is associated with EF-Tu–GTP and thus cannot enter the catalytic center in the 50S subunit. EF-Tu is released only after hydrolysis of GTP; the acceptor stem can then be placed into the A-site.Many antibiotics target this step of the elongation cycle. Tetracycline,the first broad-spectrum antibiotic discovered, binds to multiple sites in the ribosome (Brodersen et al.,2000; Pioletti et al.,2001). Binding to the A-site in the 30S subunit sterically hinders the movement of the aa-tRNA so that it cannot simultaneously interact with the decoding site in the 30S subunit and the peptidyl transferase centre in the 50S subunit. The remaining sites may affect the assembly of the ribosomal particle but are probably not responsible for any inhibitory activity, since a single mutation in the primary binding site confers tetracycline resistance.Aminoglycosides bind directly adjacent to the decoding site in the 30S subunit, rendering translation highly inaccurate. Paromomycin, for example,induces a conformational change that enhances the affinity of the A-site for near-cognate tRNAs. Hygromycin B and neomycin act by freezing the tRNA in the A-site (Brodersen et al.,2000). Streptomycin stabilizes the so-called ram state of the 30S subunit, in which its affinity for non-cognate tRNAs is increased(Carter et al., 2000).Three antibiotics hamper positioning of the A-site tRNA by acting on EF-Tu:pulvomycin seems to reduce the GTPase activity of EF-Tu and inhibits its binding to tRNA; GE2270A blocks the tRNA-binding site on EF-Tu; and kirromycin prevents the release of EF-Tu, thereby stalling the ribosome(Hogg et al., 2002).Several antimicrobial agents inhibit decoding or A-site occupation by interacting with the 50S subunit. Sparsomycin, for example, binds in the core of the peptidyl transferase centre (PTC) by stacking to the most flexible nucleotide, which induces substantial conformational alterations(Bashan et al., 2003) and affects the correct positioning of both the A-site and P-site tRNAs(Hansen et al., 2002). Linezolid, an oxazolidinone, increases frameshifting and stop codon readthrough with nonsense suppression. It presumably binds in the vicinity of the PTC at the interface between the ribosomal subunits, thereby affecting initiation and elongation, but might also affect accuracy of decoding.During the elongation cycle, each tRNA passes through the three ribosomal binding sites (A-site→P-site→E-site), creating two conformational states. In the pre-translocational state, two tRNAs are bound to the A-site and P-site. In the post-translocational state, the P-site and E-site are occupied. Correct positioning of the aa-tRNA stimulates its CCA end to flip towards the 3′ end of the P-site tRNA in concert with peptide bond formation. This reaction is thought to induce translocation of the deacylated P-site tRNA into the E-site (Bashan et al.,2003).A variety of organisms produce antimicrobial compounds that target peptide bond formation. These are effective against many competing organisms because of the high phylogenetic conservation of the PTC; however, the originator is also attacked by its drug and must therefore develop resistance mechanisms,which are sometimes costly. Drugs inhibiting peptide bond formation act in three distinct ways: (1) by mimicking the substrate; (2) by blocking the path of the growing polypeptide chain; or (3) by alteration of tRNA positioning on the ribosome.Puromycin is a universal antibiotic and a tool for investigating the mechanism of peptide bond formation. Mimicking the aminoacylated end of the aa-tRNA, it participates in peptide bond formation, but its non-hydrolysable amide bond cannot be cleaved and peptidylpuromycin falls off the ribosome(Nissen et al., 2000; Bashan et al., 2003). Chloramphenicol, rarely used nowadays owing to its significant side effects,acts at the same site in the PTC. It occupies the position of the amino acid attached to the A-site tRNA, preventing peptide bond formation(Schlünzen et al.,2001).Lincosamides (clindamycin and lincomycin) interact with both the A-site and P-site on the 50S subunit, hampering positioning of both tRNA molecules and directly inhibiting peptide bond formation. Macrolides (e.g. erythromycin)instead bind at the entrance of the ribosomal exit tunnel(Schlünzen et al., 2001),blocking the path of the nascent chain through the ribosome. This site is some distance from the PTC; thus a polypeptide chain of 3-5 residues can be produced before protein biosynthesis stalls. Semi-synthetic derivatives of erythromycin such as carbomycin A or josamycin possess a long,mycarose-isobutyrate extension of the desosamine sugar (the crucial functional group of the macrolides) that protrudes into the PTC. These compounds thus also interfere with the correct positioning of the P-site substrate and directly affect peptide bond formation(Hansen et al., 2002). Azithromycin, a rather new erythromycin derivative, binds to two sites in the D. radiodurans 50S subunit(Schlünzen et al., 2003). The two azithromycin molecules are in direct contact; so in this case a cooperative effect might enhance its antimicrobial activity. Streptogramins also have a dual inhibitory mechanism, but this derives from two unrelated compounds: streptogramins A and streptogramins B. These enhance each other's activity probably through a direct interaction. Streptogramins A destabilize the binding of tRNAs to the A-site and P-site; streptogramins B presumably occupy the tunnel for the nascent peptide chain – similarly to the macrolides. Macrolides, lincosamides and streptogramins B all fail to inhibit MLSB-resistant strains possessing a single modification in the rRNA of the PTC, which reflects the overlapping binding site of these antibiotics.The mechanism behind the antimicrobial activity of another group of compounds, the pleuromutilins (e.g. tiamulin or valnemulin), is unclear. Since they compete with carbomycin but not erythromycin, they might prevent correct positioning of the CCA ends of tRNAs for peptide transfer.GTP-bound elongation factor EF-G binds to the A-site of the 50S subunit. Facilitated by GTP hydrolysis, it forces the anticodon stem loop of the A-site and P-site tRNAs to move into the P-site and E-site on the 30S subunit in concert with mRNA translocation. EF-G-GDP is then released from the ribosome,and the elongation process continues with the release of the deacylated E-site tRNA and the decoding of the next aa-tRNA. The antibiotic fusidic acid stalls the EFG-GDP complex on the ribosome. Thiostrepton and micrococcin also affect EF-G but by binding in the GTPaseassociated regions of the 50S subunit. Thiostrepton appears to induce a particular conformation of the ribosomal protein L11 that greatly reduces the GTPase activity of associated EF-G or EF-Tu. It also affects IF2 but in this case enhances GTPase activity. The exact mechanism of action of these antibiotics is not yet known.Viomycin acts at a slightly earlier stage: it seems to hamper translocation so that the A-site remains blocked. Viomycin has some additional effects, such as induction of misreading, inhibition of the peptidyl transferase reaction and inhibition of subunit dissociation during termination. Like many other drugs, it appears to bind at the interface between the subunits, thus altering the positioning of the A-site and P-site tRNAs, possibly affecting the affinity of the 50S subunit for them. Spectinomycin, another aminoglycoside antibiotic, is a rather rigid molecule. It binds to the head region of the 30S subunit, which undergoes a conformational change during elongation. Spectinomycin prevents this transition, inhibiting EF-G-catalysed translocation of the peptidyl-tRNA from the A-site to the P-site(Carter et al., 2000).The elongation cycle normally continues until a stop codon is detected by class 1 release factor (RF1 and RF2). The class 2 factor, RF3-GTP, facilitates the cognate binding of the other release factors. This triggers the hydrolysis of the ester bond connecting nascent peptide to tRNA and release of the peptide chain. Ribosome recycling factor (RRF) in conjunction with EF-G-promoted translocation then drives dissociation of the subunits in readiness for formation of a new 30S initiation complex. To date, no antibiotic is known specifically to affect termination of protein synthesis.

High-resolution crystal structures of large ribosomal subunits from Deinococcus radiodurans complexed with tRNA-mimics indicate that precise substrate positioning, mandatory for efficient protein biosynthesis with no further conformational rearrangements, is governed by remote interactions of the tRNA helical features. Based on the peptidyl transferase center (PTC) architecture, on the placement of tRNA mimics, and on the existence of a two-fold related region consisting of about 180 nucleotides of the 23S RNA, we proposed a unified mechanism integrating peptide bond formation, A-to-P site translocation, and the entrance of the nascent protein into its exit tunnel. This mechanism implies sovereign, albeit correlated, motions of the tRNA termini and includes a spiral rotation of the A-site tRNA-3' end around a local two-fold rotation axis, identified within the PTC. PTC features, ensuring the precise orientation required for the A-site nucleophilic attack on the P-site carbonyl-carbon, guide these motions. Solvent mediated hydrogen transfer appears to facilitate peptide bond formation in conjunction with the spiral rotation. The detection of similar two-fold symmetry-related regions in all known structures of the large ribosomal subunit, indicate the universality of this mechanism, and emphasizes the significance of the ribosomal template for the precise alignment of the substrates as well as for accurate and efficient translocation. The symmetry-related region may also be involved in regulatory tasks, such as signal transmission between the ribosomal features facilitating the entrance and the release of the tRNA molecules. The protein exit tunnel is an additional feature that has a role in cellular regulation. We showed by crystallographic methods that this tunnel is capable of undergoing conformational oscillations and correlated the tunnel mobility with sequence discrimination, gating and intracellular regulation.

High‐resolution crystal structures of large ribosomal subunits from Deinococcus radiodurans complexed with tRNA‐mimics indicate that precise substrate positioning, mandatory for efficient protein biosynthesis with no further conformational rearrangements, is governed by remote interactions of the tRNA helical features. Based on the peptidyl transferase center (PTC) architecture, on the placement of tRNA mimics, and on the existence of a two‐fold related region consisting of about 180 nucleotides of the 23S RNA, we proposed a unified mechanism integrating peptide bond formation, A‐to‐P site translocation, and the entrance of the nascent protein into its exit tunnel. This mechanism implies sovereign, albeit correlated, motions of the tRNA termini and includes a spiral rotation of the A‐site tRNA‐3′ end around a local two‐fold rotation axis, identified within the PTC. PTC features, ensuring the precise orientation required for the A‐site nucleophilic attack on the P‐site carbonyl‐carbon, guide these motions. Solvent mediated hydrogen transfer appears to facilitate peptide bond formation in conjunction with the spiral rotation. The detection of similar two‐fold symmetry‐related regions in all known structures of the large ribosomal subunit, indicate the universality of this mechanism, and emphasizes the significance of the ribosomal template for the precise alignment of the substrates as well as for accurate and efficient translocation. The symmetry‐related region may also be involved in regulatory tasks, such as signal transmission between the ribosomal features facilitating the entrance and the release of the tRNA molecules. The protein exit tunnel is an additional feature that has a role in cellular regulation. We showed by crystallographic methods that this tunnel is capable of undergoing conformational oscillations and correlated the tunnel mobility with sequence discrimination, gating and intracellular regulation.

Ribosomes, the universal cellular organelles catalyzing the translation of genetic code into proteins, are protein/RNA assemblies, of a molecular weight 2.5 mega Daltons or higher. They are built of two subunits that associate for performing protein biosynthesis. The large subunit creates the peptide bond and provides the path for emerging proteins. The small has key roles in initiating the process and controlling its fidelity. Crystallographic studies on complexes of the small and the large eubacterial ribosomal subunits with substrate analogs, antibiotics, and inhibitors confirmed that the ribosomal RNA governs most of its activities, and indicated that the main catalytic contribution of the ribosome is the precise positioning and alignment of its substrates, the tRNA molecules. A symmetry-related region of a significant size, containing about two hundred nucleotides, was revealed in all known structures of the large ribosomal subunit, despite the asymmetric nature of the ribosome. The symmetry rotation axis, identified in the middle of the peptide-bond formation site, coincides with the bond connecting the tRNA double-helical features with its single-stranded 3' end, which is the moiety carrying the amino acids. This thus implies sovereign movements of tRNA features and suggests that tRNA translocation involves a rotatory motion within the ribosomal active site. This motion is guided and anchored by ribosomal nucleotides belonging to the active site walls, and results in geometry suitable for peptide-bond formation with no significant rearrangements. The sole geometrical requirement for this proposed mechanism is that the initial P-site tRNA adopts the flipped orientation. The rotatory motion is the major component of unified machinery for peptide-bond formation, translocation, and nascent protein progression, since its spiral nature ensures the entrance of the nascent peptide into the ribosomal exit tunnel. This tunnel, assumed to be a passive path for the growing chains, was found to be involved dynamically in gating and discrimination.

Human serum albumin (HSA) is an abundant plasma protein that is responsible for the transport of a range of insoluble endogenous compounds including fatty acids, hormones and toxic metabolites such as bilirubin.Albumin also binds an impressive array of drugs in two primary sites (I and II) and much of the clinical and pharmaceutical interest in the protein derives from its effect on drug pharmacokinetics.HSA is a monomer that contains three homologous helical domains (I-III) each divided into two subdomains (A and B).Previously we determined the first high-resolution structures of HSA complexed with fatty acids and revealed the molecular interactions between HSA and its primary ligand.Here we present a crystallographic study of site I drugs and drug-analogues complexed with HSA-myristate.The structures confirm that phenylbutazone, oxyphenbutazone, azapropazone, iodipamide, dansyl-L-arginine, indomethacin, warfarin, triiodobenzoic acid and di-iodosalicyclic acid all bind at site I (subdomain IIA) in the presence of fatty acids.Our studies have also revealed the presence of additional drug sites for some of these compounds in subdomains IB and IIIB.The crystal structures throw new light on the molecular details of HSA-drug interaction and provide a new understanding of the structural basis of the very broad drug-binding specificity of the protein.By using a range of site I drugs we have been able to probe both common and distinct modes of interaction at site I; it is apparent that side-chain flexibility is important for accommodation of a range of different compounds in this binding site.

Resistance to antibiotics is a major problem in modern therapeutics. Ribosomes, the cellular organelle catalyzing the translation of the genetic code into proteins, are targets for several clinically relevant antibiotics. The ribosomes from eubacteria are excellent pathogen models. High resolution structures of the large and small ribosomal subunits were used as references that allowed unambiguous localization of almost a dozen antibiotic drugs, most of which are clinically relevant. Analyses of these structures showed a great diversity in the antibiotics' modes of action, such as interference with substrate binding, hindrance of the mobility required for the biosynthetic process and the blockage of tunnel which provides the path of exit for nascent proteins. All antibiotics studied by us were found to bind primarily to ribosomal RNA and, except for one allosteric effect, their binding did not cause major conformational changes. Antibiotics of the small ribosomal subunit may hinder tRNA binding, decoding, translocation, and the initiation of the entire biosynthetic process. The large subunit agents may target the GTPase center, interfere with peptide bond formation, or block the entrance of the nascent protein exit tunnel. The overall structure of the peptidyl transferase center and the modes of action of the antibiotic agents indicate that the ribosome serves as a template for proper positioning of tRNAs, rather than participating actively in the catalytic events associated with the creation of peptide bonds.

We describe the high resolution structure of the large ribosomal subunit from Deinococcus radiodurans (D50S), a gram-positive mesophile suitable for binding of antibiotics and functionally relevant ligands. The over-all structure of D50S is similar to that from the archae bacterium Haloarcula marismortui (H50S); however, a detailed comparison revealed significant differences, for example, in the orientation of nucleotides in peptidyl transferase center and in the structures of many ribosomal proteins. Analysis of ribosomal features involved in dynamic aspects of protein biosynthesis that are partially or fully disordered in H50S revealed the conformations of intersubunit bridges in unbound subunits, suggesting how they may change upon subunit association and how movements of the L1-stalk may facilitate the exit of tRNA.

The small ribosomal subunit is responsible for the decoding of genetic information and plays a key role in the initiation of protein synthesis. We analyzed by X‐ray crystallography the structures of three different complexes of the small ribosomal subunit of Thermus thermophilus with the A‐site inhibitor tetracycline, the universal initiation inhibitor edeine and the C‐terminal domain of the translation initiation factor IF3. The crystal structure analysis of the complex with tetracycline revealed the functionally important site responsible for the blockage of the A‐site. Five additional tetracycline sites resolve most of the controversial biochemical data on the location of tetracycline. The interaction of edeine with the small subunit indicates its role in inhibiting initiation and shows its involvement with P‐site tRNA. The location of the C‐terminal domain of IF3, at the solvent side of the platform, sheds light on the formation of the initiation complex, and implies that the anti‐association activity of IF3 is due to its influence on the conformational dynamics of the small ribosomal subunit.