The intermolecular complex of erythromycin and ribosome

Direct entry to erythronolides via a cyclic bis[allene]

The complexity and low tractability of antibiotic macrolides pose serious challenges to addressing the problem of resistance through semi- or total synthesis. Here we describe a new strategy involving the preparation of a complex yet tractable macrocycle and the transformation of this macrocycle into a range of erythronolide congeners. These compounds represent valuable sectors of erythromycinoid structure space and constitute intermediates with the potential to provide further purchase in this space. The routes are short. The erythronolides were prepared in three or fewer steps from the macrocycle, which was prepared in a longest linear sequence of 11 steps.

Preparation of cyclic 2′,3′-carbamate derivatives of erythromycin macrolide antibiotics

Tricarbonylation of clarithromycin has been effected in a one-pot reaction with phosgene. The 11,12-diol moiety was closed into a cyclic carbonate, while the dimethylamino alcohol of the desosamine sugar was cyclised with loss of a methyl group to form a cyclic 2',3'-carbamate. The 4'' hydroxyl group in clarithromycin was converted into a chloroformate group and subsequently to an allyl carbonate which on Pd-catalysis furnished a novel N-demethylclarithromycin 2',3'-carbamate-11,12-carbonate. Hydrolytic removal of the cladinose sugar and a subsequent oxidation furnished the corresponding ketolide. The 11,12-cyclic carbonate moiety was cleaved by sodium azide to the 10,11-anhydro-9-ketone. 11-N-Arylated cyclic 11,12:2',3'-dicarbamate derivatives were prepared in a copper(I) chloride aided reaction between aryl isocyanates and 10,11-anhydro 9-ketones. The products are novel N-arylated-N'-demethylated 11,12:2',3'-dicarbamate ketolides derived from clarithromycin.

Mechanism of action of dipropofol and synergistic action with other antibacterial agents in vitro

The mode of action of dipropofol and its antibacterial activity in combination with other antibiotics against Gram-positive and -negative bacteria were investigated. Dipropofol showed a bactericidal effect against Staphylococcus aureus 209P, and inhibited the incorporations of 3H-glutamate and 3H-leucine into S. aureus 209P and Bacillus subtilis PCI219 cells. These results indicated that the mechanism of action of dipropofol was mediated by the inhibition of protein synthesis or amino acid incorporation. A synergistic effect was performed by checkerboard titration with Muller-Hinton agar plates containing dipropofol (0.39 microg/ml, 1/4 x MIC) and various concentrations of nine other antibiotics in vitro. The synergism against vancomycin resistant Enterococcus faecium was confirmed in the combination of dipropofol with rifampicin. The MIC of rifampicin was decreased from 0.39 microg/ml to <0.005 microg/ml by the addition of dipropofol. This combinational effect in reversing vancomycin resistance of enterococci highlights novel drug targets and has importance in the design of new therapeutic regimens against resistant pathogens.

Inhibition of [14C]chloramphenicol binding to Escherichia coli ribosomes by erythromycin derivatives

The effect of erythromycin A and 35 analogues of erythromycin A on [(14)C]chloramphenicol binding to Escherichia coli ribosomes was evaluated. Substitutions on various portions of the erythromycin molecule were made with retention of ability to bind to ribosomes. Specifically, substantial activity in interference with [(14)C]chloramphenicol binding was retained upon removal of the cladinose and various substitutions on the 3-hydroxyl, the oxime, and 2-hydroxyl groups. Erythromycin analogues with relatively poor binding activity to ribosomes could be detected. This assay can be used alone or in conjunction with microbiological assays for screening of active analogues. It permits an estimate of the general binding activity of compounds rapidly and directly. The assay reflects the ability of the compounds to interact with their target organelle, the ribosome, and may serve as a useful adjunct in developing new compounds.

New research in macrolides and ketolides since 1997

Macrolide and ketolide antibacterials remain a very dynamically active group. To overcome erythromycin A resistance within Gram-positive cocci and bacteria, novel compounds have been semi-synthesised, such as ketolides and C-4'' carbamate erythromycylamine derivatives. The continual efforts of those studying macrolides have led to molecular level investigations into the mechanism of action of these antibacterials. Among all novel derivatives, only telithromycin and AB-773 are currently under development. No real novel developments have been seen with the 15- and 16-membered ring macrolides, however, research is also continuing in this area. This review is an update of our knowledge in the field of macrolides.

Synthesis of two and antibacterial activity of one novel oxime ether derivatives of erythromycin A

The synthesis of novel erythromycin A 9-O-(2-ethenesulfony-ethyl)-oxime and erythromycin A 9-O-(3-oxo-butyl)-oxime from erythromycin A (EA) by the Michael reaction is described and to describe the effects of transformation of ketone in position 9 of EA to an oxime ether. This transformation occurred in a single step without protecting of any functional moiety of erythromycin oxime and zero waste manner in good yield. The antibacterial screen of EA 9-O-(2-ethenesulfony-ethyl)-oxime is also reported.

Purification and characterization of an erythromycin esterase from an erythromycin-resistant Pseudomonas sp

An erythromycin esterase (molecular mass 51200 Da) was purified from Pseudomonas sp. GD100, which was isolated from a salmon hatchery sediment sample from Washington State. The pI of the protein was 4.5-4.8. The enzyme was inhibited by 1 mM mercuric acid, and had the substrate specificity for structurally related 14-membered macrolides, which decreased in the order of oleandomycin, erythromycin A and erythromycin A enol ether. The activity for erythromycin A varied with temperature, but the effect of pH was minimal at pH 6.0-9.0. The half-life of the enzyme was estimated to be 8.9 h at 35 degrees C and 0.23 h at 55 degrees C, and the activation energy of the catalytic reaction of erythromycin A was estimated at 16.2 kJ mol(-1).

Rokitamycin: bacterial resistance to a 16-membered ring macrolide differs from that to 14- and 15-membered ring macrolides

Rokitamycin is the latest semi-synthetic 16-membered ring macrolide introduced into clinical practice. It is characterized by greater hydrophobicity, better bacterial uptake and a slower release, more cohesive ribosomal binding, and a longer post-antibiotic-effect (PAE) than can be observed with other available 14-, 15- and 16-membered ring macrolides. Rokitamycin exerts its activity on strains that harbor inducible erm genes or the efflux gene, mef(A). It has also been reported to be more active in vitro than other 16-membered ring macrolides. However, these recognized features are not fully exploited yet because current automated test procedures used in many microbiological laboratories determine susceptibility only to erythromycin or clarithromycin. Resistance to 16-membered ring macrolides cannot be predicted solely on the basis of known resistance to erythromycin or clarithromycin as revealed by an automated susceptibility assay. At least equally important is the knowledge of the bacterial resistance phenotype. This is underlined by the existence of Gram-positive coccal strains resistant to erythromycin and other 14-,15-membered ring macrolides but susceptible to 16-membered ring macrolides. Since the local prevalence of erythromycin phenotypes is generally unknown but might determine the outcome of treatment, the procedure for identifying the phenotypes in erythromycin-resistant strains (which can be easily and cheaply performed using the two- or three-disk assay) should become routine, at least in the countries in which 16-membered ring macrolides are used. This approach should help to optimize the use of macrolides, improve our knowledge of the local prevalence of phenotypes resistant to erythromycin, and offer the possibility of treating infections caused by certain types of erythromycin-resistant pathogens.

Structural basis for the interaction of antibiotics with the peptidyl transferase centre in eubacteria

Ribosomes, the site of protein synthesis, are a major target for natural and synthetic antibiotics. Detailed knowledge of antibiotic binding sites is central to understanding the mechanisms of drug action. Conversely, drugs are excellent tools for studying the ribosome function. To elucidate the structural basis of ribosome-antibiotic interactions, we determined the high-resolution X-ray structures of the 50S ribosomal subunit of the eubacterium Deinococcus radiodurans, complexed with the clinically relevant antibiotics chloramphenicol, clindamycin and the three macrolides erythromycin, clarithromycin and roxithromycin. We found that antibiotic binding sites are composed exclusively of segments of 23S ribosomal RNA at the peptidyl transferase cavity and do not involve any interaction of the drugs with ribosomal proteins. Here we report the details of antibiotic interactions with the components of their binding sites. Our results also show the importance of putative Mg+2 ions for the binding of some drugs. This structural analysis should facilitate rational drug design.

Macrolide and ketolide antibiotic separation by reversed phase high performance liquid chromatography

Twenty different macrolide and ketolide antibiotics were analyzed by reversed phase high performance liquid chromatography on an ODS-2 cartridge column. Each of these compounds was uniquely separated and purified by varying the flow rate. Retention times of the individual drugs were proportional to the flow rate of the mobile phase. Recovery of antimicrobial activity for most of the drugs was greater than 90% based on a microbiological assay of material recovered from the column. Retention times were related to structural differences between these antimicrobial agents.

Mechanisms of bacterial resistance to macrolide antibiotics

Macrolides have been used in the treatment of infectious diseases since the late 1950s. Since that time, a finding of antagonistic action between erythromycin and spiramycin in clinical isolates1 led to evidence of the biochemical mechanism and to the current understanding of inducible or constitutive resistance to macrolides mediated by erm genes containing, respectively, the functional regulation mechanism or constitutively mutated regulatory region. These resistant mechanisms to macrolides are recognized in clinically isolated bacteria. (1) A methylase encoded by the erm gene can transform an adenine residue at 2058 (Escherichia coli equivalent) position of 23S rRNA into an 6N, 6N-dimethyladenine. Position 2058 is known to reside either in peptidyltransferase or in the vicinity of the enzyme region of domain V. Dimethylation renders the ribosome resistant to macrolides (MLS). Moreover, another finding adduced as evidence is that a mutation in the domain plays an important role in MLS resistance: one of several mutations (transition and transversion) such as A2058G, A2058C or U, and A2059G, is usually associated with MLS resistance in a few genera of bacteria. (2) M (macrolide antibiotics)- and MS (macrolide and streptogramin type B antibiotics)- or PMS (partial macrolide and streptogramin type B antibiotics)-phenotype resistant bacteria cause decreased accumulation of macrolides, occasionally including streptogramin type B antibiotics. The decreased accumulation, probably via enhanced efflux, is usually inferred from two findings: (i) the extent of the accumulated drug in a resistant cell increases as much as that in a susceptible cell in the presence of an uncoupling agent such as carbonylcyanide-m-chlorophenylhydrazone (CCCP), 2,4-dinitrophenol (DNP), and arsenate; (ii) transporter proteins, in M-type resistants, have mutual similarity to the 12-transmembrane domain present in efflux protein driven by proton-motive force, and in MS- or PMS-type resistants, transporter proteins have mutual homology to one or two ATP-binding segments in efflux protein driven by ATP. (3) Two major macrolide mechanisms based on antibiotic inactivation are dealt with here: degradation due to hydrolysis of the macrolide lactone ring by an esterase encoded by the ere gene; and modification due to macrolide phosphorylation and lincosamide nucleotidylation mediated by the mph and lin genes, respectively. But enzymatic mechanisms that hydrolyze or modify macrolide and lincosamide antibiotics appear to be relatively rare in clinically isolated bacteria at present. (4) Important developments in macrolide antibiotics are briefly featured. On the basis of information obtained from extensive references and studies of resistance mechanisms to macrolide antibiotics, the mode of action of the drugs, as effectors, and a hypothetical explanation of the regulation of the mechanism with regard to induction of macrolide resistance are discussed.

Molecular investigation of the postantibiotic effects of clarithromycin and erythromycin on Staphylococcus aureus cells

The kinetics of recovery after inhibition of growth by erythromycin and clarithromycin were examined in Staphylococcus aureus cells. After inhibition for one mass doubling by 0.5 microg of the antibiotics/ml, a postantibiotic effect (PAE) of 3 and 4 h duration was observed for the two drugs before growth resumed. Cell viability was reduced by 25% with erythromycin and 45% with clarithromycin compared with control cells. Erythromycin and clarithromycin treatment reduced the number of 50S ribosomal subunits to 24 and 13% of the number found in untreated cells. 30S subunit formation was not affected. Ninety minutes was required for resynthesis to give the control level of 50S particles. Protein synthesis rates were diminished for up to 4 h after the removal of the macrolides. This continuing inhibition of translation was the result of prolonged binding of the antibiotics to the 50S subunit as measured by 14C-erythromycin binding to ribosomes in treated cells. The limiting factors in recovery from macrolide inhibition in these cells, reflected as a PAE, are the time required for the synthesis of new 50S subunits and the slow loss of the antibiotics from ribosomes in inhibited cells.

A ketolide resistance mutation in domain II of 23S rRNA reveals the proximity of hairpin 35 to the peptidyl transferase centre

Ketolides represent a new generation of macrolide antibiotics. In order to identify the ketolide-binding site on the ribosome, a library of Escherichia coli clones, transformed with a plasmid carrying randomly mutagenized rRNA operon, was screened for mutants exhibiting resistance to the ketolide HMR3647. Sequencing of the plasmid isolated from one of the resistant clones and fragment exchange demonstrated that a single U754A mutation in hairpin 35 of domain II of the E. coli 23S rRNA was sufficient to confer resistance to low concentrations of the ketolide. The same mutation also conferred erythromycin resistance. Both the ketolide and erythromycin protected A2058 and A2059 in domain V of 23S rRNA from modification with dimethyl sulphate, whereas, in domain II, the ketolide protected, while erythromycin enhanced, modification of A752 in the loop of the hairpin 35. Thus, mutational and footprinting results strongly suggest that the hairpin 35 constitutes part of the macrolide binding site on the ribosome. Strong interaction of ketolides with the hairpin 35 in 23S rRNA may account for the high activity of ketolides against erythromycin-resistant strains containing rRNA methylated at A2058. The existence of macrolide resistance mutations in the central loop of domain V and in hairpin 35 in domain II together with antibiotic footprinting data suggest that these rRNA segments may be in close proximity in the ribosome and that hairpin 35 may be a constituent part of the ribosomal peptidyl transferase centre.

Transferred nuclear Overhauser effect study of macrolide-ribosome interactions: correlation between antibiotic activities and bound conformations

The study of macrolide-ribosome interactions has been investigated using two-dimensional transferred nuclear Overhauser effect spectroscopy (TRNOESY). A new medically important macrolide antibiotic, roxithromycin, with the replacement of the 9-keto group in erythromycin by a 9-oxime chain, was studied in the complex state with the bacterial ribosome. Analysis of transferred nuclear Overhauser effect (TRNOE) experiment resulted in a set of constraints for all protons pairs. These constraints were used in structure determination procedures based on molecular modelling to obtain a bound structure compatible with the experimental NMR data. The results compared with the conformational analysis of the substrate in solution indicate that only one specific conformation is preferred in the bound state while in the free state the sugar ring moities were relatively disordered. The bioactive macrolide antibiotics studied roxithromycin and erythromycin which displayed a strong NMR response, are metabolized in RU39001 and erythralosamine respectively which do not retain antimicrobial activity. The inactive major metabolites were used to define if TRNOEs observation may be characteristic of a biological activity. These control experiments gave essentially blank TRNOESY spectra. This study shows that Mg2+ does not play a direct role for the low affinity binding site studied by TRNOE what is in agreement with an hypothesis of two distinct binding levels, with a low affinity binding level necessary for the tight binding one.

Macrolide antibiotics inhibit 50S ribosomal subunit assembly in Bacillus subtilis and Staphylococcus aureus

Macrolide antibiotics are clinically important antibiotics which are effective inhibitors of protein biosynthesis in bacterial cells. We have recently shown that some of these compounds also inhibit 50S ribosomal subunit formation in Escherichia coli. Now we show that certain macrolides have the same effect in two gram-positive organisms, Bacillus subtilis and Staphylococcus aureus. Assembly in B. subtilis was prevented by erythromycin, clarithromycin, and azithromycin but not by oleandomycin. 50S subunit formation in S. aureus was prevented by each of seven structurally related 14-membered macrolides but not by lincomycin or two streptogramin antibiotics. Erythromycin treatment did not stimulate the breakdown of performed 50S subunits in either organism. The formation of the 30S ribosomal subunit was also unaffected by these compounds. Assembly was also inhibited in a B. subtilis strain carrying a plasmid with the ermC gene that confers macrolide resistance by rRNA methylation. These results suggest that ribosomes contain an additional site for the inhibitory functions of macrolide antibiotics.

Ribosomal protein gene sequence changes in erythromycin-resistant mutants of Escherichia coli

The genes for ribosomal proteins L4 and L22 from two erythromycin-resistant mutants of Escherichia coli have been isolated and sequenced. In the L4 mutant, an A-to-G transition in codon 63 predicted a Lys-to-Glu change in the protein. In the L22 strain, a 9-bp deletion removed codons 82 to 84, eliminating the sequence Met-Lys-Arg from the protein. Consistent with these DNA changes, in comparison with wild-type proteins, both mutant proteins had reduced first-dimension mobilities in two-dimensional polyacrylamide gels. Complementation of each mutation by a wild-type gene on a plasmid vector resulted in increased erythromycin sensitivity in the partial-diploid strains. The fraction of ribosomes containing the mutant form of the protein was increased by growth in the presence of erythromycin. Erythromycin binding was increased by the fraction of wild-type protein present in the ribosome population. The strain with the L4 mutation was found to be cold sensitive for growth at 20 degrees C, and 50S-subunit assembly was impaired at this temperature. The mutated sequences are highly conserved in the corresponding proteins from a number of species. The results indicate the participation of these proteins in the interaction of erythromycin with the ribosome.

Tight binding of clarithromycin, its 14-(R)-hydroxy metabolite, and erythromycin to Helicobacter pylori ribosomes

Clarithromycin is a recently approved macrolide with improved pharmacokinetics, antibacterial activity, and efficacy in treating bacterial infections including those caused by Helicobacter pylori, an agent implicated in various forms of gastric disease. We successfully isolated ribosomes from H. pylori and present the results of a study of their interaction with macrolides. Kinetic data were obtained by using 14C-labeled macrolides to probe the ribosomal binding site. Clarithromycin, its parent compound erythromycin, and its 14-(R)-hydroxy metabolite all bound tightly to H. pylori ribosomes. Kd values were in the range of 2 x 10(-10) M, which is the tightest binding interaction observed to date for a macrolide-ribosome complex. This tight binding was due to very slow dissociation rate constants of 7.07 x 10(-4), 6.83 x 10(-4), and 16.6 x 10(-4) min-1 for clarithromycin, erythromycin, and 14-hydroxyclarithromycin, respectively, giving half-times of dissociation ranging from 7 to 16 h, the slowest yet measured for a macrolide-ribosome complex. These dissociation rate constants are 2 orders of magnitude slower than the dissociation rate constants of macrolides from other gram-negative ribosomes. [14C]clarithromycin was bound stoichiometrically to 50S ribosomal subunits following incubation with 70S ribosomes and subsequent separation of the 30S and 50S subunits by sucrose density gradient centrifugation. These data predict that the lower MIC of clarithromycin compared with that of erythromycin for H. pylori is likely due to a faster rate of intracellular accumulation, possibly because of increased hydrophobicity.

Role of protonated and neutral forms of macrolides in binding to ribosomes from gram-positive and gram-negative bacteria

Erythromycin binds to a single site on the bacterial 50S ribosomal subunit and perturbs protein synthesis. However, erythromycin contains desosamine and thus exists in both protonated (greater than 96%) and neutral (less than 4%) forms at physiological pH because of the pKa of the dimethylamino group. We therefore examined the relative roles of both forms in binding to ribosomes isolated from two species each of gram-positive and gram-negative bacteria. We developed a system to directly measure the forward (association) rate constant of formation of the macrolide-ribosome complex, and we have measured both the forward and reverse (dissociation) rate constants as a function of pH. Forward rate constants and binding affinity did not correlate with pH when the interaction of erythromycin with ribosomes from both gram-positive and gram-negative bacteria was examined, demonstrating that the protonated form of this macrolide binds to ribosomes. Conversely, the neutral form of macrolide cannot be the sole binding species and appears to bind with the same kinetics as the protonated form. Forward rate constants were 3- to 4-fold greater at physiological pH, and binding affinity calculated from rate constants was 5- to 10-fold greater than previously estimated. Similar results were obtained with azithromycin, a novel 15-membered macrolide that contains an additional tertiary amine in the macrolide ring. Ribosome- and macrolide-specific kinetic parameters were demonstrated at neutral pH and may be related to the potency of the two macrolides against gram-positive and gram-negative bacteria.

Reductive methods for isotopic labeling of antibiotics

Methods for the reductive methylation of the amino groups of eight different antibiotics using 3HCOH or H14COH are presented. The reductive labeling of an additional seven antibiotics by NaB3H4 is also described. The specific activity of the methyl-labeled drugs was determined by a phosphocellulose paper binding assay. Two quantitative assays for these compounds based on the reactivity of the antibiotic amino groups with fluorescamine and of the aldehyde and ketone groups with 2,4-dinitrophenylhydrazine are also presented. Data on the cellular uptake and ribosome binding of these labeled compounds are also presented.

Binding of novel macrolide structures to macrolides-lincosamides-streptogramin B-resistant ribosomes inhibits protein synthesis and bacterial growth

Dimethylation of adenine 2058 in 23S rRNA renders bacteria resistant to macrolides, lincosamides, and streptogramin B (MLS resistance), because the antibiotic binding site on the altered 50S ribosomal subunit is no longer accessible. We now report that certain 6-O-methyl-11,12-cyclic carbamate derivatives of erythromycin are able to bind to dimethylated MLS-resistant 50S ribosomal subunits, thus inhibiting protein synthesis and cell growth. One of these novel structures, an 11-deoxy-11-(carboxyamino)-6-O-methylerythromycin A 11,12-(cyclic ester) derivative, structure 1a, was studied in detail. It inhibited in vitro protein synthesis in extracts prepared from both susceptible and MLS-resistant Bacillus subtilis with 50% inhibitory concentrations of 0.4 and 20 microM, respectively. The derivative bound specifically to a single site on the 50S subunit of MLS-resistant ribosomes prepared from B. subtilis and Staphylococcus aureus, and no binding to 30S subunits was observed. The association rate constant of derivative 1a with sensitive and resistant ribosomes was 100- and 500-fold slower, respectively, than that of the parent compound, erythromycin, with sensitive ribosomes. The dissociation rate constant of 1a from sensitive and resistant ribosomes was 50- to 100-fold slower than the rate of erythromycin dissociation from sensitive ribosomes. Furthermore, 1a binding to sensitive 50S subunits led to induction of ermC and ermD, while binding to resistant 50S subunits did not, showing that perturbation of sensitive and resistant 50S subunit function by 1a differs. These data demonstrated that 1a is unique in its interaction with MLS-resistant ribosomes and that this interaction causes a novel allosteric perturbation of ribosome function.

Interferon-gamma and antibiotics fail to act synergistically to kill Legionella pneumophila in human monocytes

Legionella pneumophila, the agent of Legionnaires' disease, is a gram-negative facultative intracellular bacterial pathogen that multiplies in human blood monocytes and alveolar macrophages. Interferon-gamma (IFN-gamma)-activated human monocytes inhibit the intracellular multiplication of L. pneumophila but fail to kill the organism. Similarly, erythromycin and rifampin, the drugs of choice in the treatment of Legionnaires' disease, inhibit the growth of L. pneumophila within monocytes without exerting a cidal effect. In this study, we examined the combined effects of IFN-gamma and antibiotics (erythromycin, rifampin, and clindamycin) to determine whether these independently acting agents would synergistically mediate the killing of intracellular L. pneumophila. Each agent alone or in combination was effective in inhibiting the intracellular multiplication of L. pneumophila. However, IFN-gamma and antibiotics together were unable to kill intracellular L. pneumophila, regardless of the sequence in which they were administered to monocytes. Like erythromycin and rifampin, clindamycin, which is highly concentrated in human alveolar macrophages, was capable of inhibiting the intracellular multiplication of L. pneumophila but failed to kill the bacteria in nonactivated or IFN-gamma-activated monocytes. These results demonstrate that intracellular L. pneumophila are highly resistant to the bactericidal effects of both activated monocytes and antibiotics, alone or in combination.

Elongating ribosomes in vivo are refractory to erythromycin

We have studied the kinetics of erythromycin inhibition of translation in growing bacteria. In order to simplify the interpretation of our data, we have used a mutant (envA), known to have an increased permeability to several antibiotics, including erythromycin. The data clearly show that an initial stage of translation is sensitive to erythromycin, but that the elongating ribosome is insensitive to the antibiotic.

Novel mechanism for plasmid-mediated erythromycin resistance by pNE24 from Staphylococcus epidermidis

We describe an unusual type of erythromycin resistance (Emr) mediated by a plasmid designated pNE24 from Staphylococcus epidermidis. This 26.5-kilobase plasmid encodes resistance strictly to 14-membered macrolide antibiotics, erythromycin, and oleandomycin. Resistance to other macrolide-lincosamide-streptogramin B (MLS) antibiotics was not observed even after a prior induction stimulus with various MLS antibiotics. Plasmid pNE24 was found to express resistance constitutively and manifested a low to intermediate MIC (62.5 micrograms/ml) for erythromycin. The resistance gene, designated erpA, appears to mediate resistance by altering the permeability of the host cell for erythromycin, because the measured uptake of 14C-labeled erythromycin by strain 958-2 (containing pNE24) was lower than for the erythromycin-susceptible, isogenic strain 958-1. No inactivation of erythromycin in overnight broth culture supernatants could be detected. In addition, no significant loss in binding affinity between [14C]erythromycin and ribosome could be detected for ribosomes isolated from strain 958-2 relative to 958-1, indicating that pNE24 probably does not produce a modification of the bacterial ribosome. No other selectable marker was found associated with pNE24; however, a 60,000-dalton protein was present only in the membrane fractions of cells (958-2) containing pNE24 and may play a role in mediating resistance to erythromycin.

In vitro [3H]-erythromycin binding to Staphylococcus aureus

Characteristics of erythromycin binding to Staphylococcus aureus were determined by using kinetics and equilibrium binding experiments. Both methods yielded identical values of the dissociation constant, i.e. 0.1 muM. This value was in accord with that found with a bacterial extract of ribosomes which are the organelles where erythromycin exerts its action. This good agreement shows that the dissociation constant of erythromycin determined with intact bacteria is a good reflect of specific bacterial receptors of macrolides, i.e. ribosomes. In addition, mechanism of uptake of the antibiotic by Staphylococcus aureus was investigated. Passive diffusion process was shown to be mainly responsible for this phenomenon.

Translational attenuation: the regulation of bacterial resistance to the macrolide-lincosamide-streptogramin B antibiotics

The regulation of ermC is described in detail as an example of regulation on the level of translation. ermC specifies a ribosomal RNA methylase which confers resistance to the macrolide-lincosamide-streptogramin B group of antibiotics. Synthesis of the ermC gene product is induced by erythromycin, a macrolide antibiotic. Stimulation of methylase synthesis is mediated by binding of erythromycin to an unmethylated ribosome. The translational attenuation model, supported by sequencing data and by mutational analysis, proposes that binding of erythromycin causes stalling of a ribosome during translation of a "leader peptide", resulting in isomerization of the ermC transcript from an inactive to an active conformer. The ermC system is analogous to the transcriptional attenuation systems described for certain biosynthetic operons. ermC is unique in that interaction with a small molecule inducer mediates regulation on the translational level. However, it is but one example of nontranscriptional -level control of protein synthesis. Other systems are discussed in which control is also exerted through alterations of RNA conformation and an attempt is made to understand ermC in this more general context. Finally, other positive examples of translational attenuation are presented.

Ribosomal binding region for the antibiotic tiamulin: stoichiometry, subunit location, and affinity for various analogs

Equilibrium dialysis experiments with a highly purified preparation of labeled tiamulin, a semisynthetic derivative of the antibiotic pleuromutilin, and Escherichia coli ribosomes allowed the determination of two binding sites for the drug. The binding reaction showed a cooperative effect. Of the two subunits, the 50S particle was able to bind the antibiotic in a 1:1 stoichiometry. Hence, the 50S subunit contributed predominantly to the binding energy which held the antibiotic to the ribosomes. The 30S subunit, showing no strong affinity for the drug, may be needed for the generation of the second binding site in the 70S particle. If depleted of ammonium ions, 70S ribosomes lost their binding capacity for the antibiotic. The attachment sites for tiamulin could be restored by heating the ribosomes to 40 degrees C in the presence of either ammonium ions or the antibiotic. Other pleuromutilin derivatives displaced labeled tiamulin from its ribosomal binding sites. By quantifying this competition, the relative affinity of various pleuromutilin derivatives for E. coli ribosomes was determined. The binding correlated with the minimal inhibitory concentrations of these compounds against E. coli. When compared with the minimal inhibitory concentrations of these compounds against E. coli. When compared with the minimal inhibitory concentrations against E. coli. When compared with the minimal inhibitory concentrations against Staphylococcus aureus, the correlation was less strict, but the same trend prevailed. These results suggest that the antibacterial activities of various pleuromutilin derivatives on a given test organism are mainly determined by the strength of binding to the ribosomes within the bacterial cell.

Competition between trichodermin and several other sesquiterpene antibiotics for binding to their receptor site(s) on eukaryotic ribosomes

1. Of the five sesquiterpene antibiotics tested and found to inhibit protein synthesis in yeast spheroplasts, trichothecin, trichodermol or trichodermin stabilized polyribosomes whereas, in contrast, verrucarin A or T-2 toxin induced 'run off' of polyribosomes with a corresponding increase in 80S monoribosomes. The effect of fusarenon X on the system could not be determined as the drug failed to enter the cells. 2. [acetyl-14C]Trichodermin bound to yeast polyribosomes with a dissociation constant of 2.10 muM and to yeast 'run off' ribosomes with a dissociation constant of 0.72 muM. 3. Trichothecin, trichodermol, fusarenon X, T-2 toxin and verrucarin A competed with [acetyl-14C]trichodermin for binding to its receptor site on 'run off' ribosomes. The observed competition was quantitatively similar for all drugs tested. In contrast, the five drugs competed to different extents with trichodermin for binding to its receptor site on polyribosomes. Thus trichothecin competed with relative efficiency, whereas verrucarin A competed poorly, and the other drugs occupied intermediate positions between these two extremes. 4. Studies were also carried out with yeast 'run off' ribosomes prepared from both a wild-type strain and a strain resistant to trichodermin. Competition experiments between verrucarin A and [3H]anisomycin indicated that verrucarin A bound to 'run off' ribosomes from the mutant strain less efficiently than to those from the wild-type.

Binding of erythromycin to the 50S ribosomal subunit is affected by alterations in the 30S ribosomal subunit

Expression of resistance to erythromycin in Escherichia coli, caused by an altered L4 protein in the 50S ribosomal subunit, can be masked when two additional ribosomal mutations affecting the 30S proteins S5 and S12 are introduced into the strain (Saltzman, Brown, and Apriion, 1974). Ribosomes from such strains bind erythromycin to the same extent as ribosomes from erythromycin sensitive parental strains (Apirion and Saltzman, 1974). Among mutants isolated for the reappearance of erythromycin resistance, kasugamycin resistant mutants were found. One such mutant was analysed and found to be due to undermethylation of the rRNA. The ribosomes of this strain do not bind erythromycin, thus there is a complete correlation between phenotype of cells with respect to erythromycin resistance and binding of erythromycin to ribosomes. Furthermore, by separating the ribosomal subunits we showed that 50S ribosomes bind or do not bind erythromycin according to their L4 protein; 50S with normal L4 bind and 50S with altered L4 do not bind erythromycin. However, the 30S ribosomes with altered S5 and S12 can restore binding in resistant 50S ribosomes while the 30S ribosomes in which the rRNA also became undermethylated did not allow erythromycin binding to occur. Thus, evidence for an intimate functional relationship between 30S and 50S ribosomal elements in the function of the ribosome could be demonstrated. These functional interrelationships concerns four ribosomal components, two proteins from the 30S ribosomal subunit, S5, and S12, one protein from the 50S subunit L4, and 16S rRNA.

The mode of action of pleuromutilin derivatives. Location and properties of the pleuromutilin binding site on Escherichia coli ribosomes

Using equilibrium dialysis techniques it could be demonstrated that (a) the pleuromutilin derivative 14-deoxy-14[(2-diethylaminoethyl)-mercaptoacetoxy] dihydromutilin HCl binds to one site per ribosome specifically, (b) the binding constant is 1.3 times 10(7) M(-1) and (c) chloramphenicol and puromycin compete with binding of the pleuromutilin derivative. Similarly the nucleotides CpA and CpCpA also displace the unsaturated derivative of the above-mentioned pleuromutilin compound. These findings suggest that the ribosomal binding site for pleuromutilin overlaps with that for chloramphenicol and analogs of the 3'-terminus of a tRNA, like puromycin or the nucleotides CpA and CpCpA.

Reversal by syngeneic spleen cells of inhibitory effects of drugs and irradiation on Friend virus

The dissociation constants for binding to ribosomes from Escherichia coli and concentrations at which 50% inhibition of [¹⁴C]erythromycin binding to ribosomes occurred were determined for 45 erythromycin analogues. These values were correlated with their antibacterial activities against Bacillus subtilis. Compounds which bound to ribosomes best showed the greatest activities; those which were poorly bound to ribosomes showed little or no antibacterial activity. The ribosomal binding assays therefore reflected the general antibacterial potential of the erythromycin analogues.

Properties of ribosomes from Streptomyces erythreus and Streptomyces griseus

Ribosomes from an erythromycin-producing strain, Streptomyces erythreus, lacked affinity for erythromycin and were also resistant to other macrolide antibiotics (leucomycin, spiramycin, and tylosin) and to lincomycin, whereas Streptomyces griseus B(3) ribosomes were susceptible to all of these antibiotics.

Binding of [14C]erythromycin to Escherichia coli ribosomes

Erythromycin binding to Escherichia coli ribosomes required K(+) and Mg(2+). Under optimal conditions, the dissociation constant for erythromycin binding to E. coli ribosomes was found to be 1.0 x 10(-8) M and 1.4 x 10(-8) M at 24 C and 5 C, respectively. One molecule of [(14)C]erythromycin was bound to each 70S ribosome at equilibrium. Binding of erythromycin to ribosomes was rapid and reversible. The specific rate constants for the forward and reverse reactions were 1.7 x 10(7) liters per mol per min and 0.15 per min, respectively.

Effect of erythromycin analogues on binding of [14C]erythromycin to Escherichia coli ribosomes

The relative ability of 44 erythromycin analogues to bind to ribosomes was determined by their effect on [(14)C]erythromycin binding to Escherichia coli ribosomes. The association and dissociation constants of each of these erythromycin derivatives were determined as well as their interaction coefficient for their binding to ribosomes. Substitutions were made on various portions of the erythromycin molecule with retention of substantial activity as measured by inhibition of [(14)C]erythromycin binding to ribosomes. Since the effect of erythromycin analogues on [(14)C]erythromycin binding to ribosomes provides a relatively sensitive assay for these compounds, erythromycin analogues with relatively little affinity for ribosomes could be detected. Compounds with association constants of 10(4) M(-1) were detectable; the association constant for erythromycin binding to ribosomes was approximately 10(8) M(-1). Thus, compounds with 0.0001 the association constant of erythromycin were detectable. This assay could be used alone or in conjunction with microbiological assays for primary screening of active analogues or other compounds which interfere with [(14)C]erythromycin binding to ribosomes. It permits an estimate of the general activity of compounds rapidly and directly. Variables such as metabolic modifications of the compounds and permeability are excluded. The present assay reflects the ability of the compounds to interact directly with their target organelle and may serve as a useful adjunct in developing new compounds.

Antibiotics as probes of ribosome structure: binding of chloramphenicol and erythromycin to polyribosomes; effect of other antibiotics

Antibiotics were used as probes of ribosome topology and function. Studies of [(14)C]chloramphenicol and [(14)C]erythromycin binding to ribosomes and polyribosomes revealed the following features. The requirement of high K(+) concentration (150 mM) for [(14)C]chloramphenicol binding to NH(4)Cl-washed ribosomes resulted from the washing procedure. Neither native 70S ribosomes nor polyribosomes require K(+) greater than 30 mM for [(14)C]chloramphenicol binding. Whereas [(14)C]chloramphenicol binds to both ribosomes and polyribosomes, [(14)C]erythromycin binds essentially only to ribosomes. After removal of peptidyl-transfer ribonucleic acid (tRNA) from polyribosomes, [(14)C]erythromycin could then be bound. The effects of a number of antibiotics on [(14)C]chloramphenicol binding to ribosomes and polyribosomes was assessed. It was found that most of the macrolides (erythromycin, carbomycin, spiramycin III, niddamycin, oleandomycin, and tylosin) and streptogramins A and B (vernamycin A, PA114A, vernamycin Balpha, and PA114B) inhibited chloramphenicol binding to NH(4)Cl-washed and native 70S ribosomes, but not to polyribosomes. After removal of peptidyl-tRNA from polyribosomes, [(14)C]chloramphenicol binding was then inhibited. In contrast, sparsomycin and althiomycin inhibited chloram-phenicol binding to polyribosomes, but not to ribosomes. After removal of peptidyl-tRNA from polyribosomes, sparsomycin and althiomycin were then ineffective. The presence of peptidyl-tRNA on polyribosomes apparently is required for binding of sparsomycin and althiomycin, but prevents binding of most macrolides and streptogramins. The lincosaminides (lincomycin and celesticetin) and methymycin (a small macrolide) inhibited [(14)C]chloramphenicol binding to NH(4)Cl-washed and native 70S ribosomes best, but also inhibited the binding to polyribosomes. The amino nucleosides and other antibiotics tested do not seem to interact strongly with the major chloramphenicol-binding site. These results provide knowledge of the interrelationships between antibiotic and substrate ribosome binding sites which should eventually contribute to a map of ribosomal topology.

Mutation to erythromycin dependence in Escherichia coli K-12

A nitrosoguanidine-induced mutant of Escherichia coli K-12 strain JC12 was absolutely dependent on erythromycin or related macrolide antibiotics for growth. The only other drugs which permitted growth (lincomycin and chloramphenicol) are, like the macrolides, inhibitors of the 50S ribosome. The order of relative effectiveness of these drugs was macrolides > lincomycin > chloramphenicol. Rates of growth with all drugs were concentration dependent. Erythromycin starvation was followed by normal rates of increase in cell mass and macromolecular synthesis for approximately one mass-doubling time, after which macromolecular synthesis abruptly ceased and cell lysis and death occurred. The dependent mutant gave rise spontaneously to revertants to independence with very high frequency (10(-4)). The gene (mac) for macrolide dependence is located near minute 25 on the E. coli chromosome; it does not result in increased resistance to these drugs. A separate gene for erythromycin resistance (eryA) is located in the cluster of ribosomal structural genes near spc, close to minute 63. Dependence on macrolides was most clearly evident in strains carrying mutations at both eryA and mac.