Virginiamycin M, a specific inhibitor of the acceptor site of ribosomes

Total synthesis of (-)-virginiamycin M2: application of crotylsilanes accessed by enantioselective Rh(II) or Cu(I) promoted carbenoid Si-H insertion

A stereoselective synthesis of the antibiotic (-)-virginiamycin M(2) is detailed. A convergent strategy was utilized that proceeded in 10 steps (longest linear sequence) from enantioenriched silane (S)-15. This reagent, which was prepared via a Rh(II)- or Cu(I)-catalyzed carbenoid Si-H insertion, was used to introduce the desired olefin geometry and stereocenters of the C1-C5 propionate subunit. A modified Negishi cross-coupling or an efficient alkoxide-directed titanium-mediated alkyne-alkyne reductive coupling strategy was utilized to assemble the trisubstituted (E,E)-diene. An underutilized late-stage SmI(2)-mediated macrocyclization was employed to construct the 23-membered macrocycle scaffold of the natural product.

Macromolecule metabolism and photosynthetic functions in blue-green algae treated with virginiamycin, an inhibitor of protein synthesis

The M component of virginiamycin inhibited growth of Plectonema boryanum under both photoautotrophic and heterotrophic conditions. Though the S component of this antibiotic had no apparent activity per se, it enhanced the inhibitory action of its partner. Cells incubated with suitable concentrations of either M or M + S stopped growing and lysed. Loss of the colony-forming capacity occurred quickly in the presence of M + S and slowly in the presence of M alone. Virginiamycin M inhibited protein synthesis in autotrophically and heterotrophically growing Plectonema. This effect was very rapid and could be reversed by removing the antibiotic. The S component did not block the incorporation of amino acids into proteins, but prevented the reversibility of the inhibitory effect of M. Virginiamycin M or S did not affect the photosynthetic oxygen development (Hill's reaction) in Plectonema. Moreover, carbon dioxide photoassimilation and formation of chlorophyll were inhibited only after an appreciable lag. Deoxyribonucleic acid synthesis was blocked virtually without delay by virginiamycin M. Since virginiamycin inhibited protein synthesis in a similar fashion in the unicellular Anacystis nidulans, as well as in the filamentous P. boryanum, the mechanism of action of this antibiotic is probably the same in all blue-green algae.

Novel streptogramin antibiotics

Streptogramins represent a unique class of antibiotics remarkable for their antibacterial activity and their unique mechanism of action. These antibiotics are produced naturally as secondary metabolites by a number of Streptomyces species and have been classified into two main groups. They consist of at least two structurally unrelated compounds, group A or M (macrolactones) and group B or S (cyclic hexadepsipeptides). Both groups bind bacterial ribosomes and inhibit protein synthesis at the elongation step and they act synergistically in vitro against many microorganisms. Streptogramins A and B act synergistically in vivo; the mixture of the two compounds is more powerful than the individual components and their combined action is irreversible. The pharmacokinetic parameters of group A and B streptogramins in blood are similar. The major gap, limiting the therapeutic use of the natural compounds, was represented by the lack dissolution in water. The synthesis of water-soluble derivatives of pristinamycin I(A) and II(B) has allowed the development of injectable, first represented by quinupristin/dalfopristin (Synercid) and oral formulations, represented by RPR-106972, streptogramins with fixed compositions. Streptogramins have demonstrated activity against Gram-positive microorganisms in vitro and in vivo, including those with multi-drug resistance. Moreover, the absence of cross-resistance to macrolides in many of these microorganisms and the rarity of cross-resistance between the two groups of antibiotics associated with the rapid bacterial killing are the principal features of the streptogramins, offering the possibility for treating the rising number of infections that are caused by multi-resistant Gram-positive bacteria.

Action of erythromycin and virginiamycin S on polypeptide synthesis in cell-free systems

Erythromycin (a 14-membered macrolide) and virginiamycin S (a type B synergimycin) block protein biosynthesis in bacteria, but are virtually inactive on poly(U)-directed poly(Phe) synthesis. We have recently shown, however, that these antibiotics inhibit the in vitro polypeptide synthesis directed by synthetic copolymers: this effect is analyzed further in the present work. We were unable to find any consistent alteration produced by these antibiotics on coupled and uncoupled EF-G- and EF-Tu-dependent GTPases, on the EF-Tu-directed binding of aminoacyl-tRNA to ribosomes, and on the EF-G- and GTP-mediated translocation of peptidyl-tRNA bound to poly(U,C).ribosome complexes. With these complexes, the peptidyl transfer reaction, as measured by peptidylpuromycin synthesis, was 10-30% inhibited by virginiamycin S and erythromycin. A direct relationship between the virginiamycin S- and erythromycin-promoted inhibition of poly(A,C)-directed polypeptide synthesis, on the one hand, and the EF-G concentration and the rate of the polymerization reaction, on the other hand, was observed, in agreement with a postulated reversible inhibitor action of these antibiotics. The increased inhibitory activity, which was observed during the first 4-6 rounds of elongation, in the presence of virginiamycin S or erythromycin, was suggestive of a specific action of these antibiotics on the correct positioning of peptidyl-tRNA at the P site. The marked stimulation of premature release of peptidyl-tRNA from poly(A,C).ribosome complexes can be referred to an altered interaction of the C-terminal aminoacyl residue of the growing peptidyl chain with the ribosome. We conclude that the action of virginiamycin S and erythromycin entails a template-dependent alteration of the interaction of peptidyl-tRNA with the donor site of peptidyltransferase, which may lead to a transient functional block of the ribosome and in some instances to a premature release of peptidyl-tRNA and termination of the elongation process.

Action of virginiamycin M on the stability of different ribosomal complexes to ultracentrifugation

It was previously shown that virginiamycin M produces in vivo an accumulation of pressure-sensitive (60 S) ribosomes, and in vitro an inactivation of the donor and acceptor sites of peptidyl transferase. The latter action, however, is expected to cause the accumulation in vivo of ribosome complexes carrying acylated tRNA species: such complexes are usually endowed with pressure resistance. However, present data indicate that poly(U).ribosome complexes carrying Phe-tRNA, Ac-Phe-tRNA or Ac-Phe-Phe-tRNA at either the A or the P site become pressure-sensitive after exposure to virginiamycin M in vitro. It is known also that uncoupled EF-G GTPase is stimulated by P-site-bound unacylated tRNA, not by the acylated species. Our data show, however, a stimulation of EF-G GTPase, when ribosomal complexes carrying Ac-Phe-tRNA or Ac-Phe-Phe-tRNA at the P site are incubated with virginiamycin M. The interpretation proposed to account for all these findings is that complexes carrying A- and P-site-bound aminoacyl-tRNA derivatives, which undergo a stable interaction with the peptidyl transferase, are endowed with ultracentrifugal stability, whereas complexes with unacylated tRNA (which does not interact with the enzyme) are pressure-sensitive. By inactivating the donor and acceptor sites of peptidyltransferase, virginiamycin M causes aminoacyl-tRNA.ribosome complexes to mimic tRNA.ribosome complexes in their pressure-lability and competence in EF-G GTPase stimulation. This interpretation is supported by the finding that the ribosome-promoted protection of aminoacyl-tRNA against spontaneous hydrolysis is suppressed by virginiamycin M.

Inhibition of polypeptide synthesis in cell-free systems by virginiamycin S and erythromycin. Evidence for a common mode of action of type B synergimycins and 14-membered macrolides

Macrolides, lincosamides and type B synergimycins are powerful inhibitors of protein synthesis in vivo, but many of them were found to be inactive in vitro. In the present work, we confirm that virginiamycin S (a type B synergimycin) and erythromycin (a 14-membered macrolide) have no effect on poly(U)-directed poly(Phe) synthesis. However, the amino-acid polymerization reactions directed by poly(U,G), poly(U,C), poly(A,G) and poly(A,C) were increasingly inhibited (20-50%) by both antibiotics. The action of these inhibitors proved to be template-dependent and favored by the incorporation of proline and of basic amino acids into peptides. Under these conditions, virginiamycin S and erythromycin markedly stimulated a release of peptidyl-tRNA from the ribosomes. In the poly(A,C) model system, these antibiotics produced a 50% inhibition of amino-acid incorporation into total peptides, a 70% release of ribosome-bound peptidyl-tRNA, and a 95% repression of the synthesis of long peptide chains. The production of equivalent effects at saturating concentrations of these antibiotics in the four model systems examined is suggestive of a similarity in their mode of action. Our results indicate that 14-membered macrolides and type B synergimycins can act on ribosomes during the whole elongation process. The functional block produced by both antibiotics is usually reversible, but may result in a premature release of peptidyl-tRNA when the stability of ribosomal complexes is lowered by the incorporation of basic amino acids.

Ribosome protection by tRNA derivatives against inactivation by virginiamycin M: evidence for two types of interaction of tRNA with the donor site of peptidyl transferase

Virginiamycin M (VM) was previously shown to interfere with the function of both the A and P sites of ribosomes and to inactivate tRNA-free ribosomes but not particles bearing peptidyl-tRNA. To explain these findings, the shielding ability afforded by tRNA derivatives positioned at the A and P sites against VM-produced inactivation was explored. Unacylated tRNA(Phe) was ineffective, irrespective of its position on the ribosome. Phe-tRNA and Ac-Phe-tRNA provided little protection when bound directly to the P site but were active when present at the A site. Protection by these tRNA derivatives was markedly enhanced by the formation of the first peptide bond and increased further upon elongation of peptide chains. Most of the shielding ability of Ac-Phe-tRNA and Phe-tRNA positioned at the A site was conserved when these tRNAs were translocated to the P site by the action of elongation factor G and GTP. Thus, a 5-10-fold difference in the protection afforded by these tRNAs was observed, depending on their mode of entry to the P site. This indicates the occurrence of two types of interaction of tRNA derivatives with the donor site of peptidyl transferase: one shared by acylated tRNAs directly bound to the ribosomal P site (no protection against VM) and the other characteristic of aminoacyl- or peptidyl-tRNA translocated from the A site (protection of peptidyl transferase against VM). To explain these data and previous observations with other protein synthesis inhibitors, a new model of peptidyl transferase is proposed.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory action of virginiamycin components on cell-free systems for polypeptide formation from Bacillus subtilis

Although virginiamycin components VM and VS are known to exert in vivo a synergistic inhibition of bacterial growth and viability, in cell-free systems only VM has proven active. In the present work, the in vivo and in vitro activities of VM and VS on Bacillus subtilis have been compared. Peptide formation in homogenates of bacteria previously incubated with either VM or VS was found strongly repressed; the 2 components acted synergistically. Ribosomes were fully responsible for this effect, as shown by mixed reconstitution experiments. On the other hand, cytoplasm from control bacteria disrupted in 10 mM Mg2+ buffer was refractory to in vitro inhibition by virginiamycin, whereas ribosomes prepared in 1 mM Mg2+ were sensitive to VM. VS was inactive on poly(U)-directed poly(phenylalanine) formation, and displayed some activity on the poly(A)-poly(lysine) system. In a cell-free system from Bacillus subtilis infected with phage 2C, both VM and VS were active and blocked synergistically protein synthesis in vitro. When the host cells were incubated with VS and the corresponding homogenate was then treated with VM, a complete inhibition of protein synthesis was observed. The present work, thus, describes the techniques for investigating the in vivo and in vitro action of synergimycins on the same organism, and for reproducing in vitro the synergistic interaction of type A and B components previously observed only in vivo.

The lasting ribosome alteration produced by virginiamycin M disappears upon removal of certain ribosomal proteins

Transient incubation of bacterial ribosomes with virginiamycin M produces a lasting damage of 50 S ribosomal subunits, whereby the elongation of peptide chains is still blocked after removal of the antibiotic. To elucidate the mechanism of this inactivation, ribosomal proteins were stepwise removed from 50 S subunits previously incubated with virginiamycin M, and cores were submitted to three functional tests. Total removal of proteins L7, L8, L12 and L16, and partial removal of L6, L9, L10 and L11, resulted in a loss of the virginiamycin M-induced alteration. When the split protein fractions were added back to these cores, unaltered functional particles were obtained. The reconstituted subunits, on the other hand, proved fully sensitive to virginiamycin M in vitro as they underwent, upon transient contact with the antibiotic, an alteration comparable to that of native particles. It is concluded that the virginiamycin M-induced ribosome damage is due to the production of a stable conformational change of the 50 S subunit. These data parallel those of an accompanying paper (Cocito, C., Vanlinden, F. and Branlant, C. (1983) Biochim. Biophys. Acta 739, 158-163) showing the intactness of all rRNA species from ribosomes treated in vivo and in vitro with virginiamycin M.

Competition between erythromycin and virginiamycin for in vitro binding to the large ribosomal subunit

When the S component of virginiamycin binds in vitro to the 50 S ribosomal subunit, a change of fluorescence intensity proportional to the amount of complex formed occurs. Erythromycin competes with virginiamycin S for attachment to ribosomes, and removes previously bound virginiamycin S from its target, as revealed by spectrofluorimetric analysis. The 50 S subunits which are incubated with the M component of virginiamycin (50 S*) have an increased affinity for virginiamycin S (the association constants of virginiamycin S with ribosomes are 2.5 x 10(6) M-1 in the absence of virginiamycin M, and 15 x 10(6) M-1 in its presence). Erythromycin does not compete with virginiamycin S for attachment to 50 S* subunits nor is it able to remove virginiamycin S previously bound to the 50 S* subunit. Thus, virginiamycin M produces a change in ribosomes, which results in a tighter complex virginiamycin S-50 S* subunit. Such change does not require the presence of virginiamycin M, however, as shown by the observation that ribosomes to which labeled virginiamycin M is transiently linked bind virginiamycin S in a form that cannot be removed by erythromycin.

Mapping of virginiamycin S resistance in Bacillus subtilis

Resistance to virginiamycin S (VS, a type B synergimycin) has been mapped in Bacillus subtilis. Transduction experiments with phage PBS1 indicate that the gene for virginiamycin S resistance (VSR) is cotransduced with the markers pur A16 (20%) and cys A14 (46-49%). Transformation experiments indicate that the gene for virginiamycin S resistance maps between the markers for elongation factor G and erythromycin resistance, and is therefore located within the gene cluster of ribosomal proteins.

Lasting damage to bacterial ribosomes by reversibly bound virginiamycin M

The M and S components of virginiamycin (VM and VS) inhibit protein synthesis in bacteria--reversibly when a single component is present and irreversibly when both are present. In cell-free systems, each factor binds to the large ribosomal subunit, and the affinity of ribosomes for VS is enhanced in the presence of VM. The present work shows that the action of VM (a 500-dalton modified depsipeptide) in vivo and in vitro persists upon its removal. The in vivo demonstration is based on the loss of viability of uninfected bacteria, and on the irreversible inactivation of virus-infected cells, that are caused by a sequential incubation with VM and VS (the inhibitory action of either component alone is reversible). In vitro, the binding of labeled VM to ribosomes, followed by its detachment, yields particles unable to perform poly(U)-directed polyphenylalanine synthesis. Also, the association constant for the binding of VS to these particles is equal to that of particles incubated with a mixture of VM and VS. Our findings indicate that VM action is catalytic rather than stoichiometric, and suggest the occurrence of two states of the large ribosomal subunit, a situation leading to a complex equilibrium with multiple transitional steps in the presence of virginiamycin.

Ribosomal function and its inhibition by antibiotics in prokaryotes

Most of the known antibiotics act at the level of protein biosynthesis probably due to the extraordinary complexity of the translation machinery which can be interfered with at many points. At first a survey is given of our present knowledge covering the structure and function of the prokaryotic ribosome. The most important antibiotics acting at the translational level are integrated into this network of data. The binding sites and the inhibition mechanisms of the drugs, together with the ribosomal components altered in resistant mutants are described. Finally, the points of interference with the translational machinery are indicated in an extended scheme of ribosomal functions.

A spectrofluorimetric study of the interaction between virginiamycin S and bacterial ribosomes

Virginiamycin S (VS, a type B component of the synergistin group of antibiotics) is fluorescent in solution: the fluorescence intensity is proportional to VS concentration. The intensity of VS fluorescence was found to increase upon addition of 50S ribosomal subunits, and this variation (deltaI 416 nm) to be proportional to the concentration of 50S subunits. This new technique was, then, used to measure the binding reaction of VS to ribosomes. Similar patterns of linkage were obtained for ribosomes and large subunits, whereas very little fixation to 30S particles was detected. The binding reaction was virtually instantaneous at any temperature, and, for saturating VS, was not influenced by Mg++ concentration in the range 1 to 20 mM, nor by the replacement of 100 mM K+ with NH+4. The association constant of VS TO 50S particles was found to be KA=2.5 X 10(6)M-1, and from the Scatchard plot a v value of 0.9 was calculated, which points to a stoichiometric reaction leading to 1 mole VS bound per mole of 50S particles. Upon fixation of virginiamycin M (VM, a type A component of the synergistin group of antibiotics), the delta I of the VS-ribosome complex was increased, and a KA=15 x 10(6)M-1 was recorded for the association constant of VS to 50S particles. Such sixfold increase in the affinity of ribosomes for VS may account for the synergistic effect of the 2 virginiamycin components in sensitive bacteria.

Characterisation of the binding of virginiamycin S to Escherichia coli ribosomes

Virginiamycin S is an inhibitor of protein synthesis in vivo. In this paper we show by equilibrium dialysis that it binds specifically to the 50-S subunit of Escherichia coli ribosomes, with one binding site per subunit. This binding is not altered by the presence of chloramphenicol, tetracycline or puromycin but is competed for by erythromycin. Using the splitting-reconstitution method, it could be demonstrated that protein L16 is absolutely required for the binding of virginiamycin S to the 50-S subunit.

Effect of virginiamycin on the growth cycle of Bdellovibrio

The two components of virginiamycin, virginiamycin M (VM) and virginiamycin S (VS), were used to explore the life cycle of symbiosis-dependent and -independent strains of Bdellovibrio bacteriovorus during multiplication in a two-membered system with either living or heat-inactivated Escherichia coli or in axenic cultures. Relatively high concentrations of these inhibitors separately were required to stop growth under all the conditions, but the minimum inhibitory concentration of the single components was reduced 1,000-fold by the association of VM and VS. No dissociation between mass growth and cell division was observed with VM; VS specifically halted cell division without affecting the kinetics of macromolecules formation and overall growth. This effect on cell division was only obtained when the antibiotic was added during the first half of the multiplication cycle and was reversible at any time.

Origin and metabolic properties of the RNA species formed during the replication cycle of virus 2C

When short pulses of [(3)H]uracil were administered to Bacillus subtilis infected with phage 2C, the main species of labeled RNA was a 10S component that hybridized chiefly, but not exclusively, with the heavy strand of 2C DNA. After long pulses, most of the radioactivity was found in the 23S, 16S, and 5S rRNA's, which are coded for by the cell genome. Formation of such RNA species was reduced but not suppressed upon infection, the extent of inhibition being proportional to the virus-to-cell ratio. When bacteria were incubated with virginiamycin, an inhibitor of protein synthesis, and then infected with phage 2C, formation of virus-specific RNA decreased. This antibiotic also reduced the preferential transcription of the heavy strand of 2C DNA. The methylation pattern of rRNA remained unchanged upon infection with phage 2C. Virginiamycin reduced both the methylation and stability of rRNA in uninfected cells; this effect, however, was clearly reduced during the viral cycle. It can be concluded that in 2C-infected B. subtilis, cellular and viral RNA species are simultaneously synthesized and a preferential transcription of viral message depends not only on the number of available copies of viral template, but also on their translation. Moreover, virus-dictated proteins are responsible for the inhibition of cellular RNA formation as well as for the asymmetrical transcription of phage genome. Finally, virginiamycin and phage 2C have antagonistic, nonoverlapping effects on the metabolism and function of the RNA of the host cell.