Properties of the ribosomes of antibiotic producers: effects thiostrepton and micrococcin on the organisms which produce them

Avoidance of suicide in antibiotic-producing microbes

Many microbes synthesize potentially autotoxic antibiotics, mainly as secondary metabolites, against which they need to protect themselves. This is done in various ways, ranging from target-based strategies (i.e. modification of normal drug receptors or de novo synthesis of the latter in drug-resistant form) to the adoption of metabolic shielding and/or efflux strategies that prevent drug-target interactions. These self-defence mechanisms have been studied most intensively in antibiotic-producing prokaryotes, of which the most prolific are the actinomycetes. Only a few documented examples pertain to lower eukaryotes while higher organisms have hardly been addressed in this context. Thus, many plant alkaloids, variously described as herbivore repellents or nitrogen excretion devices, are truly antibiotics-even if toxic to humans. As just one example, bulbs of Narcissus spp. (including the King Alfred daffodil) accumulate narciclasine that binds to the larger subunit of the eukaryotic ribosome and inhibits peptide bond formation. However, ribosomes in the Amaryllidaceae have not been tested for possible resistance to narciclasine and other alkaloids. Clearly, the prevalence of suicide avoidance is likely to extend well beyond the remit of the present article.

The binding of thiostrepton to 23S ribosomal RNA

The antibiotic, thiostrepton, binds to 23S ribosomal RNA from E coli with a dissociation constant (KD) of 2.4 x 10(-7) M. The specificity of the interaction was established using 16S rRNA and modified or mutationally-altered 23S rRNA. Thus, no binding was detected with rRNA from the 30S subunit nor with rRNA modified in vitro by the thiostrepton resistance methylase. Mutant 23S rRNA, altered at residue 1067 in each of the 3 possible ways, showed reduced binding affinity for thiostrepton. The KD for the G mutation was 3.5 x 10(-6) M; for the C mutation, 2.4 x 10(-5) M; and for the U mutation, 4.8 x 10(-5) M. This reduction in drug binding is compatible with functional analyses; the C or U mutation results in ribosomal particles which are poorly inhibited by the drug compared with wild-type, whereas the G mutation results in an intermediate response to the drug in protein synthesis. The smallest 23S rRNA fragment used here that was capable of binding thiostrepton, in a nitrocellulose filter binding assay, comprised residues 1052-1112 and the dissociation constant was 3.0 x 10(-7) M, ie virtually indistinguishable from that with intact 23S RNA. However, the drug was incapable of binding to the 5'-moiety of this fragment (ie residues 1052-1084) or to an RNA transcript complementary to 1052-1112.

Streptomyces relC mutants with an altered ribosomal protein ST-L11 and genetic analysis of a Streptomyces griseus relC mutant

Several relaxed (rel) mutants have been obtained from Streptomyces species by selecting colonies resistant to thiopeptin, an analogue of thiostrepton. Using two-dimensional gel electrophoresis, I compared the ribosomal proteins from rel and rel+ pairs of S. antibioticus, S. lavendulae, S. griseoflavus, and S. griseus. It was found that all of the Streptomyces rel mutants thus examined had an altered or missing ribosomal protein, designated tentatively ST-L11. These rel mutants therefore could be classified as relC mutants and were highly sensitive to erythromycin or high temperature. A relC mutant of S. griseus was defective in streptomycin production, but phenotypic reversion of this defect to normal productivity was found at high incidence among progeny of the relC mutant. This phenotypic reversion did not accompany a reappearance of ribosomal protein ST-L11, and furthermore the ability of accumulating ppGpp still remained at a low level, thus suggesting existence of a mutation (named sup) which suppresses the streptomycin deficiency phenotype exhibited by the relC mutant. Genetic analysis revealed that there is a correlation between the rel mutation and the inability to produce streptomycin or aerial mycelia. The sup mutation was found to lie at a chromosomal locus distinct from that of the relC mutation. It was therefore concluded that the dependence of streptomycin production on the normal function of the relC gene could be entirely bypassed by a mutation at the suppressor locus (sup). The suppressing effect of the sup mutation on the relC mutation was blocked when the afs mutation (defective in A-factor synthesis) was introduced into a relC sup double mutant. It is proposed that the sup gene or its product can be direct or indirect target for ppGpp.

Thiostrepton-induced gene expression in Streptomyces lividans

Thiostrepton induced the expression of four proteins (17, 19, 30, and 56 kilodaltons) of unknown function in Streptomyces lividans. The chromosomal gene which encoded the 19-kilodalton protein (tipA) was cloned and sequenced. Transcription of the tipA promoter was induced at least 200-fold by thiostrepton. The tipA 200-fold by thiostrepton. The tipA transcriptional start site (located by S1 mapping and primer extension experiments) was preceded by a 45-base-pair imperfect inverted-repeat sequence which included the -10 and -35 regions of the promoter. Under noninducing conditions in vivo, this might form a cruciform structure which is not recognized by RNA polymerase. A 143-base-pair fragment including this region was cloned into a promoter probe vector, pIJ486. In this plasmid, pAK114, the thiostrepton-inducible tipA promoter controlled the expression of a kanamycin resistance gene encoding an aminoglycoside phosphotransferase. As little as 1 ng of thiostrepton spotted on a lawn of S. lividans(pAK114) induced kanamycin-resistant growth. Other thiostreptonlike antibiotics also induced tipA, but structurally unrelated antibiotics which inhibit translation had no effect. In S. lividans, the promoter could be induced by thiostrepton during either growth or stationary phase. The tipA promoter should be a valuable tool for expression studies in streptomycetes.

Site-directed mutagenesis of Escherichia coli 23 S ribosomal RNA at position 1067 within the GTP hydrolysis centre

Site-directed mutagenesis has been used to change, specifically, residue 1067 within 23 S ribosomal RNA of Escherichia coli. This nucleoside (adenosine in the wild-type sequence) lies within the GTPase centre of the larger ribosomal subunit and is normally the target for the methylase enzyme responsible for resistance to the antibiotic thiostrepton. The performance of the altered ribosomes was not impaired in cell-free protein synthesis nor in GTP hydrolysis assays (although the 3 mutant strains grew somewhat more slowly than wild-type) but their responses to thiostrepton did vary. Thus, ribosomes containing the A to C or A to U substitution at residue 1067 of 23 S rRNA were highly resistant to the drug, whereas the A to G substitution resulted in much lesser impairment of thiostrepton binding and the ribosomes remained substantially sensitive to the antibiotic. These data reinforce the hypothesis that thiostrepton binds to 23 S rRNA at a site that includes residue A1067. They also exclude any possibility that the insensitivity of eukaryotic ribosomes to the drug might be due solely to the substitution of G at the equivalent position within eukaryotic rRNA.

Mutual and self-sensitivity among antibiotically active mutant derivatives from the inactive degenerate Aspergillus versicolor N5

Antibiotically active producer mutants derived from the spontaneous degenerate parent Aspergillus versicolor N5 possessed not only mutual but also self-sensitive activity. The producer mutants, like the inactive parent, were only 3.5-fold less sensitive than the most sensitive unrelated organism, Trichophyton rubrum. The germination of spores is generally more sensitive than growth of vegetative cells. The antifungal spectrum of these mutual and self-sensitive mutants was fairly wide, unlike the host range specificity of bacteriocinogenic strains acting on organisms closely related to the producers. The self and mutual growth inhibitory principle was finally identified as the antibiotics mycoversilin and versilin in the case of producer mutants (N5)17 and N5T10(7), respectively, or Vx, an antibiotic of unknown molecular species, in the case of another producer mutant N5T10(8). Thus self-sensitivity, instead of self-resistance, of these antibiotically active mutant derivatives is a unique property among filamentous fungi in having simultaneously expressed two loci of contradictory functions, one for synthesis of, and the other for sensitivity towards, the same or related antibiotics.

Studies of the GTPase domain of archaebacterial ribosomes

Ribosomes from the methanogens Methanococcus vannielii and Methanobacterium formicicum catalyse uncoupled hydrolysis of GTP in the presence of factor EF-2 from rat liver (but not factor EF-G from Escherichia coli). In this assay, and in poly(U)-dependent protein synthesis, they were sensitive to thiostrepton. In contrast, ribosomes from Sulfolobus solfataricus did not respond to factor EF-2 (or factor EF-G) but possessed endogenous GTPase activity, which was also sensitive to thiostrepton. Ribosomes from the methanogens did not support (p)ppGpp production, but did appear to possess the equivalent of protein L11, which in E. coli is normally required for guanosine polyphosphate synthesis. Protein L11 from E. coli bound well to 23S rRNA from all three archaebacteria (as did thiostrepton) and oligonucleotides protected by the protein were sequenced and compared with rRNA sequences from other sources.

[Secondary metabolites as endogenous effectors of microbial cytodifferentiation]

The present survey covers the regulatory role of microbial secondary metabolites and related compounds as endogenous signals of cell differentiation of the producing organisms. Several antibiotics have been shown to exert autoregulatory effects on differentiation-associated functions. The mechanisms of self-protection of the producing cells against the autotoxicity of secondary metabolites are discussed in terms of an integral part of the modulation of signal strength. As a further topic, the review deals with the hormone-like interference of particular metabolites with differentiating cells. Conclusive discussion concerns the potential use of microbial signal molecules either as tools for directed manipulations of product syntheses or as pharmaceutics.

Concerning the mode of action of micrococcin upon bacterial protein synthesis

The antibiotic, micrococcin, binds to complexes formed between bacterial 23-S ribosomal RNA and ribosomal protein L11 and, in doing so, inhibits of thiostrepton. In assay systems simulating partial reaction of protein synthesis, micrococcin inhibits a number of processes believed to involve the ribosomal A site while stimulating GTP hydrolysis dependent upon ribosomes and elongation factor EF-G. The latter effect is not observed upon ribosomes lacking a protein homologous with protein L11. Nor is it apparent upon those containing 23-S RNA previously subjected to the action of a specific methylase known to render ribosomes resistant to thiostrepton. It is concluded that stimulation by micrococcin of factor-dependent GTP hydrolysis results from the binding of the drug to its normal target site which involves 23-S RNA and protein L11.

Binding of thiostrepton to a complex of 23-S rRNA with ribosomal protein L11

Thiostrepton binds with high affinity and with a 1 : 1 stoichiometry to a complex formed between Escherichia coli 23-S ribosomal RNA and ribosomal protein L11 of E. coli or the homologous protein BM-L11 of Bacillus megaterium. In the presence of T1 ribonuclease, protein BM-L11 and thiostrepton protect from degradation a fragment of E. coli 23-S RNA estimated to be about 50 nucleotides in length.

Mechanism of resistance to thiostrepton in the producing-organism Streptomyces azureus

An antibiotic-producing organism, Streptomyces azureus, defends itself against its own toxic product by methylating the RNA of its ribosomes.

Early days of antimicrobialherapy

Sporangiomycin and micrococcin inhibit the binding of aminoacyl-transfer ribonucleic acid into the ribosomal A site in intact bacterial protoplasts. They also prevent the assembly of [ribosome-elongation factor G-guanine nucleotide] complexes in vitro and compete with [³⁵S]thiostrepton for ribosomal binding sites. We conclude that micrococcin and sporangiomycin block the ribosomal A site in the vicinity of the complex guanosine triphosphatase center and so resemble thiostrepton in their modes of action.