Properties of ribosomes from Streptomyces erythreus and Streptomyces griseus

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.

A repeated decapeptide motif in the C-terminal domain of the ribosomal RNA methyltransferase from the erythromycin producer Saccharopolyspora erythraea

Re-analysis of the primary structure of the ribosomal RNA N-methyltransferase that confers self-resistance on the erythromycin-producing bacterium Saccharopolyspora erythraea has confirmed the presence of a C-terminal domain containing extensive repeat sequences. Nine tandem repeats can be discerned, with a decapeptide consensus sequence GGRx(H/R)GDRRT, although no single residue is wholly invariant. This highly polar, potentially flexible domain, which is predicted to adopt either a random coil or a structure with beta turns, has a counterpart in the erythromycin methyltransferase of an erythromycin-producing species of Arthrobacter. It also significantly resembles a portion of the C-terminal region of the eukaryotic protein nucleolin, which is unusually rich in dimethylarginine and glycine, and which is also predicted to behave as a random coil in solution. This resemblance, despite the very different roles of these proteins in ribosome biogenesis, strengthens the idea that in both rRNA methyltransferases and nucleolin these C-terminal sequences might contribute to rRNA binding.

[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.

Biochemical characterization of resistance determinants cloned from antibiotic-producing streptomycetes

Determinants of antibiotic resistance have been cloned from four antibiotic-producing streptomycetes into Streptomyces lividans. Biochemical analyses of resistant clones revealed the presence of enzymes that had previously been characterized as likely resistance determinants in the producing organisms. These included: 23S rRNA methylases from S. azureus and S. erythreus, which confer resistance to thiostrepton and erythromycin, respectively; viomycin phosphotransferase from S. vinaceus; and aminoglycoside phosphotransferase and acetyltransferase from the neomycin producer S. fradiae. In general, the levels of antibiotic resistance of the clones were similar to those of the producing organisms. Although the two aminoglycoside-modifying enzymes from S. fradiae could independently confer only low-level resistance to neomycin, the presence of both enzymes in the same strain resulted in a level of resistance comparable with that of the producing organism.

[Possibilities for direct manipulation of gene expression of microbial secondary metabolism]

The present review deals with some theoretical and applied aspects of directed manipulations of control mechanisms governing the expression of microbial secondary metabolism. In attempting to make broad generalizations, the production of secondary metabolites is discussed in terms of the cellular differentiation of relevant organisms. On the basis of the actual information about the regulation of microbial idiolite synthesis, some potential ways for the quantitative and the qualitative improvement of secondary metabolite production are discussed. A number of examples demonstrate the effectiveness of rational strategies of strain development, e. g., the removal of non-specific repressions of secondary metabolism by environmental factors, the excessive production of precursors due to altered control of intermediary metabolism, the increased resistance of producer organism against the autotoxicity of some idiolites, the deletion of alternative pathways of the primary and secondary metabolism, manipulations concerning the product spectrum, the deletion of feedback mechanisms, and elimination of degradating pathways in the secondary metabolism etc. The scope and limitations of rational strategies of strain improvement by genetic and physiologic manipulations are subjected to final discussion.

23S ribosomal ribonucleic acid of macrolide-producing streptomycetes contains methylated adenine

Coresistance to macrolide, lincosamide, and streptogramin B-type (MLS) antibiotics by a common biochemical mechanism characterizes clinically resistant pathogens. Of 10 streptomycetes tested for resistance to macrolide, lincosamide, and streptogramin B-type antibiotics, only 1, Streptomyces erythreus, the organism used for production of erythromycin, was found resistant to all three classes; moreover, it was the only streptomycete in the series tested found to contain N6-dimethyladenine (m62A) in 23S ribosomal ribonucleic acid, the structural alteration of ribosomal ribonucleic acid associated with clinical resistance. Of the seven streptomycetes tested for the presence of m62A and N6-methyladenine (m6A), two, S. fradiae and S. cirratus, which produce the macrolide antibiotics tylosin and cirramycin, respectively, were found to contain m6A, but not m62A. The remaining strains tested, including strains which produce lincomycin and streptogramins, contained neither m6A nor m62A.

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.

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

Ribosomes of Streptomyces azureus, which produces thiostrepton, are resistant to thiostrepton by virtue of being unable to bind the antibiotic. These ribosomes are also resistant to a number of other antibiotics (the thiostrepton group) which may share a common ribosomal binding site and a common mode of action with thiostrepton. Conversely, Bacillus pumilis and a strain of micrococcus, which produce the (probably identical) antibiotics micrococcin P and micrococcin, respectively, possess ribosomes which are susceptible to these antibiotics in vitro, although the organisms themselves are resistant.