Streptomycin Biosynthesis

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.

Antibiotics

The chapter informs about different types of antibiotics, their structure, biosynthesis and their regulation. Industrial cultivation and isolation of antibiotics is described in the chapter. Search for microorganisms producing antibiotics and preparation of high-producing strains is described. Resistance against antibiotics in producing microorganisms and pathogens is discussed.

A novel protein with alkaline phosphatase and protease inhibitor activities in Streptomyces hiroshimensis

A novel alkaline phosphatase (S-ALP) was found in the culture filtrate of Streptomyces hiroshimensis IFO 12785. Purification was achieved on Sephadex G-75 column, palmitoylated gauze column, and Superdex 75 HR column chromatographies. The molecular weight of S-ALP was estimated to be 14200 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The isoelectric point is 9.2. S-ALP had maximum enzyme activity at pH 9.5. S-ALP efficiently catalyzed both p-nitrophenyl phosphate and p-nitrophenyl phosphorylcholine substrates, particularly the latter. The N-terminal amino acid sequence (25 residues) of S-ALP was 60 to 72% identical to that of Streptomyces subtilisin inhibitor-like proteins. S-ALP exhibited trypsin inhibition in addition to a strong inhibition of subtilisin.

Enzymatic synthesis of aminoglycoside antibiotics: novel adenosylmethionine:2-deoxystreptamine N-methyltransferase activities in hygromycin B- and spectinomycin-producing Streptomyces spp. and uses of the methylated products

Aminocyclitols structurally related to streptamine, a 1,3-diaminocyclitol, are common components of the RNA-binding aminoglycoside antibiotics. The respective aminocyclitol cores of hygromycin B and spectinomycin are N(3)-methyl-2-deoxy-D-streptamine and N(1),N(3)-dimethyl-2-epi-streptamine. Adenosyl[methyl-(14)C]methionine:2-deoxystreptamine N-methyltransferase activities were detected in extracts of early-stationary-phase mycelia of the hygromycin B producer Streptomyces hygroscopicus subsp. hygroscopicus ATCC 27438 and the spectinomycin producer Streptomyces flavopersicus ATCC 19756. Extracts of both strains methylated the N(1)- and N(3)-amino groups of 2-deoxystreptamine, streptamine, and 2-epi-streptamine; the N(1)-amino group of N(3)-methyl-2-deoxy-D-streptamine, and the N(3)-amino group of N(1)-ethyl-2-deoxy-D-streptamine, the semisynthetic aminocyclitol of netilmicin. The mono[(14)C]methyl derivatives of 2-deoxystreptamine, streptamine, and 2-epi-streptamine were excellent substrates for L-glutamine:aminocyclitol aminotransferase and thereby provided a sensitive assay for derepression of this key enzyme, a generic biosynthetic marker that we have shown to be the only enzyme common to the biosyntheses of all major aminoglycoside antibiotics. Other prospective uses for these methyl-labeled 2-deoxystreptamine analogs are also described.

Two glycosyltransferases and a glycosidase are involved in oleandomycin modification during its biosynthesis by Streptomyces antibioticus

A 5.2 kb region from the oleandomycin gene cluster in Streptomyces antibioticus located between the oleandomycin polyketide synthase gene and sugar biosynthetic genes was cloned. Sequence analysis revealed the presence of three open reading frames (designated oleI, oleN2 and oleR). The oleI gene product resembled glycosyltransferases involved in macrolide inactivation including the oleD product, a previously described glycosyltransferase from S. antibioticus. The oleN2 gene product showed similarities with different aminotransferases involved in the biosynthesis of 6-deoxyhexoses. The oleR gene product was similar to several glucosidases from different origins. The oleI, oleR and oleD genes were expressed in Streptomyces lividans. OleI and OleD intracellular proteins were partially purified by affinity chromatography in an UDP-glucuronic acid agarose column and OleR was detected as a major band from the culture supernatant. OleI and OleD showed oleandomycin glycosylating activity but they differ in the pattern of substrate specificity: OleI being much more specific for oleandomycin. OleR showed glycosidase activity converting glycosylated oleandomycin into active oleandomycin. A model is proposed integrating these and previously reported results for intracellular inactivation, secretion and extracellular reactivation of oleandomycin.

Regulation of spiramycin synthesis in Streptomyces ambofaciens: effects of glucose and inorganic phosphate

The production of the 16-membered macrolide antibiotic, spiramycin, in Streptomyces ambofaciens is inhibited by glucose, 2-deoxyglucose and inorganic phosphate. The role of intracellular ATP content and phosphorylated metabolites as common regulating signals of both glucose and phosphate inhibitory effects is discussed. Two enzymatic targets of the effect of phosphate on spiramycin biosynthesis were studied. Valine dehydrogenase, the first enzyme of valine catabolism (supplier of aglycone spiramycin precursors), and alkaline phosphatase, which cleaves phosphorylated intermediates, were repressed in the presence of excess phosphate.

Purification and characterization of an extracellular enzyme from Streptomyces antibioticus that converts inactive glycosylated oleandomycin into the active antibiotic

Cell-free extracts from the oleandomycin producer, Streptomyces antibioticus, possess an intracellular glycosyltransferase capable of inactivating oleandomycin by glycosylation of the 2'-hydroxyl group in the desosamine moiety of the molecule [Vilches, C., Hernández, C., Méndez, C. & Salas, J. A. (1992) J. Bacteriol. 174, 161-165]. Using a four-step purification procedure, we have purified an enzyme activity from the culture supernatants from this organism which is able to release glucose from the inactive glycosylated molecule thus reactivating the antibiotic activity. This enzyme activity appeared in the culture supernatants immediately before oleandomycin is detected. The enzyme (molecular mass 87 kDa) showed a high degree of substrate specificity, not acting on other glycosylated macrolides such as methymycin, lankamycin and rosaramicin which are substrates for the glycosyltransferase. A second activity was detected corresponding to a 34-kDa polypeptide which probably originates from proteolytic cleavage of the larger polypeptide. The 87-kDa polypeptide possibly catalyses the last biosynthetic step in oleandomycin biosynthesis by S. antibioticus.

Biosynthesis of streptomycin. Purification and properties of a dTDP-L-dihydrostreptose: streptidine-6-phosphate dihydrostreptosyltransferase from Streptomyces griseus

dTDP-L-dihydrostreptose: streptidine-6-phosphate dihydrostreptosyltransferase, an enzyme involved in the biosynthesis of streptomycin, has been purified from Streptomyces griseus to near homogeneity by a six-step procedure involving chromatography on streptidine-6-phosphate-Sepharose. By gel filtration the apparent Mr of the enzyme was found to be about 63 000. The subunit Mr found on sodium dodecylsulfate gels is about 35 000. The transferase is dependent on Mn2+ or Mg2+ ions. Co2+ is as effective as Mg2+. From the substrates tested only streptidine 6-phosphate was an acceptor for dihydrostreptose in the synthesis of O-alpha-L-dihydrostreptose(1 leads to 4)-streptidine 6-phosphate. No activity was found with streptidine, 2-deoxystreptamine and 4-deoxy-streptamine. The activity of the transferase in the course of fermentation runs parallel to the activity of dTDP-dihydrostreptose synthase and reaches a maximum after around 50 h of fermentation, just before appearance of streptomycin in the medium.