Topological studies of the membrane component of the OleC ABC transporter involved in oleandomycin resistance in Streptomyces antibioticus

The OleC ABC transporter of Streptomyces antibioticus is constituted by an ATP-binding protein (OleC) and a hydrophobic protein (OleC5). Here we present experimental evidence demonstrating that the OleC5 protein is an integral membrane protein and we propose a topological model for its integration into the membrane. This model is based on the generation of hybrid proteins between different regions of OleC5 and a Escherichia coli beta-lactamase (BlaM) and the determination of the minimal inhibitory concentrations to ampicillin in these constructions. Fusions were generated both by cloning specific fragments of oleC5 and by creating ExoIII nested deletions of the gene. In the topological model proposed there will be six alpha-helix transmembrane regions, two cytoplasmic and four periplasmic loops and a hydrophobic linker domain.

Characterization of the ATPase activity of the N-terminal nucleotide binding domain of an ABC transporter involved in oleandomycin secretion by Streptomyces antibioticus

The oleB gene of Streptomyces antibioticus, oleandomycin producer, encodes an ABC transporter containing two putative ATP-binding domains and is involved in oleandomycin resistance and secretion in this organism. We have overexpressed in Escherichia coli the N-terminal nucleotide-binding domain of OleB (OleB') as a fusion protein and purified the fusion protein by affinity chromatography. The fusion protein showed ATPase activity dependent on the presence of Mg2+ ions. ATPase activity was resistant to specific inhibitors of P-, F-, and V-type ATPase whereas sodium azide and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-C1) were strong inhibitors. The change of Lys71, located within the Walker A motif of the OleB' protein, to Gln or Glu caused a loss of ATPase activity, whereas changing to Gly did not impair the activity. The results suggest that the intrinsic ATPase activity of purified fusion protein can be clearly distinguished from other ATP-hydrolysing enzymes, including ion-translocating ATPases or ABC-traffic ATPases, both on the basis of inhibition by different agents and since it hydrolyzes ATP without interacting with a hydrophobic membrane component.

Effects of 5-azacytidine on physiological differentiation of Streptomyces antibioticus

We studied the specificity of the effect of 5-azacytidine, a DNA-methylase inhibitor that impairs Streptomyces differentiation. We showed that this compound did not affect global DNA, RNA or protein biosynthesis in submerged cultures of S. antibioticus ETHZ 7451. Among individual proteins, enzymes such as alkaline phosphatase and intracellular protease were produced in similar amounts in the presence and absence of this compound. However, the production of extracellular protease was significantly inhibited. Also DNA-methyltransferases were inhibited, indicating that DNA methylation might be involved in the regulation of differentiation. By contrast, elevated levels of the antibiotic rhodomycin resulted when 5-azacytidine was added to the culture medium. In order to determine whether there was a correlation between sporulation and altered enzymatic activities, these activities were analysed in S. antibioticus submerged cultures. Among them, alkaline phosphatase and intracellular protease activities did not show a clear correlation with sporulation. However, high levels of extracellular protease were produced during septation of hyphae. This association between extracellular protease and sporulation suggests a specific inhibitory effect of 5-azacytidine, not only on spore formation, but also on physiological differentiation.

Biosynthesis of the macrolide oleandomycin by Streptomyces antibioticus. Purification and kinetic characterization of an oleandomycin glucosyltransferase

The oleandomycin (OM) producer, Streptomyces antibioticus, possesses a mechanism involving two enzymes for the intracellular inactivation and extracellular reactivation of the antibiotic. Inactivation takes place by transfer of a glucose molecule from a donor (UDP-glucose) to OM, a process catalyzed by an intracellular glucosyltransferase. Glucosyltransferase activity is detectable in cell-free extracts concurrent with biosynthesis of OM. The enzyme has been purified 1,097-fold as a monomer, with a molecular mass of 57.1 kDa by a four-step procedure using three chromatographic columns. The reaction operates via a compulsory-order mechanism. This has been shown by steady-state kinetic studies using either OM or an alternative substrate (rosaramycin) and dead-end inhibitors, and isotopic exchange reactions at equilibrium. OM binds first to the enzyme, followed by UDP-glucose. A ternary complex is thus formed prior to transfer of glucose. UDP is then released, followed by the glycosylated oleandomycin (GS-OM).

A second ABC transporter is involved in oleandomycin resistance and its secretion by Streptomyces antibioticus

A 3.2 kb Sstl-Sphl DNA fragment of Streptomyces antibioticus, an oleandomycin producer, conferring resistance to oleandomycin was sequenced and found to contain an open reading frame of 1710 bp (oleB). Its deduced gene product (OleB) showed a high degree of similarity with other proteins belonging to the ABC-transporter superfamily including the gene product of another oleandomycin-resistance gene (OleC). The OleB protein contains two ATP-binding domains, each of approximately 200 amino acids in length, and no hydrophobic transmembrane regions. Functional analysis of the oleB gene was carried out by deleting specific regions of the gene and assaying for oleandomycin resistance. These experiments showed that either the first or the second half of the gene containing only one ATP-binding domain was sufficient to confer resistance to oleandomycin. The gene oleB was expressed in Escherichia coli fused to a maltose-binding protein (MBP) using the pMal-c2 vector. The MBP-OleB hybrid protein was purified by affinity chromatography on an amylose resin and polyclonal antibodies were raised against the fusion protein. These were used to monitor the biosynthesis and physical location of OleB during growth. By Western analysis, the OleB protein was detected both in the soluble and in the membrane fraction and its synthesis paralleled oleandomycin biosynthesis. It was also shown that a Streptomyces albus strain, containing both a glycosyltransferase (OleD) able to inactivate oleandomycin and the OleB protein, was capable of glycosylating oleandomycin and secreting the inactive glycosylated molecule. It is proposed that OleB constitutes the secretion system by which oleandomycin or its inactive glycosylated form could be secreted by S. antibioticus.

A cytochrome P450-like gene possibly involved in oleandomycin biosynthesis by Streptomyces antibioticus

A cosmid clone from an oleandomycin producer, Streptomyces antibioticus, contains a large open reading frame encoding a type I polyketide synthase subunit and an oleandomycin resistance gene (oleB). Sequencing of a 1.4-kb DNA fragment adjacent to oleB revealed the existence of an open reading frame (oleP) encoding a protein similar to several cytochrome P450 monooxygenases from different sources, including the products of the eryF and eryK genes from Saccharopolyspora erythraea that participate in erythromycin biosynthesis. The oleP gene was expressed in Escherichia coli as a fusion protein to a maltose-binding protein. Using polyclonal antibodies against this fusion protein it was observed that the synthesis of the cytochrome P450 was in parallel to that of oleandomycin. The cytochrome P450 encoded by the oleP gene could be responsible for the epoxidation of carbon 8 of the oleandomycin lactone ring.

Autoradiographic study of hyphal growth during aerial mycelium development in Streptomyces antibioticus

The pattern of growth of aerial mycelium in Streptomyces species was investigated by autoradiography. Colonies of Streptomyces antibiotics were labeled with N-acetyl-D-[1-3H] glucosamine to localize the sites of hyphal growth during the development of aerial mycelium. Autoradiographs obtained with sections of the colonies revealed that hyphal growth occurs not only at the top of the colony but also in the inner zones of the aerial mycelium.

Intracellular glycosylation and active efflux as mechanisms for resistance to oleandomycin in Streptomyces antibioticus, the producer organism

Resistance to macrolides in producing organisms can be achieved by target site modification, intracellular inactivation of the antibiotic or active efflux mechanisms for the excretion of the antibiotic. The oleandomycin producer, Streptomyces antibioticus, possesses oleandomycin-sensitive ribosomes all along the cell cycle. However, it contains an intracellular glycosyltransferase capable of inactivating oleandomycin in the presence of UDP-glucose as cofactor. The correspondent gene (oleD) has been cloned and sequenced and the glycosyltransferase purified. Two other genes (oleB and oleC) that confer oleandomycin resistance have been cloned and characterized and both encode ABC (ATP-Binding Cassette) transporters. These may constitute the excretion mechanism throughout which the glycosylated oleandomycin is excreted. A second enzyme activity has been purified from culture supernatants of the oleandomycin producer that releases the glucose from the inactive glycosylated oleandomycin generating active antibiotic. This enzyme would probably catalyse the last step in the biosynthesis of oleandomycin.

Streptomyces antibioticus contains at least three oleandomycin-resistance determinants, one of which shows similarity with proteins of the ABC-transporter superfamily

Three different DNA fragments of an oleandomycin producer, Streptomyces antibioticus, conferring oleandomycin resistance were cloned in plasmid pIJ702 and expressed in Streptomyces lividans and in Streptomyces albus. These oleandomycin resistance determinants were designated as oleA (pOR400), oleB (pOR501) and oleC (pOR800). oleA and oleC are closely linked in the chromosome as they were both obtained together in two cosmid clones that were isolated from a genomic library. Sequencing of the oleC resistance determinant revealed four complete open reading frames (ORFs) and the C-terminal end of a fifth. The functions of orf1 and orf2 are unknown since they did not show significant similarity with other sequences in the data bases. The orf3 gene product has similarity with some proteins involved in iron and vitamin B12 uptake in bacteria. The orf4 gene product had a hydrophilic profile and showed important similarity with proteins containing typical ATP-binding domains characteristic of the ABC-transporter superfamily and involved in membrane transport and, particularly, with several genes conferring resistance to various macrolide antibiotics and anticancer drugs. The last gene, orf5, is translationally coupled to orf4 and codes for a hydrophobic polypeptide containing several transmembrane domains characteristic of integral membrane proteins. Subcloning and deletion experiments limited the resistance determinant to a 0.9 kb PstI-SphI fragment and only orf4 is included in this fragment. These results suggest that resistance to oleandomycin conferred by oleC (orf4) is probably due to an efflux transport system of the ABC-transporter superfamily.

Role of glycosylation and deglycosylation in biosynthesis of and resistance to oleandomycin in the producer organism, Streptomyces antibioticus

Cell extracts of Streptomyces antibioticus, an oleandomycin producer, can inactivate oleandomycin in the presence of UDP-glucose. The inactivation can be detected through the loss of biological activity or by alteration in the chromatographic mobility of the antibiotic. This enzyme activity also inactivates other macrolides (rosaramicin, methymycin, and lankamycin) which contain a free 2'-OH group in a monosaccharide linked to the lactone ring (with the exception of erythromycin), but not those which contain a disaccharide (tylosin, spiramycin, carbomycin, josamycin, niddamycin, and relomycin). Interestingly, the culture supernatant contains another enzyme activity capable of reactivating the glycosylated oleandomycin and regenerating the biological activity through the release of a glucose molecule. It is proposed that these two enzyme activities could be an integral part of the oleandomycin biosynthetic pathway.

[The interrelation between the formation of oleandomycin and the resistance to it in different strains of Streptomyces antibioticus]

Strains, producers of oleandomycin, with different level of antibiotic-formation have been studied for their resistance to their own antibiotic. The obtained highly active strain possesses double resistance to oleandomycin and 50% higher activity. Identity of oleandomycin phosphate substances synthesized by initial and produced highly active strains is shown by the HELC method.

Features of regenerated clones with or without fusion treatment between auxotrophic mutants of Streptomyces antibioticus and their antibiotic productivity

During experiments on protoplast fusion of complementary auxotrophic mutants (194 and 11M-21) of Streptomyces antibioticus for strain improvement, the clones (typified by F-40) regenerated on minimal regeneration medium (MRM) were found to be prototrophs, and to produce an antibiotic different from those produced by the parent strain. The protoplast regeneration of each parent was examined as a negative control experiment. In the regenerated clones of 194, half of them produced actinomycins similar to those produced by the original mutant 194, but others (typified by R-20) seemed to produce antibiotics similar to those produced by F-40. In the taxonomic characterization of morphological, cultural, and physiological properties of each strain, F-40, R-20, and the parent mutant 194 had no significant differences with a few exceptions. The problem here is whether the antibiotic of R-20 is the same as that of F-40, which was first isolated and found to be a peptide antibiotic different from actinomycins, with activity against Gram-negative and Gram-positive bacteria.

Biosynthesis of oleandomycin by Streptomyces antibioticus: influence of nutritional conditions and development of resistance

The influence of different nutritional compounds on oleandomycin biosynthesis by Streptomyces antibioticus was studied, resulting in the design of a chemically defined medium for production of the antibiotic. Of the variety of carbon and nitrogen compounds tested, fructose and aspartic acid (carbon and nitrogen sources, respectively) supported the highest oleandomycin titres. Addition of propionate but not acetate, both precursors of the skeleton of the macrolide lactone ring, stimulated the biosynthesis of the antibiotic. Oleandomycin biosynthesis was repressed by glucose but not by phosphate. S. antibioticus develops oleandomycin resistance shortly before the antibiotic begins to be synthesized, showing a triphasic pattern of resistance: spores and producing mycelium are resistant, while non-producing mycelium is sensitive.

[Study of the structure of amplifying sequence of Streptomyces antibioticus]

A low productive laboratory strain of S. antibioticus and a strain with an increased productivity of oleandomycin derived from it were studied comparatively with using restriction analysis and blotting hybridization. Amplification, site specific integration and segregation of the DNA sequence 32.0 kb in size were detected in the strains. The chromosomes of the laboratory strain contained one copy of the amplifying sequence AUD. After uniting of the end sequences AUD appeared to be capable of segregating from the chromosomes and its one copy per five genomes was present in the form of an extrachromosomal genetic element eSA1. The genome of the strain with increased productivity of oleandomycin contained in its chromosomes sequence ADS-Sa1 amplified to 150 copies and the eSA1 extrachromosomal genetic element in the form of mono-, di- and trimeric structures in the quantity of approximately one copy per genome. The BamHIB fragment of the eSA1 DNA 4 kb in size was identified. The fragment was able to participate in segregation or integration of eSA1 from or into the chromosomes since its subfragments were flanking AUD and ADS-SA1 in the chromosomes. The BamHIB fragment was hybridizing with a number of fragments of the chromosomal DNA of S. antibioticus, S. erythraeus. S. lividans and other strains of streptomycetes. It probably contained an IS-like element or a dispersed genetic element of another class. The DNA sequence of the eSA1 genetic element contained regions homologous to the sequence of the Erm E gene in S. erythraeus NRRL 2338.

[Effect of the producer type on the nature of antibiotic fermentation]

Fermentation processes in production of bacitracin, a polypeptide antibiotic by Bacillus licheniformis, and oleandomycin, a macrolide antibiotic by Streptomyces antibioticus, were studied comparatively. It was shown that the antibiotic-producing actinomycete was characterized by a prolonged phase of growth retardation. The highest efficiency of the control actions was observed at the beginning of the fermentation. They were aimed at intensifying the substrate usage during the growth phase and activation of cell metabolism. Controlled cultivation of the Bacillus representative was based on its capacity of achieving the maximum growth rate possible under the certain conditions. Therefore, an increase in the quantity of the synthesized antibiotic was due, under such conditions, to inhibition of the culture growth by various means including lower mass exchange intensity.

[Mutagenic effect of mitomycin C on different strains of oleandomycin producer Streptomyces antibioticus]

Mitomycin C, a DNA-tropic antibiotic, was shown to have a lethal effect on spore sprouts of two strains of Streptomyces antibioticus, an organism producing oleandomycin. When the time of exposure to the antibiotic increased there was an almost equal decrease in the survival rate. The mutagen action on the morphological variation and antibiotic production of the two closely related strains were diverse due to their genetic differences. The strain isolated after the culture treatment with a chemical mutagen and subjected to a more prolonged maintaining selection showed lower variation with respect to its colony morphology. The other strain isolated after treatment of the culture with high concentrations of its own antibiotic showed lower variation with respect to its antibiotic production property. The shift in the antibiotic production in the direction of the low active variants was characteristic of the both highly productive strains.

Biosynthesis of 9-beta-D-arabinofuranosyladenine: hydrogen exchange at C-2' and oxygen exchange at C-3' of adenosine

The data presented here describe new findings related to the bioconversion of adenosine to 9-beta-D-arabinofuranosyladenine (ara-A) by Streptomyces antibioticus by in vivo investigations and with a partially purified enzyme. First, in double label in vivo experiments with [2'-18O]- and [U-14C]adenosine, the 18O:14C ratio of the ara-A isolated does not change appreciably, indicating a stereospecific inversion of the C-2' hydroxyl of adenosine to ara-A with retention of the 18O at C-2'. In experiments with [3'-18O]- and [U-14C]-adenosine, [U-14C]ara-A was isolated; however, the 18O at C-3' is below detection. The adenosine isolated from the RNA from both double label experiments has essentially the same ratio of 18O:14C. Second, an enzyme has been isolated and partially purified from extracts of S. antibioticus that catalyzes the conversion of adenosine, but not AMP, ADP, ATP, inosine, guanosine, or D-ribose, to ara-A. In a single label enzyme-catalyzed experiment with [U-14C]adenosine, there was a 9.9% conversion to [U-14C]ara-A; with [2'-3H]-adenosine, there was a 8.9% release of the C-2' tritium from [2'-3H]adenosine which was recovered as 3H2O. Third, the release of 3H as 3H2O from [2'-3H]adenosine was confirmed by incubations of the enzyme with 3H2O and adenosine. Ninety percent of the tritium incorporated into the D-arabinose of the isolated ara-A was in C-2 and 8% was in C-3. The enzyme-catalyzed conversion of adenosine to ara-A occurs without added cofactors, displays saturation kinetics, a pH optimum of 6.8, a Km of 8 X 10(-4) M, and an inhibition by heavy metal cations. The enzyme also catalyzes the stereospecific inversion of the C-2' hydroxyl of the nucleoside antibiotic, tubercidin to form 7-beta-D-arabinofuranosyl-4-aminopyrrolo[2,3-d]pyrimidine. The nucleoside antibiotic, sangivamycin, in which the C-5 hydrogen is replaced with a carboxamide group, is not a substrate. On the basis of the single and double label experiments in vivo and the in vitro enzyme-catalyzed experiments, two mechanisms involving either a 3'-ketonucleoside intermediate or a radical cation are proposed to explain the observed data.

[Biosynthesis of oleandomycin by cultures of Streptomyces antibioticus obtained after regeneration and fusion of protoplasts]

The ability of Streptomyces antibioticus strains to synthesize oleandomycin is studied under the effect of regeneration and fusion of protoplasts. The production of strains-regenerants with an increased (by 30-50%) synthesis of oleandomycin is possible. Regenerants of mutants resistant to the proper antibiotic retain a high level of the oleandomycin synthesis more stably. Variations in the antibiotic-production ability are considered in regenerant populations of various generations.

Metabolites of microorganisms. 247. Phenazines from Streptomyces antibioticus, strain Tü 2706

From a strain of Streptomyces antibioticus seven yellow phenazines were isolated. The antibacterially most active antibiotic was identified as (-)-saphenamycin, a second one with compound DC-86-Y (saphenic acid). Three compounds were new: Saphenic acid methyl ether, 6-acetylphenazine-1-carboxylic acid and an inseparable mixture of fatty acid esters of saphenic acid. Two simple phenazines were phenazine-1-carboxylic acid (tubermycin B) and unsubstituted phenazine, which was isolated for the first time from a microorganism.

8-Ketodeoxycoformycin and 8-ketocoformycin as intermediates in the biosynthesis of 2'-deoxycoformycin and coformycin

An enzyme has been isolated from cell-free extracts of Streptomyces antibioticus that can catalyze the reduction of 8-ketodeoxycoformycin (8-KetodCF) and 8-ketocoformycin (8-ketoCoF) to the naturally occurring nucleoside analogues 2'-deoxycoformycin (dCF) and coformycin (CoF), respectively. The partially purified reductase requires NADPH as the cofactor and stereospecifically reduces the 8-keto group of both ketonucleoside substrates to a hydroxyl group with the R configuration at C-8. This is the same configuration of the hydroxyl group as that of the dCF and CoF isolated from S. antibioticus. The reduction proceeds at the nucleoside level, and ATP is not required. The reductase is stereospecific for the NADPH cofactor in that it transfers the pro-S but not the pro-R hydrogen from C-4 of NADPH to the 8-keto group. The apparent Km for 8-ketodCF and 8-ketoCoF were 250 and 150 microM, respectively. These in vitro results, which show that 8-ketodCF and 8-ketoCoF may be intermediates in the biosynthesis of dCF and CoF, support and extend our earlier results from in vivo studies which established that adenosine and C-1 of D-ribose are the carbon-nitrogen precursors of dCF. A possible mechanism for the formation of dCF is presented.

[Effect of surface-active agents (tween-21) on indices of energy metabolism in oleandomycin producers]

The specific growth rate of Streptomyces antibioticus, a producer of oleandomycin, and the specific rate of the antibiotic accumulation in the culture medium during fermentation were investigated. On the basis of the results obtained the fermentation period was divided into 7 phases of development. The culture treated with the surfactant (Tween-21) is characterized by a higher specific growth rate during the whole fermentation and a higher specific rate of the antibiotic accumulation at the stage of the highest production as compared to the control. The ATP content, the value of the adenylate energy charge and the contents of high-molecular weight polyphosphates in the mycelium were examined. In the phase of the intensive growth St. antibioticus was characterized by a higher ATP level and a higher energy charge. More active accumulation of polyphosphates was observed in the late intensive growth phase. It was also found that after the treatment of the culture with Tween-21 it utilized polyphosphates more actively during the antibiotic biosynthesis.

Actinomycin synthesis in Streptomyces antibioticus: enzymatic conversion of 3-hydroxyanthranilic acid to 4-methyl-3-hydroxyanthranilic acid

A methyltransferase which utilizes 3-hydroxyanthranilic acid (HAA) as a substrate was identified in detergent-treated extracts of the bacterium Streptomyces antibioticus. The enzyme catalyzes the transfer of methyl groups from [14C]S-adenosylmethionine to HAA, but does not catalyze the methylation of 3-hydroxy-DL-kynurenine. Enzyme, substrate, time, and pH dependencies for the methyl transfer reaction were examined. Reaction products obtained from scaled-up reaction mixtures were fractionated by chromatography on Dowex 1, and the Dowex 1 fractions were examined by paper and thin-layer chromatography. One Dowex fraction was shown to contain a radioactive product with the chromatographic properties of 4-methyl-3-hydroxyanthranilic acid (MHA), a known intermediate in the biosynthesis of actinomycin. Available evidence indicates that the conversion of HAA to MHA is an early step in the biosynthesis of actinomycin by S. antibioticus and other actinomycin-producing streptomycetes.

[Inhibition of oleandomycin synthesis by glucoses added during fermentation]

Oleandomycin biosynthesis by Streptomyces antibioticus is repressed by glucose added to the growth medium in the process of fermentation. Phosphotransferase involved in the synthesis of acetyl CoA and propionyl CoA (the precursors of the antibiotic macrolactone ring) is neither inhibited nor repressed, and the substrate specificity of the enzyme does not change. The content of cAMP in the mycelium of S. antibioticus does not change significantly when either glucose or sucrose is added to the medium 24 h after the inoculation whereas the content of exogenous cAMP rises abruptly 24 h after glucose addition. At the same time, the medium becomes much more acidic and the content of protein in the mycelium rises noticeably. Consequently, cAMP may be involved in the regulation of the culture growth.