From roads to biobanks: Roadkill animals as a valuable source of genetic data

To protect biodiversity we must understand its structure and composition including the bacteria and microparasites associated with wildlife, which may pose risks to human health. However, acquiring this knowledge often presents challenges, particularly in areas of high biodiversity where there are many undescribed and poorly studied species and funding resources can be limited. A solution to fill this knowledge gap is sampling roadkill (animals that die on roads as a result of collisions with circulating vehicles). These specimens can help characterize local wildlife and their associated parasites with fewer ethical and logistical challenges compared to traditional specimen collection. Here we test this approach by analyzing 817 tissue samples obtained from 590 roadkill vertebrate specimens (Amphibia, Reptilia, Aves and Mammalia) collected in roads within the Tropical Andes of Ecuador. First, we tested if the quantity and quality of recovered DNA varied across roadkill specimens collected at different times since death, exploring if decomposition affected the potential to identify vertebrate species and associated microorganisms. Second, we compared DNA stability across taxa and tissues to identify potential limitations and offer recommendations for future work. Finally, we illustrate how these samples can aid in taxonomic identification and parasite detection. Our study shows that sampling roadkill can help study biodiversity. DNA was recovered and amplified (allowing species identification and parasite detection) from roadkill even 120 hours after death, although risk of degradation increased overtime. DNA was extracted from all vertebrate classes but in smaller quantities and with lower quality from amphibians. We recommend sampling liver if possible as it produced the highest amounts of DNA (muscle produced the lowest). Additional testing of this approach in areas with different environmental and traffic conditions is needed, but our results show that sampling roadkill specimens can help detect and potentially monitor biodiversity and could be a valuable approach to create biobanks and preserve genetic data.

Mamíferos medianos y grandes del Área Nacional de Recreación Isla Santay en el occidente de Ecuador

Isla Santay es una importante área ecuatoriana de conservación para especies nativas, pero que posee poca información sobre mamíferos. Entre agosto de 2018 y enero 2019, la riqueza y abundancia de mamíferos medianos y grandes fue estudiada en dos zonas con diferente uso de suelo: no disturbada, y disturbada. La riqueza de especies fue cuantificada mediante una combinación de fototrampeo, observación directa y evidencias indirectas (rastros, heces, etc.). Registramos seis especies en ambas zonas, de las cuales tres están incluidas en la lista roja de mamíferos del Ecuador. En la zona no disturbada, Leopardus pardalis fue la especie más frecuente y mostró más actividad; mientras que esta misma especie junto con Procyon cancrivorus lo fueron en la zona intervenida; Lontra longicaudis y Philander melanurus fueron registradas por primera vez para la reserva. Especies esperadas como Didelphis marsupialis, Nasua nasua, Eira barbara, Galictis vittata, y Herpailurus yaguarondi estuvieron ausentes. Isla Santay presentó una baja riqueza de especies, probablemente debido a amenazas como la contaminación ambiental y aislamiento geográfico. A pesar de esto, Isla Santay ayuda en la protección de mamíferos en la región, especialmente para especies amenazadas. Futuras investigaciones deben priorizar la preservación de los procesos ecológicos y a entender el efecto negativo de los impactos antropogénicos en su biodiversidad.

Área de vida de la especia invasora Achatina fulica Browdich, 1822 (Gastropoda: Achatinidae) en un área de conservación de bosque seco ecuatoriano

Achatina fulica es un gasterópodo terrestre invasivo, conocido como una de las 100 especies invasoras más dañinas del mundo, el cual arribó a Ecuador hace 10 años, pero no se ha evaluado aún su impacto sobre los ecosistemas nativos. El objetivo principal fue determinar el área de vida (AV) promedio de esta especie en dos zonas con distinto grado de intervención en el Bosque Protector Cerro Blanco, un remanente de bosque seco tropical. La fase de campo consistió en la captura, marcaje, recaptura, toma de medidas morfométricas y georreferenciación de los individuos; para el análisis de datos se calculó el AV mediante el polígono convexo, y se lo correlacionó con variables ambientales a través de un análisis de componentes principales (ACP). El AV promedio en la zona alterada fue de 3,58 m2 (±0,93; n=30), y en el sendero ecoturístico fue 3,27 m2 (±0,48; n=40); se determinó que la humedad fue el parámetro ambiental que influyó directamente sobre el AV y la densidad poblacional en ambas zonas de estudio. El manejo de esta especie invasora no consta como un asunto clave de manejo para esta reserva privada, por lo que se recomienda ejecutar acciones de control para su erradicación

Production of landomycins in Streptomyces globisporus 1912 and S cyanogenus S136 is regulated by genes encoding putative transcriptional activators

The regulatory genes lanI and lndI have been cloned from the landomycin A (LaA) producer Streptomyces cyanogenus S136 and from the landomycin E (LaE) producer Streptomyces globisporus 1912, respectively and both have been sequenced. A culture of S. globisporus I2-1 carrying a disrupted lndI gene did not produce LaE and other related intermediates. Complementation of S. globisporus I2-1 with either the lndI or lanI gene reconstituted LaE production indicating that LanI and LndI are involved in activation of structural genes in the respective clusters. Structural features of these regulatory genes are discussed.

Altering the glycosylation pattern of bioactive compounds

Many bioactive natural products are glycosylated compounds in which the sugars are important or essential for biological activity. The isolation of several sugar biosynthesis gene clusters and glycosyltransferases from different antibiotic-producing organisms, and the increasing knowledge about these biosynthetic pathways opens up the possibility of generating novel bioactive compounds through combinatorial biosynthesis in the near future. Recent advances in this area indicate that antibiotic glycosyltransferases show some substrate flexibility that might allow us to alter the types of sugar transferred to the different aglycons or, less frequently, to change the position of its attachment.

Functional analysis of OleY L-oleandrosyl 3-O-methyltransferase of the oleandomycin biosynthetic pathway in Streptomyces antibioticus

Oleandomycin, a macrolide antibiotic produced by Streptomyces antibioticus, contains two sugars attached to the aglycon: L-oleandrose and D-desosamine. oleY codes for a methyltransferase involved in the biosynthesis of L-oleandrose. This gene was overexpressed in Escherichia coli to form inclusion bodies and in Streptomyces lividans, producing a soluble protein. S. lividans overexpressing oleY was used as a biotransformation host, and it converted the precursor L-olivosyl-erythronolide B into its 3-O-methylated derivative, L-oleandrosyl-erythronolide B. Two other monoglycosylated derivatives were also substrates for the OleY methyltransferase: L-rhamnosyl- and L-mycarosyl-erythronolide B. OleY methyltransferase was purified yielding a 43-kDa single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The native enzyme showed a molecular mass of 87 kDa by gel filtration chromatography, indicating that the enzyme acts as a dimer. It showed a narrow pH range for optimal activity, and its activity was clearly stimulated by the presence of several divalent cations, being maximal with Co(2+). The S. antibioticus OleG2 glycosyltransferase is proposed to transfer L-olivose to the oleandolide aglycon, which is then converted into L-oleandrose by the OleY methyltransferase. This represents an alternative route for L-oleandrose biosynthesis from that in the avermectin producer Streptomyces avermitilis, in which L-oleandrose is transferred to the aglycon by a glycosyltransferase.

Deoxysugar Methylation during Biosynthesis of the Antitumor Polyketide Elloramycin by Streptomyces olivaceus

The anthracycline-like polyketide drug elloramycin is produced by Streptomyces olivaceus Tü2353. Elloramycin has antibacterial activity against Gram-positive bacteria and also exhibits antitumor activity. From a cosmid clone (cos16F4) containing part of the elloramycin biosynthesis gene cluster, three genes (elmMI, elmMII, and elmMIII) have been cloned. Sequence analysis and data base comparison showed that their deduced products resembled S-adenosylmethionine-dependent O-methyltransferases. The genes were individually expressed in Streptomyces albus and also coexpressed with genes involved in the biosynthesis of l-rhamnose, the 6-deoxysugar attached to the elloramycin aglycon. The resulting recombinant strains were used to biotransform three different elloramycin-type compounds: l-rhamnosyl-tetracenomycin C, l-olivosyl-tetracenomycin C, and l-oleandrosyl-tetracenomycin, which differ in their 2'-, 3'-, and 4'-substituents of the sugar moieties. When only the three methyltransferase-encoding genes elmMI, elmMII, and elmMIII were individually expressed in S. albus, the methylating activity of the three methyltransferases was also assayed in vitro using various externally added glycosylated substrates. From the combined results of all of these experiments, it is proposed that methyltransferases ElmMI, ElmMII, and ElmMIII are involved in the biosynthesis of the permethylated l-rhamnose moiety of elloramycin. ElmMI, ElmMII, and ElmMIII are responsible for the consecutive methylation of the hydroxy groups at the 2'-, 3'-, and 4'-position, respectively, after the sugar moiety has been attached to the aglycon.

The role of ABC transporters in antibiotic-producing organisms: drug secretion and resistance mechanisms

Knowledge about biosynthetic gene clusters from antibiotic-producing actinomycetes is continuously increasing and the presence of an ABC transporter system is a fairly general phenomenon in most of these clusters. These transporters are involved in the secretion of the antibiotic through the cell membrane and also contribute to self resistance to the produced antibiotic.

Identification of a sugar flexible glycosyltransferase from Streptomyces olivaceus, the producer of the antitumor polyketide elloramycin

BACKGROUND

Elloramycin is an anthracycline-like antitumor drug related to tetracenomycin C which is produced by Streptomyces olivaceus Tü2353. Structurally is a tetracyclic aromatic polyketide derived from the condensation of 10 acetate units. Its chromophoric aglycon is glycosylated with a permethylated L-rhamnose moiety at the C-8 hydroxy group. Only limited information is available about the genes involved in the biosynthesis of elloramycin. From a library of chromosomal DNA from S. olivaceus, a cosmid (16F4) was isolated that contains part of the elloramycin gene cluster and when expressed in Streptomyces lividans resulted in the production of a non-glycosylated intermediate in elloramycin biosynthesis, 8-demethyl-tetracenomycin C (8-DMTC).

RESULTS

The expression of cosmid 16F4 in several producers of glycosylated antibiotics has been shown to produce tetracenomycin derivatives containing different 6-deoxysugars. Different experimental approaches showed that the glycosyltransferase gene involved in these glycosylation events was located in 16F4. Using degenerated oligoprimers derived from conserved amino acid sequences in glycosyltransferases, the gene encoding this sugar flexible glycosyltransferase (elmGT) has been identified. After expression of elmGT in Streptomyces albus under the control of the erythromycin resistance promoter, ermEp, it was shown that elmG can transfer different monosaccharides (both L- and D-sugars) and a disaccharide to 8-DMTC. Formation of a diolivosyl derivative in the mithramycin producer Streptomyces argillaceus was found to require the cooperative action of two mithramycin glycosyltransferases (MtmGI and MtmGII) responsible for the formation of the diolivosyl disaccharide, which is then transferred by ElmGT to 8-DMTC.

CONCLUSIONS

The ElmGT glycosyltransferase from S. olivaceus Tü2353 can transfer different sugars into the aglycon 8-DMTC. In addition to its natural sugar substrate L-rhamnose, ElmGT can transfer several L- and D-sugars and also a diolivosyl disaccharide into the aglycon 8-DMTC. ElmGT is an example of sugar flexible glycosyltransferase and can represent an important tool for combinatorial biosynthesis.

An ATP-binding cassette transporter and two rRNA methyltransferases are involved in resistance to avilamycin in the producer organism Streptomyces viridochromogenes Tü57

Three different resistance factors from the avilamycin biosynthetic gene cluster of Streptomyces viridochromogenes Tü57, which confer avilamycin resistance when expressed in Streptomyces lividans TK66, were isolated. Analysis of the deduced amino acid sequences showed that AviABC1 is similar to a large family of ATP-binding transporter proteins and that AviABC2 resembles hydrophobic transmembrane proteins known to act jointly with the ATP-binding proteins. The deduced amino acid sequence of aviRb showed similarity to those of other rRNA methyltransferases, and AviRa did not resemble any protein in the databases. Independent expression in S. lividans TK66 of aviABC1 plus aviABC2, aviRa, or aviRb conferred different levels of resistance to avilamycin: 5, 10, or 250 microg/ml, respectively. When either aviRa plus aviRb or aviRa plus aviRb plus aviABC1 plus aviABC2 was coexpressed in S. lividans TK66, avilamycin resistance levels reached more than 250 microg/ml. Avilamycin A inhibited poly(U)-directed polyphenylalanine synthesis in an in vitro system using ribosomes of S. lividans TK66(pUWL201) (GWO), S. lividans TK66(pUWL201-Ra) (GWRa), or S. lividans TK66(pUWL201-Rb) (GWRb), whereas ribosomes of S. lividans TK66 containing pUWL201-Ra+Rb (GWRaRb) were highly resistant. aviRa and aviRb were expressed in Escherichia coli, and both enzymes were purified as fusion proteins to near homogeneity. Both enzymes showed rRNA methyltransferase activity using a mixture of 16S and 23S rRNAs from E. coli as the substrate. Coincubation experiments revealed that the enzymes methylate different positions of rRNA.

The mtmVUC genes of the mithramycin gene cluster in Streptomyces argillaceus are involved in the biosynthesis of the sugar moieties

Mithramycin is a glycosylated aromatic polyketide produced by Streptomyces argillaceus, and is used as an antitumor drug. Three genes (mtmV, mtmU and mtmC) from the mithramycin gene cluster have been cloned, and characterized by DNA sequencing and by analysis of the products that accumulate in nonproducing mutants, which were generated by insertional inactivation of these genes. The mtm V gene codes for a 2,3-dehydratase that catalyzes early and common steps in the biosynthesis of the three sugars found in mithramycin (D-olivose, D-oliose and D-mycarose); its inactivation caused the accumulation of the nonglycosylated intermediate premithramycinone. The mtmU gene codes for a 4-ketoreductase involved in D-oliose biosynthesis, and its inactivation resulted in the accumulation of premithramycinone and premithramycin A , the first glycosylated intermediate which contains a D-olivose unit. The third gene, mtmC, is involved in D-mycarose biosynthesis and codes for a C-methyltransferase. Two mutants with lesions in the mtmC gene accumulated mithramycin intermediates lacking the D-mycarose moiety but containing D-olivose units attached to C-12a in which the 4-keto group is unreduced. This suggests that mtmC could code for a second enzyme activity, probably a D-olivose 4-ketoreductase, and that the glycosyltransferase responsible for the incorporation of D-olivose (MtmGIV) shows some degree of flexibility with respect to its sugar co-substrate, since the 4-ketoanalog is also transferred. A pathway is proposed for the biosynthesis of the three sugar moieties in mithramycin.

Identification of a growth phase-dependent promoter in the rplJL operon of Streptomyces coelicolor A3(2)

A single promoter, rplJp (P(L10)), has been identified in the rplJL operon from Streptomyces coelicolor A3(2) by promoter probe and primer extension analyses. P(L10) is located upstream of the rplL gene and of the DNA encoding the mRNA leader region that contains the putative L10 (or L10.L12(4)) binding site for translational autogenous regulation. The potential start point for transcription was found 239 nucleotides upstream of the predicted translational start codon of rplJ. The promoter sequence shows -35 and -10 hexamers that resemble those of Streptomyces consensus Escherichia coli sigma(70)-like promoters and the rplJp from Streptomyces griseus. The amount of the transcript detected by primer extension analysis decreases during growth immediately after the transition phase, a slowdown in growth occurring during exponential phase associated with increases in ppGpp level. The temporal pattern of transcripts shows a clear correlation with the temporal pattern of L10 and L7/L12 protein synthesis reported in previous kinetic studies. This indicates that P(L10) is a growth phase-dependent promoter which may contribute, together with translational regulation, to the decrease in the synthesis of L10 and L7/L12 observed in liquid minimal medium. This is supported by results of promoter probe experiments. Although no significant promoter activity has been found by promoter probing in the rplJ and rplL intergenic region, an additional 5'-transcript end was detected by primer extension, probably as a result of mRNA processing event from a longer transcript. This may be required to maintain the 1:4 ratio observed for L10 and L7/L12 in the ribosomes.

Structure alteration of polyketides by recombinant DNA technology in producer organisms--prospects for the generation of novel pharmaceutical drugs

Actinomycetes are gram-positive bacteria and commercially important microorganisms. They are producers of approximately two thirds of all bioactive compounds known and they produce a great variety of compounds which have clinical application on the basis of their activity against different kinds of organisms and cells as antibacterial (macrolides, avermectins), antitumor (anthracyclines, angucyclines, aureolic acid group) and also compounds showing immunosuppresant activity (rapamycin, FK506). Most of these clinically useful pharmaceuticals produced by actinomycetes belong to the polyketide family. Polyketides comprise a wide family of chemically diverse compounds, many of which have shown bioactivity. The development of recombinant DNA technology has opened a new and exciting field of research for the generation of new bioactive compounds through genetic manipulation of the biosynthetic pathways. Researchers in this area are trying to take advantage of the enormous capability of actinomycetes to produce pharmaceutically useful compounds in order to manipulate the different biosynthetic pathways and subsequently generate novel drugs. Combinatorial biosynthesis is now emerging as a powerful tool to generate novel families of compounds by interchanging secondary metabolism genes between bioactive producing actinomycetes. Novel compounds will be the consequence of the concerted action of enzymes from different, but related, biosynthetic pathways. Insertional inactivation of selected genes and tailoring modification may also produce novel compounds that can be useful pharmaceuticals or lead compounds for further chemical modification. This minireview will present the state of the art in this field showing the different polyketides biosynthetic pathways so far characterized and how the identified genes are being used to generate structural biodiversity. Emphasis will be made on the polyketide family including type I and type II polyketides.

Inversion of the anomeric configuration of the transferred sugar during inactivation of the macrolide antibiotic oleandomycin catalyzed by a macrolide glycosyltransferase

Macrolides are a group of antibiotics structurally characterized by a macrocyclic lactone to which one or several deoxy-sugar moieties are attached. The sugar moieties are transferred to the different aglycones by glycosyltransferases (GTF). The OleI GTF of an oleandomycin producer, Streptomyces antibioticus, catalyzes the inactivation of this macrolide by glycosylation. The product of this reaction was isolated and its structure elucidated. The donor substrate of the reaction was UDP-alpha-D-glucose, but the reaction product showed a beta-glycosidic linkage. The inversion of the anomeric configuration of the transferred sugar and other data about the kinetics of the reaction and primary structure analysis of several GTFs are compatible with a reaction mechanism involving a single nucleophilic substitution at the sugar anomeric carbon in the catalytic center of the enzyme.

Generation of hybrid elloramycin analogs by combinatorial biosynthesis using genes from anthracycline-type and macrolide biosynthetic pathways

Elloramycin and oleandomycin are two polyketide compounds produced by Streptomyces olivaceus Tü2353 and Streptomyces antibioticus ATCC11891, respectively. Elloramycin is an anthracycline-like antitumor drug and oleandomycin a macrolide antibiotic. Expression in S. albus of a cosmid (cos16F4) containing part of the elloramycin biosynthetic gene cluster produced the elloramycin non-glycosylated intermediate 8-demethyl-tetracenomycin C. Several plasmid constructs harboring different gene combinations of L-oleandrose (neutral 2,6-dideoxyhexose attached to the macrolide antibiotic oleandomycin) biosynthetic genes of S. antibioticus that direct the biosynthesis of L-olivose, L-oleandrose and L-rhamnose were coexpressed with cos16F4 in S. albus. Three new hybrid elloramycin analogs were produced by these recombinant strains through combinatorial biosynthesis, containing elloramycinone or 12a-demethyl-elloramycinone (= 8-demethyl-tetracenomycin C) as aglycone moiety encoded by S. olivaceus genes and different sugar moieties, coded by the S. antibioticus genes. Among them is L-olivose, which is here described for the first time as a sugar moiety of a natural product.

Insertional inactivation of mtrX and mtrY genes from the mithramycin gene cluster affects production and growth of the producer organism Streptomyces argillaceus

Mithramycin is an antitumor aromatic polyketide synthesized by Streptomyces argillaceus. Two genes (mtrX and mtrY) of the mithramycin gene cluster were inactivated by gene replacement. Inactivation of mtrX, that encodes an ABC excision nuclease system for DNA repair, produced a mutant that was affected in the normal rate of growth. Expression of mtrX in Streptomyces albus in a multicopy plasmid vector conferred a low increase in resistance to mithramycin. Inactivation of mtrY, that encodes a protein of unknown function, produced a 50% decrease in mithramycin biosynthesis. When mtrY was expressed in the wild-type S. argillaceus in a multicopy plasmid, this caused about 47% increase in the levels of mithramycin production. It is proposed that mtrX and mtrY could code for a secondary defense mechanism and a mithramycin regulatory element, respectively.

Identification and expression of genes involved in biosynthesis of L-oleandrose and its intermediate L-olivose in the oleandomycin producer Streptomyces antibioticus

A 9.8-kb DNA region from the oleandomycin gene cluster in Streptomyces antibioticus was cloned. Sequence analysis revealed the presence of 8 open reading frames encoding different enzyme activities involved in the biosynthesis of one of the two 2, 6-deoxysugars attached to the oleandomycin aglycone: L-oleandrose (the oleW, oleV, oleL, and oleU genes) and D-desosamine (the oleNI and oleT genes), or of both (the oleS and oleE genes). A Streptomyces albus strain harboring the oleG2 glycosyltransferase gene integrated into the chromosome was constructed. This strain was transformed with two different plasmid constructs (pOLV and pOLE) containing a set of genes proposed to be required for the biosynthesis of dTDP-L-olivose and dTDP-L-oleandrose, respectively. Incubation of these recombinant strains with the erythromycin aglycon (erythronolide B) gave rise to two new glycosylated compounds, identified as L-3-O-olivosyl- and L-3-O-oleandrosyl-erythronolide B, indicating that pOLV and pOLE encode all enzyme activities required for the biosynthesis of these two 2,6-dideoxysugars. A pathway is proposed for the biosynthesis of these two deoxysugars in S. antibioticus.

Glycosylation of macrolide antibiotics. Purification and kinetic studies of a macrolide glycosyltransferase from Streptomyces antibioticus

The oleD gene has been identified in the oleandomycin producer Streptomyces antibioticus and it codes a macrolide glycosyltransferase that is able to transfer a glucose moiety from UDP-glucose (UDP-Glc) to many macrolides. The glycosyltransferase coded by the oleD gene has been purified 371-fold from a Streptomyces lividans clone expressing this protein. The reaction product was isolated, and its structure determined by NMR spectroscopy. The kinetic mechanism of the reaction was analyzed using the macrolide antibiotic lankamycin (LK) as substrate. The reaction operates via a compulsory order mechanism. This has been shown by steady-state kinetic studies and by isotopic exchange reactions at equilibrium. LK 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 lankamycin (GS-LK). A pH study of the reaction was performed to determine values for the molecular pK values, suggesting possible amino acid residues involved in the catalytic process.

Characterization of two polyketide methyltransferases involved in the biosynthesis of the antitumor drug mithramycin by Streptomyces argillaceus

A DNA chromosomal region of Streptomyces argillaceus ATCC 12596, the producer organism of the antitumor polyketide drug mithramycin, was cloned. Sequence analysis of this DNA region, located between four mithramycin glycosyltransferase genes, showed the presence of two genes (mtmMI and mtmMII) whose deduced products resembled S-adenosylmethionine-dependent methyltransferases. By independent insertional inactivation of both genes nonproducing mutants were generated that accumulated different mithramycin biosynthetic intermediates. The M3DeltaMI mutant (mtmMI-minus mutant) accumulated 4-demethylpremithramycinone (4-DPMC) which lacks the methyl groups at carbons 4 and 9. The M3DeltaM2 (mtmMII-minus mutant) accumulated 9-demethylpremithramycin A3 (9-DPMA3), premithramycin A1 (PMA1), and 7-demethylmithramycin, all of them containing the O-methyl group at C-4 and C-1', respectively, but lacking the methyl group at the aromatic position. Both genes were expressed in Streptomyces lividans TK21 under the control of the erythromycin resistance promoter (ermEp) of Saccharopolyspora erythraea. Cell-free extracts of these clones were precipitated with ammonium sulfate (90% saturation) and assayed for methylation activity using different mithramycin intermediates as substrates. Extracts of strains MJM1 (expressing the mtmMI gene) and MJM2 (expressing the mtmMII gene) catalyzed efficient transfer of tritium from [(3)H]S-adenosylmethionine into 4-DPMC and 9-DPMA3, respectively, being unable to methylate other intermediates at a detectable level. These results demonstrate that the mtmMI and mtmMII genes code for two S-adenosylmethionine-dependent methyltransferases responsible for the 4-O-methylation and 9-C-methylation steps of the biosynthetic precursors 4-DPMC and 9-DPMA3, respectively, of the antitumor drug mithramycin. A pathway is proposed for the last steps in the biosynthesis of mithramycin involving these methylation events.

Characterization of two glycosyltransferases involved in early glycosylation steps during biosynthesis of the antitumor polyketide mithramycin by Streptomyces argillaceus

A 2,580-bp region of the chromosome of Streptomyces argillaceus, the producer of the antitumor polyketide mithramycin, was sequenced. Analysis of the nucleotide sequence revealed the presence of two genes (mtmGIII and mtmGIV) encoding proteins that showed a high degree of similarity to glycosyltransferases involved in the biosynthesis of various antibiotics and antitumor drugs. Independent insertional inactivation of both genes produced mutants that did not synthesize mithramycin but accumulated several mithramycin intermediates. Both mutants accumulated premithramycinone, a non-glycosylated intermediate in mithramycin biosynthesis. The mutant affected in the mtmGIII gene also accumulated premithramycin A1, which contains premithramycinone as the aglycon unit and a D-olivose attached at C-12a-O. These experiments demonstrate that the glycosyltransferases MtmGIV and MtmGIII catalyze the first two glycosylation steps in mithramycin biosynthesis. A model is proposed for the glycosylation steps in mithramycin biosynthesis.

Interspecies complementation in Saccharopolyspora erythraea : elucidation of the function of oleP1, oleG1 and oleG2 from the oleandomycin biosynthetic gene cluster of Streptomyces antibioticus and generation of new erythromycin derivatives

Two glycosyltransferase genes, oleG1 and oleG2, and a putative isomerase gene, oleP1, have previously been identified in the oleandomycin biosynthetic gene cluster of Streptomyces antibioticus. In order to identify which of these two glycosyltransferases encodes the desosaminyltransferase and which the oleandrosyltransferase, interspecies complementation has been carried out, using two mutant strains of Saccharopolyspora erythraea, one strain carrying an internal deletion in the eryCIII (desosaminyltransferase) gene and the other an internal deletion in the eryBV (mycarosyltransferase) gene. Expression of the oleG1 gene in the eryCIII deletion mutant restored the production of erythromycin A (although at a low level), demonstrating that oleG1 encodes the desosaminyltransferase required for the biosynthesis of oleandomycin and indicating that, as in erythromycin biosynthesis, the neutral sugar is transferred before the aminosugar onto the macrocyclic ring. Significantly, when an intact oleG2 gene (presumed to encode the oleandrosyltransferase) was expressed in the eryBV deletion mutant, antibiotic activity was also restored and, in addition to erythromycin A, new bioactive compounds were produced with a good yield. The neutral sugar residue present in these compounds was identified as L-rhamnose attached at position C-3 of an erythronolide B or a 6-deoxyerythronolide B lactone ring, thus indicating a relaxed specificity of the oleandrosyltransferase, OleG2, for both the activated sugar and the macrolactone substrate. The oleP1 gene located immediately upstream of oleG1 was likewise introduced into an eryCII deletion mutant of Sac. erythraea, and production of erythromycin A was again restored, demonstrating that the function of OleP1 is identical to that of EryCII in the biosynthesis of dTDP-D-desosamine, which we have previously proposed to be a dTDP-4-keto-6-deoxy-D-glucose 3, 4-isomerase.

Analysis of two chromosomal regions adjacent to genes for a type II polyketide synthase involved in the biosynthesis of the antitumor polyketide mithramycin in Streptomyces argillaceus

Mithramycin is an aromatic antitumour polyketide synthesized by Streptomyces argillaceus. Two chromosomal regions located upstream and downstream of the locus for the mithramycin type II polyketide synthase were cloned and sequenced. Analysis of the sequence revealed the presence of eight genes encoding three oxygenases (mtmOI, mtmOII and mtmOIII), three reductases (mtmTI, mtmTII and mtmTIII), a cyclase (mtm Y) and an acyl CoA ligase (mtmL). The three oxygenase genes were each inactivated by gene replacement. Inactivation of one of them (mtmOII) generated a non-producing mutant, while inactivation of the other two (mtmOl and mtmOIII) did not affect the biosynthesis of mithramycin. The mtmOII gene may code for an oxygenase responsible for the introduction of oxygen atoms at early steps in the biosynthesis of mithramycin leading to 4-demethylpremithramycinone. One of the reductases may be responsible for reductive cleavage of an intermediate from an enzyme and another for the reduction of a keto group in the side-chain of the mithramycin aglycon moiety. A hypothetical biosynthetic pathway showing in particular the involvement of oxygenase MtmOII and of various other gene products in mithramycin biosynthesis is proposed.

The mithramycin gene cluster of Streptomyces argillaceus contains a positive regulatory gene and two repeated DNA sequences that are located at both ends of the cluster

Sequencing of a 4.3-kb DNA region from the chromosome of Streptomyces argillaceus, a mithramycin producer, revealed the presence of two open reading frames (ORFs). The first one (orfA) codes for a protein that resembles several transport proteins. The second one (mtmR) codes for a protein similar to positive regulators involved in antibiotic biosynthesis (DnrI, SnoA, ActII-orf4, CcaR, and RedD) belonging to the Streptomyces antibiotic regulatory protein (SARP) family. Both ORFs are separated by a 1.9-kb, apparently noncoding region. Replacement of the mtmR region by an antibiotic resistance cassette completely abolished mithramycin biosynthesis. Expression of mtmR in a high-copy-number vector in S. argillaceus caused a 16-fold increase in mithramycin production. The mtmR gene restored actinorhodin production in Streptomyces coelicolor JF1 mutant, in which the actinorhodin-specific activator ActII-orf4 is inactive, and also stimulated actinorhodin production by Streptomyces lividans TK21. A 241-bp region located 1.9 kb upstream of mtmR was found to be repeated approximately 50 kb downstream of mtmR at the other end of the mithramycin gene cluster. A model to explain a possible route for the acquisition of the mithramycin gene cluster by S. argillaceus is proposed.

Oxidative cleavage of premithramycin B is one of the last steps in the biosynthesis of the antitumor drug mithramycin

BACKGROUND

Mithramycin is a member of the clinically important aureolic acid group of antitumor drugs that interact with GC-rich regions of DNA nonintercalatively. These drugs contain a chromophore aglycon that is derived from condensation of ten acetate units (catalyzed by a type II polyketide synthase). The aglycones are glycosylated at two positions with different chain length deoxyoligosaccharides, which are essential for the antitumor activity. During the early stages of mithramycin biosynthesis, tetracyclic intermediates of the tetracycline-type occur, which must be converted at later stages into the tricyclic glycosylated molecule, presumably through oxidative breakage of the fourth ring.

RESULTS

Two intermediates in the mithramycin biosynthetic pathway, 4-demethyl-premithramycinone and premithramycin B, were identified in a mutant lacking the mithramycin glycosyltransferase and methyltransferase genes and in the same mutant complemented with the deleted genes, respectively. Premithramycin B contains five deoxysugars moieties (like mithramycin), but contains a tetracyclic aglycon moiety instead of a tricyclic aglycon. We hypothesized that transcription of mtmOIV (encoding an oxygenase) was impaired in this strain, preventing oxidative breakage of the fourth ring of premithramycin B. Inactivating mtmOIV generated a mithramycin nonproducing mutant that accumulated premithramycin B instead of mithramycin. In vitro assays demonstrated that MtmOIV converted premithramycin B into a tricyclic compound.

CONCLUSIONS

In the late stages of mithramycin biosynthesis by Strepyomyces argillaceus, a fully glycosylated tetracyclic tetracycline-like intermediate (premithramycin B) is converted into a tricyclic compound by the oxygenase MtmOIV. This oxygenase inserts an oxygen (Baeyer-Villiger oxidation) and opens the resulting lactone. The following decarboxylation and ketoreduction steps lead to mithramycin. Opening of the fourth ring represents one of the last steps in mithramycin biosynthesis.

The structure of mithramycin reinvestigated

A reinvestigation of the structure of mithramycin, the principal product of Streptomyces argillaceus ATCC 12956, is reported. The structure elucidation was carried out with mithramycin decaacetate (4) using 2D NMR methods, including TOCSY, HMBC, and HSQC experiments. The work resulted in structure 3being confirmed for mithramycin.

Genetic manipulation of antitumor-agent biosynthesis to produce novel drugs

Current methods of obtaining novel drugs may be complemented in the near future by the genetic engineering of antitumor-agent biosynthesis in microorganisms. Biosynthetic gene clusters from several antitumor pathways in actinomycetes are presently being characterized and expressed in order to generate novel drugs. Several novel hydroxylated and glycosylated antitumor-drug derivatives have been produced that show a relaxed substrate specificity for secondary-metabolic enzymes, which opens up the possibility of generating novel drugs by genetic manipulation.

Identification of two genes from Streptomyces argillaceus encoding glycosyltransferases involved in transfer of a disaccharide during biosynthesis of the antitumor drug mithramycin

Mithramycin is an antitumor polyketide drug produced by Streptomyces argillaceus that contains two deoxysugar chains, a disaccharide consisting of two D-olivoses and a trisaccharide consisting of a D-olivose, a D-oliose, and a D-mycarose. From a cosmid clone (cosAR3) which confers resistance to mithramycin in streptomycetes, a 3-kb PstI-XhoI fragment was sequenced, and two divergent genes (mtmGI and mtmGII) were identified. Comparison of the deduced products of both genes with proteins in databases showed similarities with glycosyltransferases and glucuronosyltransferases from different sources, including several glycosyltransferases involved in sugar transfer during antibiotic biosynthesis. Both genes were independently inactivated by gene replacement, and the mutants generated (M3G1 and M3G2) did not produce mithramycin. High-performance liquid chromatography analysis of ethyl acetate extracts of culture supernatants of both mutants showed the presence of several peaks with the characteristic spectra of mithramycin biosynthetic intermediates. Four compounds were isolated from both mutants by preparative high-performance liquid chromatography, and their structures were elucidated by physicochemical methods. The structures of these compounds were identical in both mutants, and the compounds are suggested to be glycosylated intermediates of mithramycin biosynthesis with different numbers of sugar moieties attached to C-12a-O of a tetracyclic mithramycin precursor and to C-2-O of mithramycinone: three tetracyclic intermediates containing one sugar (premithramycin A1), two sugars (premithramycin A2), or three sugars (premithramycin A3) and one tricyclic intermediate containing a trisaccharide chain (premithramycin A4). It is proposed that the glycosyltransferases encoded by mtmGI and mtmGII are responsible for forming and transferring the disaccharide during mithramycin biosynthesis. From the structures of the new metabolites, a new biosynthetic sequence regarding late steps of mithramycin biosynthesis can be suggested, a sequence which includes glycosyl transfer steps prior to the final shaping of the aglycone moiety of mithramycin.

Analysis of a Streptomyces antibioticus chromosomal region involved in oleandomycin biosynthesis, which encodes two glycosyltransferases responsible for glycosylation of the macrolactone ring

A 6-kb region from the chromosome of Streptomyces antibioticus, an oleandomycin producer, was cloned and sequenced. This region was located between the 3' end of the gene encoding the third subunit of the oleandomycin type I polyketide synthase and the oleP and oleB genes, which encode a cytochrome P450 monooxygenase and an oleandomycin resistance gene, respectively. Analysis of the nucleotide sequence revealed the presence of five genes encoding a cytochrome P450-like protein (oleP1), two glycosyltransferases (oleG1 and oleG2) involved in the transfer of the two 6-deoxysugars (L-oleandrose and D-desosamine) to the oleandomycin macrolactone ring, a methyltransferase (oleM1), and a gene (oleY) of unknown function. Insertional inactivation of this region by gene disruption generated an oleandomycin non-producing mutant which accumulated a compound that, according to mass spectrometry analysis, could correspond to the oleandomycin macrolactone ring (oleandolide), suggesting that the mutation affects oleandrosyl glycosyltransferase.

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.

ABC transporters in antibiotic-producing actinomycetes

Many antibiotic-producing actinomycetes possess at least one ABC (ATP-binding cassette) transporter which forms part of the antibiotic biosynthetic pathway and in most cases confers resistance to the drug in an heterologous host. Three types of antibiotic ABC transporters have been so far described in producer organisms. In Type I two genes are involved, one encoding a hydrophilic ATP-binding protein with one nucleotide-binding domain and the other encoding a hydrophobic membrane protein. In Type II transporters only a gene encoding the hydrophilic ATP-binding protein with two nucleotide-binding domains is present and no gene encoding a hydrophobic membrane protein has been found. In Type III only one gene is involved which encodes both the hydrophilic and hydrophobic components. Possibly these ABC transporters are responsible for secretion of the antibiotics outside the cells. A comparative analysis of the ATP-binding components of the different antibiotic ABC transporters and analysis of the amino acid distances between the so-called Walker motifs suggests that the three types of transporters have probably evolved from a common ancestor containing a single nucleotide-binding domain.

Cloning, sequencing and transcriptional analysis of a Streptomyces coelicolor operon containing the rplM and rpsI genes encoding ribosomal proteins ScoL13 and ScoS9

The N-terminal amino acid sequences of two peptides derived from a Streptomyces coelicolor ribosomal protein were determined and degenerate oligonucleotide primers derived from these sequences were used as probes for the screening of a chromosomal DNA library of S. coelicolor. Two positive clones were isolated and DNA sequencing of a 1740-bp region of these clones that hybridised with the probes revealed the presence of four genes, two of them incomplete. The deduced products of the two complete genes. rplM and rpsI, showed clear similarities to L13 and S9 ribosomal proteins from various organisms. Promoter-probe and primer extension experiments suggest that the two genes form a single transcriptional unit. The specific rate of synthesis of both proteins was high at early stages of growth but decreased later.

Cloning and insertional inactivation of Streptomyces argillaceus genes involved in the earliest steps of biosynthesis of the sugar moieties of the antitumor polyketide mithramycin

Two genes (mtmD and mtmE) were cloned and sequenced from the mithramycin producer Streptomyces argillaceus. Comparison with proteins in databases and enzymatic assays after expression in Escherichia coli showed that they encode a glucose-1-phosphate:TTP thymidylyl transferase and a TDP-D-glucose 4,6-dehydratase, respectively. The mtmD gene was inactivated by gene replacement, generating a nonproducing mutant that accumulates a tetracyclic compound designated premithramycinone. The identification of premithramycinone reveals new aspects of the mithramycin biosynthetic pathway and suggests that at least some glycosylations occur before breakage of the fourth ring.

Interaction between ATP, oleandomycin and the OleB ATP-binding cassette transporter of Streptomyces antibioticus involved in oleandomycin secretion

The OleB protein of Streptomyces antibioticus, oleandomycin (OM) producer, constitutes an ATP-binding cassette transporter containing two nucleotide-binding domains and is involved in OM resistance and its secretion in this producer strain. We have characterized some properties of the first nucleotide-binding domain of OleB using an overexpressed fusion protein (MBP-OleB') between a maltose-binding protein (MBP) and the first half of OleB (OleB'). Extrinsic fluorescence of the base-modified fluorescent nucleotide analogue 1,N6-ethenoadenosine 5'-triphosphate (epsilon ATP) and 2'(3')-o-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate was determined in the presence of MBP and the fusion protein MBP-OleB', and it was found that epsilon ATP binds to MBP-OleB' with a stoichiometry of 0.9. Measurements of the intrinsic fluorescence of the MBP-OleB' fusion protein indicated that ATP induces a decrease in the accessibility of the MBP-OleB' tryptophans to acrylamide, an indication of a folding effect. This conclusion was confirmed by the fact that ATP also induces considerable stabilization against guanidine chloride denaturation of MBP-OleB'. Two effects were found to be associated with the presence of Mg2+ ions: (1) an increase in the quenching of MBP-OleB' intrinsic fluorescence by ATP; and (2) an increase in the accessibility of MBP-OleB' tryptophans to acrylamide. Significant changes in the intrinsic fluorescence of the fusion protein were also observed in the presence of OM, demonstrating the existence of interaction between the transporter and the antibiotic in the absence of any hydrophobic membrane component.

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.

An ABC transporter is essential for resistance to the antitumor agent mithramycin in the producer Streptomyces argillaceus

Mithramycin is an antitumor antibiotic synthesized by Streptomyces argillaceus. This producer strain is highly resistant in vivo to mithramycin (MIC 100 micrograms/ml) but sensitive to the related drugs chromomycin and olivomycin (MIC 10 micrograms/ml). From a genomic library of S. argillaceus DNA two cosmid clones were isolated which confer a high level of resistance to mithramycin on S. albus. The resistance genes were mapped by subcloning to a 3.9-kb PstI-PvuII fragment. DNA sequence analysis of this fragment revealed one incomplete and three complete open reading frames. Subcloning experiments demonstrated that resistance to mithramycin is mediated by the genes mtrA and mtrB. The mtrA gene can potentially encode an ATP-binding protein of the ABC transporter superfamily, containing one nucleotide-binding domain and showing similarity with other ABC transporters involved in resistance to daunorubicin, oleandomycin and tetronasin in their respective producer strains. The mtrB gene codes for an integral membrane protein with six putative transmembrane helices. A mithramycin-sensitive mutant was generated in a gene replacement experiment by disrupting the mtrA gene, thus demonstrating that the system encoded by the mtrAB genes is essential for conferring resistance to mithramycin in S. argillaceus.

Characterization of Streptomyces argillaceus genes encoding a polyketide synthase involved in the biosynthesis of the antitumor mithramycin

Mithramycin (Mtm) is an aromatic polyketide which shows antibacterial and antitumor activity. From a chromosomal cosmid library of Streptomyces argillaceus, a Mtm producer, a clone (cosAR7) was isolated by homology to the actI/III region of S. coelicolor and the strDEM genes of S. griseus. From this clone, a 5.3-kb DNA region was sequenced and found to encode six open reading frames (designated as mtmQXPKST1), five of them transcribed in the same direction. The deduced products of five of these genes resembled components of type-II polyketide synthases. The mtm genes would code for an aromatase (mtmQ), a polypeptide of unknown function (mtmX), a beta-ketoacylsynthase (mtmP) and a related 'chain length factor' (mtmK), an acyl carrier protein (mtmS) and a beta-ketoreductase (mtmT1). The involvement of this gene cluster in Mtm biosynthesis was demonstrated by the Mtm non-producing phenotype of mutants generated in two independent insertional inactivation experiments.

Deciphering the biosynthetic origin of the aglycone of the aureolic acid group of anti-tumor agents

BACKGROUND

Mithramycin, chromomycin, and olivomycin belong to the aureolic acid family of clinically important anti-tumor agents. These natural products share a common aromatic aglycone. Although isotope labeling studies have firmly established the polyketide origin of this aglycone, they do not distinguish between alternative biosynthetic models in which the aglycone is derived from one, two or three distinct polyketide moieties. We set out to determine the biosynthetic origin of this moiety using a recombinant approach in which the ketosynthase and chain-length factor proteins from the antibiotic-producer strain, which determine the chain length of a polyketide, are produced in a heterologous bacterial host.

RESULTS

The ketosynthase and chain-length factor genes from the polyketide synthase gene cluster from the mithramycin producer, Streptomyces argillaceus ATCC 12956, and the acyl carrier protein and ketoreductase genes from the actinorhodin polyketide synthase were expressed in Streptomyces coelicolor CH999. The recombinant strain produced a 20-carbon polyketide, comprising the complete backbone of the aglycone of mithramycin.

CONCLUSIONS

The aglycone moieties of mithramycin, chromomycin, and olivomycin are derived from a single polyketide backbone. The nascent polyketide backbone must undergo a series of regiospecific cyclizations to form a tetracenomycin-like tetracyclic intermediate. The final steps in the aglycone biosynthetic pathway presumably involve decarboxylation and oxidative cleavage between C-18 and C-19, followed by additional oxidation, reduction, and methylation reactions.

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.

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.

Synthesis of ribosomal proteins during growth of Streptomyces coelicolor

Changes in expression of ribosomal protein genes during growth and stationary phase of Streptomyces coelicolor A3(2) in liquid medium were studied. Proteins being synthesized were pulse-labelled with [35S]-methionine, separated by two-dimensional polyacrylamide gel electrophoresis, and quantified using the BioImage computer software. Most of the ribosomal proteins were synthesized throughout the life cycle. Exceptions were two proteins whose synthesis drastically decreased at the approach of stationary phase. These two proteins were identified in purified ribosomes as homologues of Escherichia coli ribosomal proteins L10 and L7/L12, using antibodies raised against fusion proteins between these ribosomal proteins and Escherichia coli beta-galactosidase. The genes (rplJ and rplL) encoding the L10 and L7/L12 proteins were contained in a 1.2 kb BamHI fragment that was cloned and sequenced. The linkage and order of the genes coincide with other L10-L7/L12 operons. However, L11 and L1 genes were not present immediately upstream of the L10 gene, as is the case for E. coli and other bacteria. Instead, two open reading frames of unknown function were found immediately upstream of the L10 gene, in an adjacent 1.9 kb BamHI fragment.

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.

Characterisation of a Streptomyces antibioticus gene encoding a type I polyketide synthase which has an unusual coding sequence

A gene (ORFB) from Streptomyces antibioticus (an oleandomycin producer) encoding a large, multifunctional polyketide synthase (PKS) was cloned and sequenced. Its product shows an internal duplication and a close similarity to the third subunit of the PKS involved in erythromycin biosynthesis by Saccharopolyspora erythraea, showing the equivalent nine active site domains in the same order along the polypeptide. An unusual feature of this ORF is the GC content of most of the sequence, which is surprisingly low, for a Streptomyces gene; the large number of codons with T in the third position is particularly striking. The last 800 bp of the gene stand out as being normal in their GC content, this region corresponding almost exactly to the thioesterase domain of the gene and suggesting that this domain was a late addition to the PKS. Based on the high degree of similarity between the ORFB product and the third subunit of the erythromycin PKS and the occurrence nearby of a gene conferring oleandomycin resistance, it is possible that this gene might be involved in the biosynthesis of the oleandomycin lactone ring.

A hydroxylase-like gene product contributes to synthesis of a polyketide spore pigment in Streptomyces halstedii

A gene, schC, adjacent to the sch gene cluster encoding the biosynthesis of a polyketide spore pigment in Streptomyces halstedii was sequenced. Its deduced product resembled flavin adenine nucleotide-containing hydroxylases involved in the biosynthesis of polycyclic aromatic polyketide antibiotics and in catabolic pathways of aromatic compounds. When schC was disrupted, the normally green spores of S. halstedii became lilac. An schC-like gene was located in an equivalent position next to a large gene cluster (whiE) known to determine spore pigment in Streptomyces coelicolor A3(2).

Characterization of a Streptomyces antibioticus gene cluster encoding a glycosyltransferase involved in oleandomycin inactivation

By homology to the mgt gene (encoding a macrolide glycosyltransferase) from Streptomyces lividans, a 3.3-kb DNA fragment from the oleandomycin producer, Streptomyces antibioticus, was cloned and sequenced. Analysis of the sequence revealed the presence of the 3' end of a gene (ORF1) and two complete ORFs (ORF2 and oleD), all of them translationally coupled. The deduced product of the sequenced region of ORF1 contained the typical signature of integral membrane proteins responsible for the translocation of substrates across the membrane. The ORF2 product did not show significant similarity with proteins in databases, but contains an N-terminus leader peptide region characteristic of secreted proteins, and a lipid attachment site motif characteristic of membrane lipoproteins synthesized with a precursor signal peptide. The oleD product showed clear similarity with several UDP-glucuronosyl- and UDP-glycosyl-transferases from different origins and particularly with the mgt gene from S. lividans, and might encode a glycosyltransferase activity capable of inactivating macrolides. It is proposed that these three genes could participate in the intracellular glycosylation of oleandomycin and its secretion during antibiotic production.