Morphological-metabolic analysis in Streptomyces rimosus microparticle-enhanced cultivations (MPEC)

Streptomyces produce a broad spectrum of biologically active molecules such as oxytetracycline and rimocidin, which are widely used in human and animal treatments. microparticle-enhanced cultivation (MPEC) is one of the tools used for Streptomyces bioprocesses intensification by the control of mycelial morphology. In the present work, morphological changes of Streptomyces rimosus caused by the addition of 10 µm talc microparticles in MPEC were correlated with the biosynthetic activity of the microorganism. Comparing the runs with and without microparticles, major morphological changes were observed in MPEC, including the deformation of pellets, variation of their size, appearance of hyphae and clumps as well as the aggregation of mycelial objects. The presence of talc microparticles also influenced the levels of the studied secondary metabolites produced by S. rimosus. Comparing control and MPEC runs, the addition of talc microparticles increased the amounts of oxytetracycline (9-fold), 2-acetyl-2-decarboxamido-oxytetracycline (7-fold), milbemycin A3+4[O] (3-fold) and CE 108 (1.5-fold), while rimocidin (27-ethyl) and milbemycin β11+4[O] production was reduced. In summary, the addition of talc microparticles to S. rimosus cultivations led to the development of smaller morphological forms like hyphae and clumps as well as to the changes in the amounts of secondary metabolites.

Oxytetracycline hyper-production through targeted genome reduction of Streptomyces rimosus

Most biosynthetic gene clusters (BGC) encoding the synthesis of important microbial secondary metabolites, such as antibiotics, are either silent or poorly expressed; therefore, to ensure a strong pipeline of novel antibiotics, there is a need to develop rapid and efficient strain development approaches. This study uses comparative genome analysis to instruct rational strain improvement, using Streptomyces rimosus, the producer of the important antibiotic oxytetracycline (OTC) as a model system. Sequencing of the genomes of two industrial strains M4018 and R6-500, developed independently from a common ancestor, identified large DNA rearrangements located at the chromosome end. We evaluated the effect of these genome deletions on the parental S. rimosus Type Strain (ATCC 10970) genome where introduction of a 145 kb deletion close to the OTC BGC in the Type Strain resulted in massive OTC overproduction, achieving titers that were equivalent to M4018 and R6-500. Transcriptome data supported the hypothesis that the reason for such an increase in OTC biosynthesis was due to enhanced transcription of the OTC BGC and not due to enhanced substrate supply. We also observed changes in the expression of other cryptic BGCs; some metabolites, undetectable in ATCC 10970, were now produced at high titers. This study demonstrated for the first time that the main force behind BGC overexpression is genome rearrangement. This new approach demonstrates great potential to activate cryptic gene clusters of yet unexplored natural products of medical and industrial value.IMPORTANCEThere is a critical need to develop novel antibiotics to combat antimicrobial resistance. Streptomyces species are very rich source of antibiotics, typically encoding 20-60 biosynthetic gene clusters (BGCs). However, under laboratory conditions, most are either silent or poorly expressed so that their products are only detectable at nanogram quantities, which hampers drug development efforts. To address this subject, we used comparative genome analysis of industrial Streptomyces rimosus strains producing high titers of a broad spectrum antibiotic oxytetracycline (OTC), developed during decades of industrial strain improvement. Interestingly, large-scale chromosomal deletions were observed. Based on this information, we carried out targeted genome deletions in the native strain S. rimosus ATCC 10970, and we show that a targeted deletion in the vicinity of the OTC BGC significantly induced expression of the OTC BGC, as well as some other silent BGCs, thus suggesting that this approach may be a useful way to identify new natural products.

Molecular Biology Methods in Streptomyces rimosus, a Producer of Oxytetracycline

Streptomyces rimosus is used for production of the broad-spectrum antibiotic oxytetracycline (OTC). S. rimosus belongs to Actinomyces species, a large group of microorganisms that produce diverse set of natural metabolites of high importance in many aspects of our life. In this chapter, we describe specific molecular biology methods and a classical homologous recombination approach for targeted in-frame deletion of a target gene or entire operon in S. rimosus genome. The presented protocols will guide you through the design of experiment and construction of homology arms and their cloning into appropriate vectors, which are suitable for gene-engineering work with S. rimosus. Furthermore, two different protocols for S. rimosus transformation are described including detailed procedure for targeted gene replacement via double crossover recombination event. Gene deletion is confirmed by colony PCR, and colonies are further characterized by cultivation and metabolite analysis. As the final step, we present in trans complementation of the deleted gene, to confirm functionality of the engineering approach achieved by gene disruption. A number of methodological steps and protocols are optimized for S. rimosus strains including the use of the selected reporter genes. Protocols described in this chapter can be applied for studying function of any individual gene product in diverse OTC-producing Streptomyces rimosus strains.

[Optimization and application of chemically defined medium for 13C metabolic flux analysis of Streptomyces rimosus M4018]

The aim of this study is to develop a synthetic medium suitable for 13C metabolic flux analysis (13C-MFA) of Streptomyces rimosus. The cell growth rate and oxytetracycline production by S. rimosus M4018 were compared when M4018 cells were growth on the optimized chemically defined media with organic nitrogen sources or inorganic nitrogen sources. First, a synthetic medium contained KNO3 as the main nitrogen source was screened, then optimized by a response surface method. Using this new medium, the oxytetracycline yield was increased from 75.2 to 145.6 mg/L. Furthermore, based on the 13C-MFA, we identified that Entner-Doudoroff pathway does not exist in S. rimosus cells cultured in a chemically defined medium with feed of 100% 1-13C labeled glucose. This study is helpful for subsequent 13C-MFA application of S. rimosus.

Alternative sigma factor over-expression enables heterologous expression of a type II polyketide biosynthetic pathway in Escherichia coli

Background

Heterologous expression of bacterial biosynthetic gene clusters is currently an indispensable tool for characterizing biosynthetic pathways. Development of an effective, general heterologous expression system that can be applied to bioprospecting from metagenomic DNA will enable the discovery of a wealth of new natural products.

Methodology

We have developed a new Escherichia coli-based heterologous expression system for polyketide biosynthetic gene clusters. We have demonstrated the over-expression of the alternative sigma factor σ⁵⁴ directly and positively regulates heterologous expression of the oxytetracycline biosynthetic gene cluster in E. coli. Bioinformatics analysis indicates that σ⁵⁴ promoters are present in nearly 70% of polyketide and non-ribosomal peptide biosynthetic pathways.

Conclusions

We have demonstrated a new mechanism for heterologous expression of the oxytetracycline polyketide biosynthetic pathway, where high-level pleiotropic sigma factors from the heterologous host directly and positively regulate transcription of the non-native biosynthetic gene cluster. Our bioinformatics analysis is consistent with the hypothesis that heterologous expression mediated by the alternative sigma factor σ⁵⁴ may be a viable method for the production of additional polyketide products.

Oxytetracycline biosynthesis

Oxytetracycline (OTC) is a broad-spectrum antibiotic that acts by inhibiting protein synthesis in bacteria. It is an important member of the bacterial aromatic polyketide family, which is a structurally diverse class of natural products. OTC is synthesized by a type II polyketide synthase that generates the poly-beta-ketone backbone through successive decarboxylative condensation of malonyl-CoA extender units, followed by modifications by cyclases, oxygenases, transferases, and additional tailoring enzymes. Genetic and biochemical studies have illuminated most of the steps involved in the biosynthesis of OTC, which is detailed here as a representative case study in type II polyketide biosynthesis.

[Disruption of zwf2 gene to improve oxytetraclyline biosynthesis in Streptomyces rimosus M4018]

Genes of zwf1 and zwf2 encode two isozymes of glucose-6-phosphate dehydrogenase (G6PDH) of Streptomyces, respectively. G6PDH is the first enzyme in the oxidative pentose phosphate pathway (PPP) and the key enzyme for NADPH generation. Based on thermal sensitive plasmid pKC1139, a recombinant plasmid pKC1139-zwf2' was constructed and verified with restriction enzyme digestion. The plasmid pKC1139-zwf2' was electropolated into competent Streptomyces rimosus M4018 cells after it was demethylated by E. coli GM2929. Transformants grown on Tryptone Soya Agar (TSA) plate containing 500 ug/mL apramycin were selected, and identified using dot hybridization analysis and PCR amplification with apramycin resistant gene as primers. A positive clone was then selected and designated M4018-delta zwf2. With parent strain S. rimosus M4018 as control, mutant M4018-delta zwf2 was cultured in shaking flask. Specific acitivity of G6PDH of M4018-delta zwf2 was only half of that of parent strain whereas yield of oxytetracycline (OTC) of mutant was 27% higher to the mutant had a similar biomass profileto that of the control biosynthesis started when the growth entered stationary phase on the 4th day. However, specific oxytetracycline production of mutant was 31% higher than that of the parent strain, indicating that zwf2 disruption could enhance oxytetracycline biosynthesis in S. rimosus M4018-delta zwf2.

Investigation of early tailoring reactions in the oxytetracycline biosynthetic pathway

Tetracyclines are aromatic polyketides biosynthesized by bacterial type II polyketide synthases. The amidated tetracycline backbone is biosynthesized by the minimal polyketide synthases and an amidotransferase homologue OxyD. Biosynthesis of the key intermediate 6-methylpretetramid requires two early tailoring steps, which are cyclization of the linearly fused tetracyclic scaffold and regioselective C-methylation of the aglycon. Using a heterologous host (CH999)/vector pair, we identified the minimum set of enzymes from the oxytetracycline biosynthetic pathway that is required to afford 6-methylpretetramid in vivo. Only two cyclases (OxyK and OxyN) are necessary to completely cyclize and aromatize the amidated tetracyclic aglycon. Formation of the last ring via C-1/C-18 aldol condensation does not require a dedicated fourth-ring cyclase, in contrast to the biosynthetic mechanism of other tetracyclic aromatic polyketides, such as daunorubicin and tetracenomycin. Acetyl-derived polyketides do not undergo spontaneous fourth-ring cyclization and form only anthracene carboxylic acids as demonstrated both in vivo and in vitro. OxyF was identified to be the C-6 C-methyltransferase that regioselectively methylates pretetramid to yield 6-methylpretetramid. Reconstitution of 6-methylpretetramid in a heterologous host sets the stage for a more systematic investigation of additional tetracycline downstream tailoring enzymes and is a key step toward the engineered biosynthesis of tetracycline analogs.

Ablation of the otcC Gene Encoding a Post-polyketide Hydroxylase from the Oxytetracyline Biosynthetic Pathway in Streptomyces rimosus Results in Novel Polyketides with Altered Chain Length

Oxytetracycline (OTC) is a 19-carbon polyketide antibiotic made by Streptomyces rimosus. The otcC gene encodes an anhydrotetracycline oxygenase that catalyzes a hydroxylation of the anthracycline structure at position C-6 after biosynthesis of the polyketide backbone is completed. A recombinant strain of S. rimosus that was disrupted in the genomic copy of otcC synthesized a novel C-17 polyketide. This result indicates that the absence of the otcC gene product significantly influences the ability of the OTC "minimal" polyketide synthase to make a polyketide product of the correct chain length. A mutant copy of otcC was made by site-directed mutagenesis of three essential glycine codons located within the putative NADPH-binding domain. The mutant gene was expressed in Escherichia coli, and biochemical analysis confirmed that the gene product was catalytically inactive. When the mutant gene replaced the ablated gene in the chromosome of S. rimosus, the ability to make a 19-carbon backbone was restored, indicating that OtcC is an essential partner in the quaternary structure of the synthase complex.

The Use of Phase Inversion Temperature (PIT) Microemulsion Technology to Enhance Oil Utilisation during Streptomyces rimosus Fed-batch Fermentations to Produce Oxytetracycline

The use of a rapeseed oil emulsion feed, produced by a phase inversion temperature (PIT) process, produced more biomass, gave a 3-fold increase in oil utilisation and a higher oxytetracycline titre but a higher residual oil concentration when compared to a conventional fed-batch Streptomyces rimosus process fed with crude rapeseed oil. Importantly, microbial utilisation of the surfactant was confirmed for the first time.

Quantification of the contribution of surface outgrowth to biocatalysis in sol-gels: oxytetracycline production by Streptomyces rimosus

A technique was developed for differentiating the activity of microbes solely within sol gels by using the contribution of biomass outgrowth. Streptomyces rimosus was immobilised in colloidal silica gels and biomass growth, oxytetracycline synthesis, pH and carbohydrate consumption were compared for UV surface-sterilised gels, untreated gels, and liquid cultures. Absolute and biomass specific oxytetracycline yields were higher for non-sterile gels than for liquid culture. Biomass solely within colloidal silica gels (1.7 mg ml(-1)), and gels obtained from colloidal silica modified by addition of larger silica particles (1.2 mg ml(-1)) yielded 27 and 21 microg ml(-1) oxytetracycline compared with 97 and 104 microg ml(-1) for unsterilised gels (3.6 and 5.2 mg ml(-1) biomass) displaying outgrowth. It was therefore apparent that biomass and antibiotic production within the gels was limited and that optimisation requires gel modification.

Oxytetracycline production by immobilized Streptomyces rimosus

This study examined whether the production of oxytetracycline by Streptomyces rimosus TM-55 (CCRC 960061) would be improved with calcium alginate immobilization in submerged fermentation compared with free cells. Results showed that in 1-mL culture broth, free cells produced 121 to 124 microg of oxytetracycline, whereas immobilized cells produced 153 to 252 microg. Immobilization of the cells retarded the growth rate of S. rimosus but increased the length of the growth period and improved the oxytetracycline production. The specific oxytetracycline productivity was 33.3 to 34.2 mg in each gram of free dry cells and was 40.2 to 40.7 mg in immobilized dry cells. The optimum immobilization conditions were alginate 2% and bead diameter of 2.13 mm. Oxytetracycline production increased with increasing inoculum density but decreased with increasing bead diameter. Ethylenediaminetetraacetic acid or monovalent ions could react with calcium in the bead or replace it with sodium ion, thereby reducing the strength of the beads.

Detection of oxytetracycline production by Streptomyces rimosus in soil microcosms by combining whole-cell biosensors and flow cytometry

Combining the high specificity of bacterial biosensors and the resolution power of fluorescence-activated cell sorting (FACS) provided qualitative detection of oxytetracycline production by Streptomyces rimosus in soil microcosms. A plasmid containing a transcriptional fusion between the tetR-regulated P(tet) promoter from Tn10 and a FACS-optimized gfp gene was constructed. When harbored by Escherichia coli, this plasmid produces large amounts of green fluorescent protein (GFP) in the presence of tetracycline. This tetracycline biosensor was used to detect the production of oxytetracycline by S. rimosus introduced into sterile soil. The tetracycline-induced GFP-producing biosensors were detected by FACS analysis, enabling the detection of oxytetracycline encounters by single biosensor cells. This approach can be used to study interactions between antibiotic producers and their target organisms in soil.

Disruption of an aromatase/cyclase from the oxytetracycline gene cluster of Streptomyces rimosus results in production of novel polyketides with shorter chain lengths

Oxytetracycline is a polyketide antibiotic made by Streptomyces rimosus. From DNA sequencing, the gene product of otcD1 is deduced to function as a bifunctional cyclase/aromatase involved in ring closure of the polyketide backbone. Although otcD1 is contiguous with the ketoreductase gene, they are located an unusually large distance from the genes encoding the "minimal polyketide synthase" of the oxytetracycline gene cluster. A recombinant, disrupted in the genomic copy of otcD1, made four novel polyketides, all of shorter chain length (by up to 10 carbons) than oxytetracycline. All four novel structures contained the unusual carboxamido group, typical of oxytetracycline. This implies that the carboxamido group is present at the start of biosynthesis of oxytetracycline, a topic that has been debated in the literature. Loss of the cyclase protein has a profound influence on the length of polyketide chain assembled, implying that OtcD1 plays a greater role in the overall integrity of the quaternary structure of the polyketide complex than hitherto imagined.

Phosphate control of oxytetracycline production by Streptomyces rimosus is at the level of transcription from promoters overlapped by tandem repeats similar to those of the DNA-binding sites of the OmpR family

Physiological studies have shown that Streptomyces rimosus produces the polyketide antibiotic oxytetracycline abundantly when its mycelial growth is limited by phosphate starvation. We show here that transcripts originating from the promoter for one of the biosynthetic genes, otcC (encoding anhydrotetracycline oxygenase), and from a promoter for the divergent otcX genes peak in abundance at the onset of antibiotic production induced by phosphate starvation, indicating that the synthesis of oxytetracycline is controlled, at least in part, at the level of transcription. Furthermore, analysis of the sequences of the promoters for otcC, otcX, and the polyketide synthase (otcY) genes revealed tandem repeats having significant similarity to the DNA-binding sites of ActII-Orf4 and DnrI, which are Streptomyces antibiotic regulatory proteins (SARPs) related to the OmpR family of transcription activators. Together, the above results suggest that oxytetracycline production by S. rimosus requires a SARP-like transcription factor that is either produced or activated or both under conditions of low phosphate concentrations. We also provide evidence consistent with the otrA resistance gene being cotranscribed with otcC as part of a polycistronic message, suggesting a simple mechanism of coordinate regulation which ensures that resistance to the antibiotic increases in proportion to production.

Recombination between the linear plasmid pPZG101 and the linear chromosome of Streptomyces rimosus can lead to exchange of ends

The 387kb linear plasmid pPZG101 of Streptomyces rimosus R6 can integrate into the chromosome or form a prime plasmid carrying the oxytetracycline biosynthesis cluster. The integration of plasmid pPZG101 into the linear chromosome of S. rimosus R6-501 in mutant MV25 was shown to be due to a single cross-over at a 4 bp common sequence. pPZG101 had integrated into a 250 kb DNA sequence that was reiterated at a low level. This sequence includes the oxytetracycline biosynthesis cluster, so that homologous recombination generated a mixed population carrying different copy numbers of the region. The 1 Mb linear plasmid pPZG103 in mutant MV17 had also arisen from a cross-over between pPZG101 and the chromosome, so that one end of pPZG103 consists of c. 850 kb of chromosomal sequence including the oxytetracycline biosynthesis cluster. The plasmid pPZG101 was shown to consist of a unique central region of about 30 kb flanked by terminal inverted repeats of about 180 kb. Analysis of a presumed ancestor plasmid pPZG102 suggested that the long terminal repeats had arisen by a recombination event during the strain development programme.

Physical mapping shows that the unstable oxytetracycline gene cluster of Streptomyces rimosus lies close to one end of the linear chromosome

A restriction map of the 8 Mb linear chromosome of Streptomyces rimosus R6-501 was constructed for the enzymes Asel (13 fragments) and Dral (7 fragments). Linking clones for all 12 Asel sites and 5 of the 6 Dral sites were isolated. The chromosome has terminal inverted repeats of 550 kb, which are the longest yet reported for a Streptomyces species. The oxytetracycline gene cluster lies about 600 kb from one end, which might account for its frequent spontaneous amplification and deletion. Several other markers were localized on the chromosome (dnaA and recA, the rrn operons, the attachment site for pSAM2 and prophages RP2 and RP3). Comparison of the conserved markers with the map of Streptomyces coelicolor A3(2) suggested there are differences in genome organization between the two species.

[Effect of redox potential of culture broth on oxytetracycline biosynthesis]

The influence of the redox potential on the biosynthesis of oxytetracycline was studied. It was shown that during the first hours of the Streptomyces rimosus cultivation Eh decreased to the negative values. There was detected a relationship between Eh within the negative values and the antibiotic production: the lower Eh, the lower the antibiotic content in the fermentation broth by the end of the process. It was noted that the Eh value during the first days of the fermentation determined the level of the antibiotic production for the entire cycle of the biosynthesis regardless of the further cultivation conditions.

Endo-deoxyribonuclease from Streptomyces rimosus

From filtrates of an oxytetracycline-producing culture of Streptomyces rimosus a deoxyribonuclease was purified to homogeneity and determined to be a potent endo-DNase. It is a monomeric, basic protein (M(r) approximately 21,000; pI approximately 9.5) stable in a broad pH range but unstable to higher temperature. The enzyme has an absolute requirement for Mg2+ or Mn2+, and for its full activity requires free SH groups and a low-ionic-strength environment. Its N-terminal primary structure differs from that of other nucleases.

Conserved organization of genes for biosynthesis of chlortetracycline in Streptomyces strains

By genomic Southern blot analysis, the DNA sequences homologous to the gene cluster responsible for biosynthesis of 6-demethyl-chlortetracycline in Streptomyces aureofaciens NRRL3203 were shown to be highly conserved in independent chlortetracycline- or tetracycline producing Streptomyces strains. By contrast, oxytetracycline-producing Streptomyces strains had no hybridization with the cluster DNA.

Morphogenesis, biomass and oxytetracycline production of Streptomyces rimosus in submerged cultivation

The minimal concentration of adenosine triphosphate (ATP) which could be detected with spectrophotometry, HPLC and luciferin-luciferase methods was 1.0 microM, 3.3 microM and 100 nM, respectively. In submerged cultivation, most Streptomyces rimosus TM-55 was in hyphae fragment form at 65 h, became short-rod mycelia at 166 h, and lysed at 504 h incubation. The ATP content had maximal value at 24 h, then gradually decreased during cultivation. The oxytetracycline potency increased as incubation occurred, had maximal potency 178.9 micrograms/ml at 166 h, and then gradually decreased. Morphogenesis was very important in oxytetracycline production in submerged cultivation of Streptomyces; short-rod mycelia had high oxytetracycline production.

The 387 kb linear plasmid pPZG101 of Streptomyces rimosus and its interactions with the chromosome

The linear plasmid pPZG101 of Streptomyces rimosus R6 was restriction mapped with the enzymes AseI, BfrI, DraI and XbaI. It is 387 kb in size and the ends are inverted repeats of at least 95 kb in length. Twenty spontaneous morphological variants and seventeen auxotrophic mutants were screened for changes in the plasmid. Two strains were found that had lost all plasmid sequences. Four strains had integrated parts of the plasmid into the chromosome. Restriction analysis suggested that at least three of the integrated strains had retained free plasmid ends. If it is assumed that the chromosome of S. rimosus R6 is linear, this might be explained by replacement of one or both chromosome ends by a plasmid end. One strain, which overproduced oxytetracycline, carried an enlarged linear plasmid of 1 Mb in size that had acquired chromosomal sequences from the oxytetracycline biosynthesis cluster.

Sequences of the oxytetracycline polyketide synthase-encoding otc genes from Streptomyces rimosus

The complete nucleotide sequences of the Streptomyces rimosus oxytetracycline (oxyTc) polyketide synthase (PKS)-encoding genes (otcY) has been determined, revealing three open reading frames. The deduced amino-acid sequences correspond to the presumed heterodimeric beta-ketoacyl synthase and acyl carrier protein found in other type-II (multicomponent) PKS systems that specify construction of acetate-derived polyketide antibiotics.

[Dynamics of the content of glycolysis enzymes in Streptomyces rimosus as it relates to the problem of regulation of oxytetracycline biosynthesis]

The activities of the key glycolytic enzymes i.e. hexokinase, phosphofructokinase, pyruvate kinase and lactate dehydrogenase were detected and investigated in 3 strains of Streptomyces rimosus differing in the level of the oxytetracycline production. The samples were assayed at different fermentation stages and under different aeration conditions. Correlation between the level of the pyruvate kinase activity within the first days of the fermentation process and the strain capacity for the antibiotic accumulation at the final stage of the fermentation process was observed.

Utilization of date products in production of oxytetracycline by Streptomyces rimosus

The carbon and nitrogen sources of a synthetic fermentation medium were substituted by equivalent amounts of date-coat sugar extract and date-seed hydrolysate. The addition of 1.0 g dm-3 urea and date-seed lipid into the medium increased the efficacy for antibiotic formation. When the MgSO4, MnSO4, FeSO4 and ZnSO4 components of the medium were replaced by 0.5 g dm-3 date-seed ash, antibiotic output was also increased.

[Selection of the optimal conditions for the procurement and regeneration of protoplasts of the industrial strain of Streptomyces rimosus, the producer of oxytetracycline, and the effect of the protoplasting process on the antibiotic activity]

Optimal conditions for protoplasting of the Streptomyces rimosus industrial strain No. 1 producing oxytetracycline were developed. Observation of the early stages of the protoplast regeneration in microchambers showed that there were two regeneration types: normal and anomalous. The latter was likely defined by the glycine effect on cell wall synthesis. It was accompanied by the stage in which the protoplasts had the form of multiplying protoplast-like cells. The protoplasting of the S. rimosus culture producing oxytetracycline resulted in an increase in the variability of an antibiotic producing property and the frequency of low active variants.

Oxytetracycline production in solid state and submerged fermentation by protoplast fusants of Streptomyces rimosus

Streptomyces rimosus TM-55 was treated with 3% EMS, and 29 auxotrophic mutants (AM-1 - AM-29) were isolated from 5457 colonies at a survival rate between 6.6-66.7%. Three sets of the auxotrophic mutants, AM-3 and AM-27, AM-7 and AM-28, and AM-3 and AM-21, were chosen for protoplast fusion with 50% PEG 1000 for 30 minutes at 25 degrees C, and 25 fusants were isolated (f-1 - f-25). In solid substrate, oxytetracycline production of 20% fusants was higher than that of the wild strain, while in submerged fermentation, it was 44%. Oxytetracycline productions of fusants f-1, f-6, f-11, f-12, f-20 and f-21 were lower than that of the wild strain in solid substrate, but this was reversed in submerged fermentation. On the other hand, OTC production of fusant f-8 was higher than that of the wild strain in solid substrate, and this was reversed in submerged fermentation.

[Molecular cloning of chlortetracycline resistance gene from chlortetracycline producer Streptomyces aureofaciens]

The chlortetracycline (CT) resistance gene ctr was cloned from S. aureofaciens 633, a strain producing the antibiotic. The 6.6-kb DNA Bam HI fragment containing the resistance gene was cloned with the plasmid vector pIJ699. Comparison of the restriction maps of the cloned gene and the oxytetracycline (OT) resistance gene otrA from S. rimosus revealed their similarity which enabled identification of the cloned resistance gene as otrA. Investigation of the resistance determinants in S. aureofaciens 633 made it possible to identify a mtr gene(s). It was demonstrated that introduction of a ctrA gene into S. lividance provided a simultaneous increase in the resistance of the recipient strain to CT and a number of macrolide antibiotics. The CT resistance determinants in S. lividans TK64 showed properties of exogenous induction by CT and the macrolide antibiotics similar to the properties of the mtr gene(s) of S. aureofaciens. Possible adaptation properties of mtr genes are discussed.

[Theoretical analysis of the continuous two-stage process of the biosynthesis of a metabolic product]

A mathematical model for continuous biosynthesis of a metabolite in a battery of two apparatuses with ideal agitation is described and analysed. In the first apparatus of the battery it is advisable to maintain a high specific rate of culture growth for continuous accumulation of young active biomass while in the second apparatus a low growth rate is expedient which provides a change in the culture metabolism to biosynthesis of the required product. To make the continuous two-stage process efficient, it is necessary to add an extremely concentrated solution of the nutrients to the second apparatus of the battery. Influence of the oxygen transport velocity on the maximum attainable concentration of the biomass and the process capacity by the required product was studied.

Cloning and heterologous expression in Streptomyces lividans of Streptomyces rimosus genes involved in oxytetracycline biosynthesis

The anhydrotetracycline (ATC) oxygenase enzyme which carries out the conversion of ATC to dehydrotetracycline was purified and the N-terminal amino acid sequence was determined. The sequence displays a significant similarity to that of the p-hydroxybenzoate hydroxylase from Pseudomonas fluorescens. This is consistent with the activity of the oxygenase, i.e., addition of a hydroxyl moiety to an aromatic ring structure. Oligonucleotide probes were designed and used to clone the corresponding fragment of chromosomal DNA from Streptomyces rimosus. This DNA fragment was used to screen a cosmid library, allowing the isolation of flanking DNA sequences. Surprisingly, the gene was located within the previously cloned cluster of genes involved in the synthesis of the biosynthetic intermediate ATC and not as had been expected (P. M. Rhodes, N. Winskill, E. J. Friend, and M. Warren, J. Gen. Microbiol. 124:329-338, 1981) at a separate locus on the other side of the chromosome. Subcloning of an appropriate DNA fragment from one of the cosmid clones onto pIJ916 produced Streptomyces lividans transformants which synthesized oxytetracycline.

Physiology of a wild strain and high yielding mutants of Streptomyces rimosus, producing oxytetracycline

A wild-type Streptomyces strain, yielding 1 g/L of oxytetracycline was compared with mutants giving up to 7 g/L, using complex media in stirred and shaken culture. Increased production of oxytetracycline was associated with high specific production rates and a longer production period. The superiority of the mutants was associated with changes in morphological behaviour during growth in submerged culture, and in their patterns of growth and respiration, coupled with increased resistance to the product. The productivity of the mutants was sensitive to the rate of stirring, the type of calcium carbonate used in the medium and the type of inoculum. Careful control of these factors was necessary to obtain high yields of oxytetracycline. With the exception of acetyl-CoA carboxylase, the levels of enzymes measured and of amounts of adenylates in the mycelium did not appear to be related to the degree of antibiotic production.

Homology between Streptomyces genes coding for synthesis of different polyketides used to clone antibiotic biosynthetic genes

Many important antibiotics such as tetracyclines, erythromycin, adriamycin, monensin, rifamycin and avermectins are polyketides. In their biosynthesis, multifunctional synthases catalyse iterated condensation of thio-esters derived from acetate, propionate or butyrate to yield aliphatic chains of varying length and carrying different alkyl substituents. Subsequent modifications, including aromatic or macrolide ring closure or specific methylations or glycosylations, generate further chemical diversity. It has been suggested that, if different polyketide synthases had a common evolutionary origin, cloned DNA coding for one synthase might be used as a hybridization probe for the isolation of others. We show here that this is indeed possible. Study of a range of such synthase genes and their products should help to elucidate what determines the choice and order of condensation of different residues in polyketide assembly, and might yield, by in vitro recombination or mutagenesis, synthase genes capable of producing novel antibiotics. Moreover, because genes for entire antibiotic pathways are usually clustered in Streptomyces, cloned polyketide synthase genes are valuable in giving access to groups of linked biosynthetic genes.

A comparative study of protoplast preparation and regeneration in Streptomyces rimosus and Streptomyces lividans

One of the basic techniques for DNA cloning in Streptomyces is the preparation of protoplasts and the efficient regeneration of normal mycelia. In order to develop an oxytetracycline producing S. rimosus strain as a host for molecular cloning, the efficiencies of protoplast preparation and cell wall regeneration were compared with those obtained using S. lividans, the host for the majority of cloning experiments. The results presented suggest that the S. rimosus strain selected is a convenient host for the cloning of recombinant DNA, at least in relation to the preparation and regeneration of protoplasts. Interestingly, the size of the protoplasts was dependent upon the physiological age of the mycelium in S. rimosus; no such dependence was observed in S. lividans.

Identification of some streptomycetes producing oxytetracycline

A study was made of Streptomyces rimosus and mutant strains to compare the phenotype of high and low oxytetracycline producers. Strains were identified using a probabilistic identification matrix for the genus Streptomyces. Mutant strains separated into two groups: high-titre strains and blocked mutants. The former identified with the S. rimosus cluster whereas the latter were not identified. Two further oxytetracycline producers identified with the Streptomyces lydicus cluster.

[Characteristics of the process of regulated biosynthesis of oxytetracycline]

Investigation of the process of oxytetracycline biosynthesis in a model medium showed that addition of maltose during the phase of antibiotic biosynthesis resulted in an increase in the rate of the culture growth, prolongation of the antibiotic production phase and increased productivity of the mycelium. The productive mycelium was characterized by a higher content of total nitrogen and phosphorus.

[Effect of aeration conditions on carbon dioxide mass transfer in antibiotic biosynthetic processes]

The effect of the aeration conditions on the mass exchange according to carbon dioxide in biosynthesis of various antibiotics was studied. On the basis of the results an equation for its estimation was developed. It was also shown that the effect of the aeration conditions on the intensity of the mass exchange according to carbon dioxide was much higher than that of the mass exchange according to oxygen.

Genetic interactions in Streptomyces rimosus mediated by conjugation and by protoplast fusion

The development of a protoplast fusion technique for oxytetracycline-producing Streptomyces rimosus strains, and its evaluation for the application for a breeding programme, has been described. Treatment of S. rimosus protoplasts with 40% (w/v) PEG 1550 for 30 min gave optimal numbers of recombinants ranging from 1 to 10% of the total progeny. Therefore, by comparison with conjugation, protoplast fusion increased the frequency of recombination by two to three orders of magnitude. The proportion of multiple crossover classes amongst recombinants was higher, by a factor of ten, after protoplast fusion (13.3%) than after conjugation (1.5%). Participation of less frequent complementary genotype doubled from 9.0% in conjugation to 17.9% in protoplast fusion. Overall, this suggested that the opportunities for crossing over in a fusion of S. rimosus protoplasts were spatially and/or temporally extended leading to a loosening of linkage with a near-random assortment of genotypes in a cross. However, by minimizing the multiple crossover classes and calculating allele frequency gradients, it was shown that the protoplast fusion technique allows arrangement of genetic markers on the S. rimosus chromosome. These are ideal characteristics for the recombination of divergent lines in a strain improvement programme.

[Effect of the aeration and agitation parameters on the oxytetracycline biosynthesis process under microkinetic conditions]

The effect of the aeration levels in flasks on the rate of oxytetracycline biosynthesis and other kinetic characteristics was studied. It was shown that changes in the medium volume in flasks, dilution of the fermentation broth with water or its filtrate and the use of oxygen for aeration had a significant effect on the characteristics studied. Special experiments with low concentrations of the biomass were performed for investigation of the effect of dissolved carbon dioxide on the kinetics of the process.

[Effect of mass exchange conditions on the biosynthesis of oxytetracycline]

An increase in the power input for agitation from 0.4 to 2.7 kw/m3 in oxytetracycline biosynthesis stimulated the antibiotic production due to an accelerated use of carbohydrate and nitrogen and an earlier transition of the culture to the phase of the antibiotic biosynthesis. The procedure for determination of the aeration and agitation conditions by the maximum rate of the oxygen uptake is described. The procedure is recommended for the use in the studies on determination of such conditions for the biosynthesis of of oxytetracycline in large apparatus.

[Effect of temperature on the kinetics of the oxytetracycline biosynthesis process]

Short-term experiments on the effect of temperature on the rate of oxytetracycline biosynthesis and consumption of carbohydrates, ammonium nitrogen and inorganic phosphorus were performed under various conditions of mass exchange. Complete fermentation cycles were also carried out at the same temperatures. It was shown that the temperature optimal under conditions of a short-term experiment was not optimal for the complete fermentation cycle. In the latter case the results also depended on the aeration and agitation conditions. It is suggested that temperature has a nonspecific effect by changing the conditions of oxygen supply to the microbial culture.

[Effect of chemical defoaming agents on the mass exchange rate in the oxytetracycline biosynthesis process]

The effect of chemical foam suppressors of various classes on the intensity of Streptomyces rimosus respiration, concentrations of dissolved oxygen and carbon dioxide and gas levels in the fermentation broth of oxytetracycline was studied. Its was shown that addition of the foam suppressors, as a rule, resulted in a decreased rate of the oxygen mass transfer due to the decreased surface of the phase contact gas-liquid. This decrease was not so significant as that in case of uninhibited intensity of the fermentation broth foaming. A different character of the effect of different foam suppressors on the biosynthetic process was noted. The effect of the foam suppressors was decreased by their regular addition in low amounts.

[Cytomorphological changes in a culture of Streptomyces rimosus, the producer of oxytetracycline, in the presence of different foam retardants]

The data on the effect of chemical foam suppressors of various classes on the morphogenesis of 2 strains of Streptomyces rimosus are presented. Correlation between the age of the culture and its reaction to unfavourable conditions is confirmed. During the first 24 hours of the growth the culture is most sensitive to low aeration rates, addition of the foam suppressors, fat deficiency in the medium. Among the foam suppressors used propinol B-400 had the most significant effect on the cells: destruction of the cell coating, marked changes in the morphogenesis and nucleocytosis of the hyphae of the second generation responsible mainly for the antibiotic biosynthesis. A more active strain was more subject to the foam suppressor toxic effect.

[Experimental study of the dynamics in the biochemical parameter changes in the oxytetracycline biosynthesis process in apparatus of varying capacity]

An increase in the input of the mechanical power speeds up the culture growth, which correlates with the rate of carbohydrate and nitrogen consumption during the first phase of the process, the time of the biosynthesis phase onset and the level of the antibiotic production during this phase. During the second half of the process the antibiotic biosynthesis markedly decreases, especially in the apparatus with a high input of the mechanical power. Deficiency in carbon and nitrogen is one of the causes of this decrease. The activity level at the end of the biosynthesis in such apparatus does not differ significantly, since it depends on the biosynthesis rate during the whole cultivation period.

Production of oxytetracycline by Streptomyces rimosus 12,907 as an animal feed supplement

Production of oxytetracycline by Streptomyces rimosus 12907 as an animal feed supplement was achieved with a fermentation medium, containing the following ingredients (g/litre): black strap molasses, 30; fodder yeast, 20; rice bran, 10; KH2PO4, 0.2; and tap water to 1,000 ml. The dry mash, containing oxytetracycline, was 50 g/litre of fermentation medium. The amount of oxytetracycline, present in the dry mash, was about 4.0%. The dry mash, containing oxytetracycline, contained the following amino acids: L-arginine, L-histidine, L-lysine, L-isoleucine, L-leucine, L-phenylalanine, Dl-methionine, and L-tryptophan.

Characterization and persistence of actinophage RP2 isolated from Streptomyces rimosus ATCC 10970

While searching for true lysogens among oxytetracycline-producing Streptomyces rimosus strains, free phage particles were detected and isolated from a liquid culture of S. rimosus ATCC 10970 (R7). The actinophage, designated RP2, appears to be a typical temperate DNA phage producing turbid plaques on the sensitive strain S. rimosus R6. Electron microscopic examination of RP2 lysates showed that it belongs to group B of Bradley's morphological classification. The rate of RP2 adsoprption at 28 degrees C appeared to be low. The length of the latent period was about 6 h and the average burst size about 120 phage particles. The lysogenic nature of the host-virus system described was established on the basis of the following characteristics: spontaneous lysis frequency of 2 X 10(-6) per cell, resistance to curing with phage-specific antiserum, spontaneous curing frequency of less than 0.05% and immunity to superinfection with the homologous phage. Clear-plaque mutants of RP2, which failed to lysogenize sensitive cultures, arose at a frequency of 10(-5).

[Methodological approaches to developing a method of regulated antibiotic biosynthesis]

Methodologic approaches to the development of methods for controlled biosynthesis of antibiotics are discussed with reference to oxytetracycline biosynthesis. The method of acute experiments in conjunction with mathematical methods of experiment planning was used for determination of the substrate concentrations optimal for the culture growth and biosynthesis of oxytetracycline. It was found that such mathematic procedure provided simultaneous determination of optimal concentrations of all the substrates.