Substantial improvement of toyocamycin production in Streptomyces diastatochromogenes by cumulative drug-resistance mutations

Unconventional approaches for the induction of microbial natural products

Expansion of the microbial drug discovery pipeline has been impeded by a limited and skewed appreciation of the microbial world and its full chemical capabilities and by an inability to induce silent biosynthetic gene clusters (BGCs). Typically, these silent genes are not expressed under standard laboratory conditions, instead requiring particular interventions to activate them. Genetic, physical, and chemical strategies have been employed to trigger these BGCs, and some have resulted in the induction of novel secondary metabolites. This review encompasses a wide range of literature and emphasizes selected successful induction of microbial secondary metabolites examples through unconventional approaches such as quorum sensing, epigenetic modulation, and ribosome engineering. Whenever applicable, we will also discuss their mechanisms and optimizations to improve the microbial drug discovery process.

Chromosome architecture as a determinant for biosynthetic diversity in Micromonospora

Natural products - small molecules generated by organisms to facilitate ecological interactions - are of great importance to society and are used as antibacterial, antiviral, antifungal and anticancer drugs. However, the role and evolution of these molecules and the fitness benefits they provide to their hosts in their natural habitat remain an outstanding question. In bacteria, the genes that encode the biosynthetic proteins that generate these molecules are organised into discrete loci termed biosynthetic gene clusters (BGCs). In this work, we asked the following question: How are biosynthetic gene clusters organised at the chromosomal level? We sought to answer this using publicly available high-quality assemblies of Micromonospora, an actinomycete genus with members responsible for biosynthesizing notable natural products, such as gentamicin and calicheamicin. By orienting the Micromonospora chromosome around the origin of replication, we demonstrated that Micromonospora has a conserved origin-proximal region, which becomes progressively more disordered towards the antipodes of the origin. We then demonstrated through genome mining of these organisms that the conserved origin-proximal region and the origin-distal region of Micromonospora have distinct populations of BGCs and, in this regard, parallel the organization of Streptomyces, which possesses linear chromosomes. Specifically, the origin-proximal region contains highly syntenous, conserved BGCs predicted to biosynthesize terpenes and a type III polyketide synthase. In contrast, the ori-distal region contains a highly diverse population of BGCs, with many BGCs belonging to unique gene cluster families. These data highlight that genomic plasticity in Micromonospora is locus-specific, and highlight the importance of using high-quality genome assemblies for natural product discovery and guide future natural product discovery by highlighting that biosynthetic novelty may be enriched in specific chromosomal neighbourhoods.

Physiology and Transcriptional Analysis of ppGpp-Related Regulatory Effects in Streptomyces diastatochromogenes 1628

ppGpp is a ubiquitous small nucleotide messenger that mediates cellular self-protective responses under environmental stress. However, the mechanisms of ppGpp that control transcription and other metabolic processes depend on the species, and ppGpp regulates the same process via different mechanisms. The level of ppGpp is regulated by RelA/SpoT homolog (RSH) enzymes that synthesize and hydrolyze the alarmone. Here, we constructed a ppGpp⁰ strain and monitored the effects of ppGpp on the transcriptional level, physiology, and secondary metabiotic production in the antibiotic producer Streptomyces diastatochromogenes 1628. The results showed the cell division and growth of ppGpp⁰ increased by measurement of gene transcription and DCWs. The utilization of nitrogen was affected depending on the nitrogen type with a significantly higher DCW of the ppGpp⁰ mutant in the medium supplied with the yeast extract and a lower growth rate in the inorganic nitrogen ammonium salt. The ppGpp-mediated stringent response could not affect the usage of carbon resources. More importantly, ppGpp⁰ inhibited the expression of antibiotic clusters and the production of toyocamycin and tetramycin P. The antibiotic resistance was also significantly downregulated in the ppGpp⁰ mutant. In conclusion, this study showed detailed changes in ppGpp-mediated stringent responses on S. diastatochromogenes 1628 cell growth, nutrient utilization, morphological characteristics, antibiotic production, and resistance, which will provide insights into the role of ppGpp in Streptomyces. IMPORTANCE The ppGpp-mediated stringent response is widely distributed in Escherichia coli, Bacillus subtilis, Streptomyces, Staphylococcus aureus, etc. Stringent responses give strains the ability to resist environmental stresses, and survival from nutrition starvation, virulence, long-term persistence, biofilm formation, and gut colonization. ppGpp has many targets in cells and can reprogram DNA replication, transcription, ribosome biogenesis and function, and lipid metabolism. However, the mechanism of ppGpp to control transcription and other metabolic processes depends on the bacterial species and regulates the same process via a different mechanism. In Streptomyces, how ppGpp regulates the transcription remains to be elucidated. However, because ppGpp regulates many genes involved in primary and secondary metabolism, we compared the transcription and cell division, cell growth, morphological differentiation, antibiotic resistance, and secondary synthesis in the wild-type S. diastatochromogenes and ppGpp⁰ strains.

Genome Shuffling Mutant of Streptomyces diastatochromogenes for Substantial Improvement of Toyocamycin Production

Toyocamycin, a nucleoside antibiotic, is a fungicide with the potential to control plant pathogens. In this study, three rounds of genome shuffling screening were applied to enhance the toyocamycin production in Streptomyces diastatochromogenes 1628. After three rounds of genome-shuffling screening, the toyocamycin production increased by 10.8-fold that of wild-type, and 2.64-fold of its parental strain. By optimization of its nutrition condition in medium, the highest production of toyocamycin reached 1173.6 mg/L in TY-producing medium. In addition, the mechanism for the improvement of shuffled strains was investigated. Recombinants with increased toyocamycin production exhibited higher transcriptional level of the toy cluster and product resistance. Furthermore, the rise of ATP hydrolysis rate indicated that intracellular ATP exhibit a significant role in tuning the toy cluster by an ATP-binding pathway-specific regulator. In all, we obtained S. diastatochromogenes mutants with enhanced toyocamycin production, and provided a valuable clue for the activation of secondary metabolites.

The intricate molecular identification of Streptomyces: a case study on Antarctic soil isolates

Despite the worldwide use of 16S rRNA to identify bacterial species, the use of this gene does not discriminate the 750 species in the genus Streptomyces. A MLST scheme was constructed with rpoB, gyrB, recA, trpB and atpD genes to access the genomic variances in Streptomyces species evolution. We analyze the housekeeping genes in 49 Streptomyces isolates from Antarctic soil. It was used two different databases, GenBank and EzBioCloud to compare the 16S sequences. The species founded in both databases are not the same, but in both cases, a few isolates achieve the necessary high percentage to consider the identification. There is a lack of deposited sequences in the other genes, as the data in GenBank proved to be insufficient. Isolate LMA323St_9 has the potential to be studied as a novel species. Besides that, the use of housekeeping genes gives robust phylogenetic information to understand in group relationships.

Use of elicitors to enhance or activate the antibiotic production in streptomyces

Streptomyces is the largest and most significant genus of Actinobacteria, comprising 961 species. These Gram-positive bacteria produce many versatile and important bioactive compounds; of these, antibiotics, specifically the enhancement or activation of their production, have received extensive research attention. Recently, various biotic and abiotic elicitors have been reported to modify the antibiotic metabolism of Streptomyces, which promotes the production of new antibiotics and bioactive metabolites for improvement in the yields of endogenous products. However, some elicitors that obviously contribute to secondary metabolite production have not yet received sufficient attention. In this study, we have reviewed the functions and mechanisms of chemicals, novel microbial metabolic elicitors, microbial interactions, enzymes, enzyme inhibitors, environmental factors, and novel combination methods regarding antibiotic production in Streptomyces. This review has aimed to identify potentially valuable elicitors for stimulating the production of latent antibiotics or enhancing the synthesis of subsistent antibiotics in Streptomyces. Future applications and challenges in the discovery of new antibiotics and enhancement of existing antibiotic production using elicitors are discussed.

Isolation and Evaluation of Rhizosphere Actinomycetes With Potential Application for Biocontrolling Fusarium Wilt of Banana Caused by Fusarium oxysporum f. sp. cubense Tropical Race 4

Fusarium wilt of banana caused by Fusarium oxysporum f. sp. cubense tropical race 4 (TR4) is globally one of the most destructive soil-borne fungal diseases. Biological control using environmental microorganisms is considered as an alternative and sustainable strategy. Actinomycetes have the potential to explore biocontrol agents due to their production of diverse metabolites. The isolation and identification of high-efficiency and broad-spectrum antagonistic actinomycetes are the key for the application of biocontrol agents. In the present study, 60 actinomycetes were obtained from the rhizosphere soil of Machilus pingii in the primitive ecological natural reserve of Hainan province, China. Seventeen isolates and their extracts exhibited significant antifungal activity against F. oxysporum TR4. Particularly, strain BITDG-11 with the strongest inhibition ability had a broad-spectrum antifungal activity. The assay of its physiological and biochemical profiles showed that strain BITDG-11 had the ability to produce IAA and siderophores and had a positive response to gelatin liquefaction and nitrate reduction. Enzyme activities of chitinase, β-1,3-glucanase, lipase, and urease were also detected. Average nucleotide identity calculated by comparison with the standard strain genome of Streptomyces albospinus JCM3399 was 86.55% below the novel species threshold, suggesting that the strain could be a novel species. In addition, Streptomyces BITDG-11 obviously reduced the disease index of banana plantlets and promoted plant growth at 45 days post inoculation. The higher and lasting expression levels of defense genes and activities of antioxidant enzymes were induced in the roots of banana. Genome sequencing revealed that the Streptomyces BITDG-11 chromosome contained large numbers of conserved biosynthesis gene clusters encoding terpenes, non-ribosomal peptides, polyketides, siderophores, and ectoines. Fifteen bioactive secondary metabolites were further identified from Streptomyces BITDG-11 extract by gas chromatography-mass spectrometry. Dibutyl phthalate demonstrating a strong antifungal activity was the major compound with the highest peak area. Hence, Streptomyces sp. BITDG-11 has a great potential to become an essential constituent of modern agricultural practice as biofertilizers and biocontrol agents.

Harnessing synthetic biology-based strategies for engineered biosynthesis of nucleoside natural products in actinobacteria

Antibiotic resistance poses an increasing threat to global health, and it is urgent to reverse the present trend by accelerating development of new natural product derived drugs. Nucleoside antibiotics, a valuable family of promising natural products with remarkable structural features and diverse biological activities, have played significant roles in healthcare and for plant protection. Understanding the biosynthesis of these intricate molecules has provided a foundation for bioengineering the microbial cell factory towards yield enhancement and structural diversification. In this review, we summarize the recent progresses in employing synthetic biology-based strategies to improve the production of target nucleoside antibiotics. Moreover, we delineate the advances on rationally accessing the chemical diversities of natural nucleoside antibiotics.

Enhancement of angucycline production by combined UV mutagenesis and ribosome engineering and fermentation optimization in Streptomyces dengpaensis XZHG99T

Strain improvement of Streptomyces dengpaensis XZHG99^T was performed by combined UV mutagenesis and ribosome engineering, as well as fermentation optimization for enhanced angucycline production (rabelomycin and saquayamycin B1). First, four streptomycin-resistant mutants were obtained after screening of UV mutagenesis and ribosome engineering. Then a rpsL mutant (HTT7) with higher productivity of rabelomycin and saquayamycin B1 was selected according to genetic screening and HPLC/LC-MS analyses, whose maximum titers of rabelomycin and saquayamycin B1 were 3.6 ± 0.02 mg/L and 7.5 ± 0.04 mg/L, respectively, about fourfold higher than those produced by XZHG99^T. Next, fermentation optimization of HTT7 was successively carried out by single-factor experiments in shake flasks. The titers of rabelomycin and saquayamycin B1 were increased to 11.2 ± 0.04 mg/L and 20.5 ± 0.02 mg/L after optimization of shake flask fermentation conditions, respectively, which was increased about sixfold compared with those produced by XZHG99^T. Finally, the titers of rabelomycin and saquayamycin B1 reached 15.7 ± 0.05 mg/L and 39.9 ± 0.05 mg/L after the scaled-up fermentation, which was 7.8-fold and 11.4-fold higher than those produced by XZHG99^T, respectively. These data demonstrate that the combined empirical strain-breeding approaches are still an effective and convenient pathway to improve strain production ability.

Substantial improvement of tetraene macrolide production in Streptomyces diastatochromogenes by cumulative drug resistance mutations

Tetraene macrolides remain one of the most reliable fungicidal agents as resistance of fungal pathogens to these antibiotics is relatively rare. The modes of action and biosynthesis of polyene macrolides had been the focus of research over the past few years. However, few studies have been carried out on the overproduction of polyene macrolides. In the present study, cumulative drug-resistance mutation was used to obtain a quintuple mutant G5-59 with huge tetraene macrolide overproduction from the starting strain Streptomyces diastatochromogenes 1628. Through DNA sequence analysis, the mutation points in the genes of rsmG, rpsL and rpoB were identified. Additionally, the growth characteristic and expression level of tetrRI gene (belonging to the large ATP binding regulator of LuxR family) involved in the biosynthesis of tetraene macrolides were analyzed. As examined with 5L fermentor, the quintuple mutant G5-59 grew very well and the maximum productivity of tetramycin A, tetramycin P and tetrin B was as high as 1735, 2811 and 1500 mg/L, which was 8.7-, 16- and 25-fold higher than that of the wild-type strain 1628, respectively. The quintuple mutant G5-59 could be useful for further improvement of tetraene macrolides production at industrial level.

The Antitumor Agent Ansamitocin P-3 Binds to Cell Division Protein FtsZ in Actinosynnema pretiosum

Ansamitocin P-3 (AP-3) is an important antitumor agent. The antitumor activity of AP-3 is a result of its affinity towards β-tubulin in eukaryotic cells. In this study, in order to improve AP-3 production, the reason for severe growth inhibition of the AP-3 producing strain Actinosynnema pretiosum WXR-24 under high concentrations of exogenous AP-3 was investigated. The cell division protein FtsZ, which is the analogue of β-tubulin in bacteria, was discovered to be the AP-3 target through structural comparison followed by a SPR biosensor assay. AP-3 was trapped into a less hydrophilic groove near the GTPase pocket on FtsZ by hydrogen bounding and hydrophobic interactions, as revealed by docking analysis. After overexpression of the APASM_5716 gene coding for FtsZ in WXR-30, the resistance to AP-3 was significantly improved. Moreover, AP-3 yield was increased from 250.66 mg/L to 327.37 mg/L. After increasing the concentration of supplemented yeast extract, the final yield of AP-3 reached 371.16 mg/L. In summary, we demonstrate that the cell division protein FtsZ is newly identified as the bacterial target of AP-3, and improving resistance is an effective strategy to enhance AP-3 production.

The Link between Purine Metabolism and Production of Antibiotics in Streptomyces

Stress and starvation causes bacterial cells to activate the stringent response. This results in down-regulation of energy-requiring processes related to growth, as well as an upregulation of genes associated with survival and stress responses. Guanosine tetra- and pentaphosphates (collectively referred to as (p)ppGpp) are critical for this process. In Gram-positive bacteria, a main function of (p)ppGpp is to limit cellular levels of GTP, one consequence of which is reduced transcription of genes that require GTP as the initiating nucleotide, such as rRNA genes. In Streptomycetes, the stringent response is also linked to complex morphological differentiation and to production of secondary metabolites, including antibiotics. These processes are also influenced by the second messenger c-di-GMP. Since GTP is a substrate for both (p)ppGpp and c-di-GMP, a finely tuned regulation of cellular GTP levels is required to ensure adequate synthesis of these guanosine derivatives. Here, we discuss mechanisms that operate to control guanosine metabolism and how they impinge on the production of antibiotics in Streptomyces species.

Sequential improvement of rimocidin production in Streptomyces rimosus M527 by introduction of cumulative drug-resistance mutations

Rimocidin is a polyene macrolide that exhibits a strong inhibitory activity against a broad range of plant-pathogenic fungi. In this study, fermentation optimization and ribosome engineering technology were employed to enhance rimocidin production in Streptomyces rimosus M527. After the optimization of fermentation, rimocidin production in S. rimosus M527 increased from 0.11 ± 0.01 to 0.23 ± 0.02 g/L during shake-flask experiments and reached 0.41 ± 0.05 g/L using 5-L fermentor. Fermentation optimization was followed by the generation of mutants of S. rimosus M527 through treatment of the strain with different concentrations of gentamycin (Gen) or rifamycin. One Gen^r mutant named S. rimosus M527-G37 and one Rif^r mutant named S. rimosus M527-R5 showed increased rimocidin production. Double-resistant (Gen^r and Rif^r) mutants were selected using S. rimosus M527-G37 and S. rimosus M527-R5, and subsequently tested. One mutant, S. rimosus M527-GR7, which was derived from M527-G37, achieved the greatest cumulative improvement in rimocidin production. In the 5-L fermentor, the maximum rimocidin production achieved by S. rimosus M527-GR7 was 25.36% and 62.89% greater than those achieved by S. rimosus M527-G37 and the wild-type strain S. rimosus M527, respectively. Moreover, in the mutants S. rimosus M527-G37 and S. rimosus M527-GR7 the transcriptional levels of ten genes (rimA_sr to rimK_sr) located in the gene cluster involved in rimocidin biosynthesis were all higher than those in the parental strain M527 to varying degrees. In addition, after expression of the single rimocidin biosynthetic genes in S. rimosus M527 a few recombinants showed an increase in rimocidin production. Expression of rimE led to the highest production.