Streptomyces clavuligerus relA-null mutants overproduce clavulanic acid and cephamycin C: negative regulation of secondary metabolism by (p)ppGpp

ppGpp is a dual-role regulator involved in balancing iron absorption and prodiginine biosynthesis in Pseudoalteromonas

Iron is an essential element for microbial survival and secondary metabolism. However, excess iron availability and overloaded secondary metabolites can hinder microbial growth and survival. Microorganisms must tightly control iron homeostasis and secondary metabolism. Our previous studies have found that the stringent starvation protein A (SspA) positively regulates prodiginine biosynthesis by activating iron uptake in Pseudoalteromonas sp. strain R3. It is believed that the interaction between SspA and the small nucleotide ppGpp is important for iron to exert regulation functions. However, the roles of ppGpp in iron absorption and prodiginine biosynthesis, and the underlying relationship between ppGpp and SspA in strain R3 remain unclear. In this study, we found that ppGpp accumulation in strain R3 could be induced by limiting iron. In addition, ppGpp not only positively regulated iron uptake and prodiginine biosynthesis via increasing the SspA level but also directly repressed iron uptake and prodiginine biosynthesis independent of SspA, highlighting the finding that ppGpp can stabilize both iron levels and prodiginine production. Notably, the abolishment of ppGpp significantly increased prodiginine production, thus providing a theoretical basis for manipulating prodiginine production in the future. This dynamic ppGpp-mediated interaction between iron uptake and prodiginine biosynthesis has significant implications for understanding the roles of nutrient uptake and secondary metabolism for the survival of bacteria in unfavorable environments.

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 editing reveals that pSCL4 is required for chromosome linearity in Streptomyces clavuligerus

Streptomyces clavuligerus is an industrially important actinomycete whose genetic manipulation is limited by low transformation and conjugation efficiencies, low levels of recombination of introduced DNA, and difficulty in obtaining consistent sporulation. We describe the construction and application of versatile vectors for Cas9-mediated genome editing of this strain. To design spacer sequences with confidence, we derived a highly accurate genome assembly for an isolate of the type strain (ATCC 27064). This yielded a chromosome assembly (6.75 Mb) plus assemblies for pSCL4 (1795 kb) and pSCL2 (149 kb). The strain also carries pSCL1 (12 kb), but its small size resulted in only partial sequence coverage. The previously described pSCL3 (444 kb) is not present in this isolate. Using our Cas9 vectors, we cured pSCL4 with high efficiency by targeting the plasmid's parB gene. Five of the resulting pSCL4-cured isolates were characterized and all showed impaired sporulation. Shotgun genome sequencing of each of these derivatives revealed large deletions at the ends of the chromosomes in all of them, and for two clones sufficient sequence data was obtained to show that the chromosome had circularized. Taken together, these data indicate that pSCL4 is essential for the structural stability of the linear chromosome.

Streptomyces clavuligerus: The Omics Era

The Streptomyces clavuligerus genome consists in a linear chromosome of about 6.7 Mb and four plasmids (pSCL1 to pSCL4), the latter one of 1.8 Mb. Deletion of pSCL4, results in viable mutants with high instability in the chromosome arms, which may lead to chromosome circularisation. Transcriptomic and proteomic studies comparing different mutants with the wild-type strain improved our knowledge on the biosynthesis and regulation of clavulanic acid, cephamycin C and holomycin. Additional knowledge has been obtained on the SARP-type CcaR activator and the network of connections with other regulators (Brp, AreB, AdpA, BldG, RelA) controlling ccaR expression. The transcriptional pattern of the cephamycin and clavulanic acid clusters is supported by the binding of CcaR to different promoters and confirmed that ClaR is a CcaR-dependent activator that controls the late steps of clavulanic biosynthesis. Metabolomic studies allowed the detection of new metabolites produced by S. clavuligerus such as naringenin, desferroxamines, several N-acyl tunicamycins, the terpenes carveol and cuminyl alcohol or bafilomycin J. Heterologous expression of S. clavuligerus terpene synthases resulted in the formation of no less than 15 different terpenes, although none of them was detected in S. clavuligerus culture broth. In summary, application of the Omic tools results in a better understanding of the molecular biology of S. clavuligerus, that allows the use of this strain as an industrial actinobacterial platform and helps to improve CA production.

Proteomic analysis of a hom-disrupted, cephamycin C overproducing Streptomyces clavuligerus

BACKGROUND

Streptomyces clavuligerus is prolific producer of cephamycin C, a medically important antibiotic. In our former study, cephamycin C titer was 2-fold improved by disrupting homoserine dehydrogenase (hom) gene of aspartate pahway in Streptomyces clavuligerus NRRL 3585.

OBJECTIVE

In this article, we aimed to provide a comprehensive understanding at the proteome level on potential complex metabolic changes as a consequence of hom disruption in Streptomyces clavuligerus AK39.

METHODS

A comparative proteomics study was carried out between the wild type and its hom disrupted AK39 strain by 2 Dimensional Electrophoresis-Matrix Assisted Laser Desorption and Ionization Time-Of-Flight Mass Spectrometry (2DE MALDI-TOF/MS) and Nanoscale Liquid Chromatography- Tandem Mass Spectrometry (nanoLC-MS/MS) analyses. Clusters of Orthologous Groups (COG) database was used to determine the functional categories of the proteins. The theoretical pI and Mw values of the proteins were calculated using Expasy pI/Mw tool.

RESULTS

"Hypothetical/Unknown" and "Secondary Metabolism" were the most prominent categories of the differentially expressed proteins. Upto 8.7-fold increased level of the positive regulator CcaR was a key finding since CcaR was shown to bind to cefF promoter thereby direcly controlling its expression. Consistently, CeaS2, the first enzyme of CA biosynthetic pathway, was 3.3- fold elevated. There were also many underrepresented proteins associated with the biosynthesis of several Non-Ribosomal Peptide Synthases (NRPSs), clavams, hybrid NRPS/Polyketide synthases (PKSs) and tunicamycin. The most conspicuously underrepresented protein of amino acid metabolism was 4-Hydroxyphenylpyruvate dioxygenase (HppD) acting in tyrosine catabolism. The levels of a Two Component System (TCS) response regulator containing a CheY-like receiver domain and an HTH DNA-binding domain as well as DNA-binding protein HU were elevated while a TetR-family transcriptional regulator was underexpressed.

CONCLUSION

The results obtained herein will aid in finding out new targets for further improvement of cephamycin C production in Streptomyces clavuligerus.

Regulatory Control of Rishirilide(s) Biosynthesis in Streptomyces bottropensis

Streptomycetes are well-known producers of numerous bioactive secondary metabolites widely used in medicine, agriculture, and veterinary. Usually, their genomes encode 20-30 clusters for the biosynthesis of natural products. Generally, the onset and production of these compounds are tightly coordinated at multiple regulatory levels, including cluster-situated transcriptional factors. Rishirilides are biologically active type II polyketides produced by Streptomyces bottropensis. The complex regulation of rishirilides biosynthesis includes the interplay of four regulatory proteins encoded by the rsl-gene cluster: three SARP family regulators (RslR1-R3) and one MarR-type transcriptional factor (RslR4). In this work, employing gene deletion and overexpression experiments we revealed RslR1-R3 to be positive regulators of the biosynthetic pathway. Additionally, transcriptional analysis indicated that rslR2 is regulated by RslR1 and RslR3. Furthermore, RslR3 directly activates the transcription of rslR2, which stems from binding of RslR3 to the rslR2 promoter. Genetic and biochemical analyses demonstrated that RslR4 represses the transcription of the MFS transporter rslT4 and of its own gene. Moreover, DNA-binding affinity of RslR4 is strictly controlled by specific interaction with rishirilides and some of their biosynthetic precursors. Altogether, our findings revealed the intricate regulatory network of teamworking cluster-situated regulators governing the biosynthesis of rishirilides and strain self-immunity.

Clavulanic Acid Production by Streptomyces clavuligerus: Insights from Systems Biology, Strain Engineering, and Downstream Processing

Clavulanic acid (CA) is an irreversible β-lactamase enzyme inhibitor with a weak antibacterial activity produced by Streptomyces clavuligerus (S. clavuligerus). CA is typically co-formulated with broad-spectrum β‑lactam antibiotics such as amoxicillin, conferring them high potential to treat diseases caused by bacteria that possess β‑lactam resistance. The clinical importance of CA and the complexity of the production process motivate improvements from an interdisciplinary standpoint by integrating metabolic engineering strategies and knowledge on metabolic and regulatory events through systems biology and multi-omics approaches. In the large-scale bioprocessing, optimization of culture conditions, bioreactor design, agitation regime, as well as advances in CA separation and purification are required to improve the cost structure associated to CA production. This review presents the recent insights in CA production by S. clavuligerus, emphasizing on systems biology approaches, strain engineering, and downstream processing.

(p)ppGpp Metabolism and Antimicrobial Resistance in Bacterial Pathogens

Single cell microorganisms including pathogens relentlessly face myriads of physicochemical stresses in their living environment. In order to survive and multiply under such unfavorable conditions, microbes have evolved with complex genetic networks, which allow them to sense and respond against these stresses. Stringent response is one such adaptive mechanism where bacteria can survive under nutrient starvation and other related stresses. The effector molecules for the stringent response are guanosine-5'-triphosphate 3'-diphosphate (pppGpp) and guanosine-3', 5'-bis(diphosphate) (ppGpp), together called (p)ppGpp. These effector molecules are now emerging as master regulators for several physiological processes of bacteria including virulence, persistence, and antimicrobial resistance. (p)ppGpp may work independently or along with its cofactor DksA to modulate the activities of its prime target RNA polymerase and other metabolic enzymes, which are involved in different biosynthetic pathways. Enzymes involved in (p)ppGpp metabolisms are ubiquitously present in bacteria and categorized them into three classes, i.e., canonical (p)ppGpp synthetase (RelA), (p)ppGpp hydrolase/synthetase (SpoT/Rel/RSH), and small alarmone synthetases (SAS). While RelA gets activated in response to amino acid starvation, enzymes belonging to SpoT/Rel/RSH and SAS family can synthesize (p)ppGpp in response to glucose starvation and several other stress conditions. In this review, we will discuss about the current status of the following aspects: (i) diversity of (p)ppGpp biosynthetic enzymes among different bacterial species including enteropathogens, (ii) signals that modulate the activity of (p)ppGpp synthetase and hydrolase, (iii) effect of (p)ppGpp in the production of antibiotics, and (iv) role of (p)ppGpp in the emergence of antibiotic resistant pathogens. Emphasis has been given to the cholera pathogen Vibrio cholerae due to its sophisticated and complex (p)ppGpp metabolic pathways, rapid mutational rate, and acquisition of antimicrobial resistance determinants through horizontal gene transfer. Finally, we discuss the prospect of (p)ppGpp metabolic enzymes as potential targets for developing antibiotic adjuvants and tackling persistence of infections.

The Balance Metabolism Safety Net: Integration of Stress Signals by Interacting Transcriptional Factors in Streptomyces and Related Actinobacteria

Soil dwelling Streptomyces species are faced with large variations in carbon or nitrogen sources, phosphate, oxygen, iron, sulfur, and other nutrients. These drastic changes in key nutrients result in an unbalanced metabolism that have undesirable consequences for growth, cell differentiation, reproduction, and secondary metabolites biosynthesis. In the last decades evidence has accumulated indicating that mechanisms to correct metabolic unbalances in Streptomyces species take place at the transcriptional level, mediated by different transcriptional factors. For example, the master regulator PhoP and the large SARP-type regulator AfsR bind to overlapping sequences in the afsS promoter and, therefore, compete in the integration of signals of phosphate starvation and S-adenosylmethionine (SAM) concentrations. The cross-talk between phosphate control of metabolism, mediated by the PhoR-PhoP system, and the pleiotropic orphan nitrogen regulator GlnR, is very interesting; PhoP represses GlnR and other nitrogen metabolism genes. The mechanisms of control by GlnR of several promoters of ATP binding cassettes (ABC) sugar transporters and carbon metabolism are highly elaborated. Another important cross-talk that governs nitrogen metabolism involves the competition between GlnR and the transcriptional factor MtrA. GlnR and MtrA exert opposite effects on expression of nitrogen metabolism genes. MtrA, under nitrogen rich conditions, represses expression of nitrogen assimilation and regulatory genes, including GlnR, and competes with GlnR for the GlnR binding sites. Strikingly, these sites also bind to PhoP. Novel examples of interacting transcriptional factors, discovered recently, are discussed to provide a broad view of this interactions. Altogether, these findings indicate that cross-talks between the major transcriptional factors protect the cell metabolic balance. A detailed analysis of the transcriptional factors binding sequences suggests that the transcriptional factors interact with specific regions, either by overlapping the recognition sequence of other factors or by binding to adjacent sites in those regions. Additional interactions on the regulatory backbone are provided by sigma factors, highly phosphorylated nucleotides, cyclic dinucleotides, and small ligands that interact with cognate receptor proteins and with TetR-type transcriptional regulators. We propose to define the signal integration DNA regions (so called integrator sites) that assemble responses to different stress, nutritional or environmental signals. These integrator sites constitute nodes recognized by two, three, or more transcriptional factors to compensate the unbalances produced by metabolic stresses. This interplay mechanism acts as a safety net to prevent major damage to the metabolism under extreme nutritional and environmental conditions.

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.

The CagRS Two-Component System Regulates Clavulanic Acid Metabolism via Multiple Pathways in Streptomyces clavuligerus F613-1

Streptomyces clavuligerus F613-1 produces a clinically important β-lactamase inhibitor, clavulanic acid (CA). Although the biosynthesis pathway of CA has essentially been elucidated, the global regulatory mechanisms of CA biosynthesis remain unclear. The paired genes cagS and cagR, which are annotated, respectively, as orf22 and orf23 in S. clavuligerus ATCC 27064, encode a bacterial two-component regulatory system (TCS) and were found next to the CA biosynthetic gene cluster of S. clavuligerus F613-1. To further elucidate the regulatory mechanism of CA biosynthesis, the CagRS TCS was deleted from S. clavuligerus F613-1. Deletion of cagRS resulted in decreased production of CA, but the strain phenotype was not otherwise affected. Both transcriptome and ChIP-seq data revealed that, in addition to CA biosynthesis, the CagRS TCS mainly regulates genes involved in primary metabolism, such as glyceraldehyde 3-phosphate (G3P) metabolism and arginine biosynthesis. Notably, both G3P and arginine are precursors of CA. Electrophoretic mobility shift assays demonstrated that the response regulator CagR could bind to the intergenic regions of argG, argC, oat1, oat2, ceaS1, and claR in vitro, suggesting that CagR can directly regulate genes involved in arginine and CA biosynthesis. This study indicated that CagRS is a pleiotropic regulator that can directly affect the biosynthesis of CA and indirectly affect CA production by regulating the metabolism of arginine and G3P. Our findings provide new insights into the regulation of CA biosynthetic pathways and provide an innovative approach for future metabolic engineering efforts for CA production in S. clavuligerus.

Chromomycin, an antibiotic produced by Streptomyces flaviscleroticus might play a role in the resistance to oxidative stress and is essential for viability in stationary phase

The well-known role of antibiotics in killing sensitive organisms has been challenged by the effects they exert at subinhibitory concentrations. Unfortunately, there are very few published reports on the advantages these molecules may confer to their producers. This study describes the construction of a genetically verified deletion mutant of Streptomyces flaviscleroticus unable to synthesize chromomycin. This mutant was characterized by a rapid loss of viability in stationary phase that was correlated with high oxidative stress and altered antioxidant defences. Altered levels of key metabolites in the mutant signalled a redistribution of the glycolytic flux toward the PPP to generate NADPH to fight oxidative stress as well as reduction of ATP-phosphofructokinase and Krebs cycle enzymes activities. These changes were correlated with a shift in the preference for carbon utilization from glucose to amino acids. Remarkably, chromomycin at subinhibitory concentration increased longevity of the non-producer and restored most of the phenotypic features' characteristic of the wild type strain. Altogether these observations suggest that chromomycin may have antioxidant properties that would explain, at least in part, some of the phenotypes of the mutant. Our observations warrant reconsideration of the secondary metabolite definition and raise the possibility of crucial roles for their producers.

Antibacterial Discovery and Development: From Gene to Product and Back

Concern over the reports of antibiotic-resistant bacterial infections in hospitals and in the community has been publicized in the media, accompanied by comments on the risk that we may soon run out of antibiotics as a way to control infectious disease. Infections caused by Enterococcus faecium, Staphylococcus aureus, Klebsiella species, Clostridium difficile, Acinetobacter baumannii, Pseudomonas aeruginosa, Escherichia coli, and other Enterobacteriaceae species represent a major public health burden. Despite the pharmaceutical sector's lack of interest in the topic in the last decade, microbial natural products continue to represent one of the most interesting sources for discovering and developing novel antibacterials. Research in microbial natural product screening and development is currently benefiting from progress that has been made in other related fields (microbial ecology, analytical chemistry, genomics, molecular biology, and synthetic biology). In this paper, we review how novel and classical approaches can be integrated in the current processes for microbial product screening, fermentation, and strain improvement.

The Pathway-Specific Regulator ClaR of Streptomyces clavuligerus Has a Global Effect on the Expression of Genes for Secondary Metabolism and Differentiation

Streptomyces clavuligerus claR::aph is a claR-defective mutant, but in addition to its claR defect it also carries fewer copies of the resident linear plasmids pSCL2 and pSCL4 (on the order of 4 × 10(5)-fold lower than the wild-type strain), as shown by qPCR. To determine the function of ClaR without potential interference due to plasmid copy number, a new strain, S. clavuligerus ΔclaR::aac, with claR deleted and carrying the wild-type level of plasmids, was constructed. Transcriptomic analyses were performed in S. clavuligerus ΔclaR::aac and S. clavuligerus ATCC 27064 as the control strain. The new ΔclaR mutant did not produce clavulanic acid (CA) and showed a partial expression of genes for the early steps of the CA biosynthesis pathway and a very poor expression (1 to 8%) of the genes for the late steps of the CA pathway. Genes for cephamycin C biosynthesis were weakly upregulated (1.7-fold at 22.5 h of culture) in the ΔclaR mutant, but genes for holomycin biosynthesis were expressed at levels from 3- to 572-fold higher than in the wild-type strain, supporting the observed overproduction of holomycin by S. clavuligerus ΔclaR::aac. Interestingly, three secondary metabolites produced by gene clusters SMCp20, SMCp22, and SMCp24, encoding still-cryptic compounds, had partially or totally downregulated their genes in the mutant, suggesting a regulatory role for ClaR wider than previously reported. In addition, the amfR gene was downregulated, and consequently, the mutant did not produce aerial mycelium. Expression levels of about 100 genes in the genome were partially up- or downregulated in the ΔclaR mutant, many of them related to the upregulation of the sigma factor-encoding rpoE gene.

Enhancing effect of lysine combined with other compounds on cephamycin C production in Streptomyces clavuligerus

BACKGROUND

Lysine plays an important role in Streptomyces clavuligerus metabolism; it takes part in its catabolism, via cadaverine, and in its secondary metabolism, in which lysine is converted via 1-piperideine-6-carboxylate to alpha-aminoadipic acid, a beta-lactam antibiotic precursor. The role of lysine as an enhancer of cephamycin C production, when added to production medium at concentrations above 50 mmol l(-1), has already been reported in the literature, with some studies attributing a positive influence to multifunctional diamines, among other compounds. However, there is a lack of research on the combined effect of these compounds on antibiotic production.

RESULTS

Results from experimental design-based tests were used to conduct response surface-based optimization studies in order to investigate the synergistic effect of combining lysine with cadaverine, putrescine, 1,3-diaminopropane, or alpha-aminoadipic acid on cephamycin C volumetric production. Lysine combined with cadaverine influenced production positively, but only at low lysine concentrations. On the whole, higher putrescine concentrations (0.4 g l(-1)) affected negatively cephamycin C volumetric production. In comparison to culture media containing only lysine as additive, combinations of this amino acid with alpha-aminoadipic acid or 1,3-diaminopropane increased cephamycin C production by more than 100%.

CONCLUSION

This study demonstrated that different combinations of lysine with diamines or lysine with alpha-aminoadipic acid engender significant differences with respect to antibiotic volumetric production, with emphasis on the benefits observed for lysine combined with alpha-aminoadipic acid or 1,3-diaminopropane. This increase is explained by mathematical models and demonstrated by means of bioreactor cultivations. Moreover, it is consistent with the positive influence of these compounds on lysine conversion to alpha-aminoadipic acid, a limiting step in cephamycin C production.

ppGpp metabolism is involved in heterocyst development in the cyanobacterium Anabaena sp. strain PCC 7120

When deprived of a combined-nitrogen source in the growth medium, the filamentous cyanobacterium Anabaena sp. PCC 7120 (Anabaena) can form heterocysts capable of nitrogen fixation. The process of heterocyst differentiation takes about 20 to 24 h, during which extensive metabolic and morphological changes take place. Guanosine tetraphosphate (ppGpp) is the signal of the stringent response that ensures cell survival by adjusting major cellular activities in response to nutrient starvation in bacteria, and ppGpp accumulates at the early stage of heterocyst differentiation (J. Akinyanju, R. J. Smith, FEBS Lett. 107:173-176, 1979; J Akinyanju, R. J. Smith, New Phytol. 105:117-122, 1987). Here we show that all1549 (here designated relana) in Anabaena, homologous to relA/spoT, is upregulated in response to nitrogen deprivation and predominantly localized in vegetative cells. The disruption of relana strongly affects the synthesis of ppGpp, and the resulting mutant, all1549Ωsp/sm, fails to form heterocysts and to grow in the absence of a combined-nitrogen source. This phenotype can be complemented by a wild-type copy of relana. Although the upregulation of hetR is affected in the mutant, ectopic overexpression of hetR cannot rescue the phenotype. However, we found that the mutant rapidly loses its viability, within a time window of 3 to 6 h, following the deprivation of combined nitrogen. We propose that ppGpp plays a major role in rebalancing the metabolic activities of the cells in the absence of the nitrogen source supply and that this regulation is necessary for filament survival and consequently for the success of heterocyst differentiation.

A novel TetR family transcriptional regulator, SAV576, negatively controls avermectin biosynthesis in Streptomyces avermitilis

Avermectins produced by Streptomyces avermitilis are potent anti-parasitic agents that are useful in animal health care, agriculture, and the treatment of human infections. In a search for novel regulators that affect avermectin biosynthesis, comparative transcriptome analysis was performed between wild-type strain ATCC31267 and avermectin overproducing strain 76-02-e, revealing some differentially expressed genes. SAV576, which is downregulated in 76-02-e and encodes a TetR family transcriptional regulator (TFR), was shown to inhibit avermectin production by indirectly affecting the expression of ave genes. SAV576 directly repressed the transcription of its gene SAV576 and of adjacent genes SAV575 (encodes cytochrome P450/NADPH-ferrihemoprotein reductase) and SAV574. The SAV576-binding sites within the bidirectional SAV575-SAV576 promoter region were determined by DNase I footprinting assays. A consensus 15-bp palindromic sequence CCRTACRVYGTATGS was found in these binding sites and shown to be important for SAV576-binding activity. SAV575, an important target gene of SAV576, was shown to exert a positive effect on avermectin production. The study findings extend our limited knowledge of the complex regulation of avermectin biosynthesis and provide a basis for rational genetic manipulation of S. avermitilis to improve avermectin production through control of SAV576 and its target gene.

A simplified diphenylamine colorimetric method for growth quantification

Cell growth needs to be monitored in biological studies and bioprocess optimization. In special circumstances, such as microbial fermentations in media containing insoluble particles, accurate cell growth quantification is a challenge with current methods. Only the Burton method is applicable in such circumstances. The original Burton method was previously simplified by adopting a two-step sample pretreatment in perchloric acid procedure to eliminate the need for DNA extraction. Here, we further simplified the Burton method by replacing the previous two-step perchloric acid pretreatment with a new and one-step diphenylamine reagent pretreatment. The reliability and accuracy of this simplified method were assessed by measuring the biomass of four model microorganisms: Escherichia coli, Streptomyces clavuligerus, Saccharomyces cerevisiae, and Trichoderma reesei grown in normal media or those containing solid particles. The results demonstrate that this new simplified method performs comparably to the conventional methods, such as OD600 or the previously modified Burton method, and is much more sensitive than the dry weight method. Overall, the new method is simple, reliable, easy to perform, and generally applicable in most circumstances, and it reduces the operation time from more than 12 h (for the previously simplified Burton method) to about 2 h.

Molecular regulation of antibiotic biosynthesis in streptomyces

Streptomycetes are the most abundant source of antibiotics. Typically, each species produces several antibiotics, with the profile being species specific. Streptomyces coelicolor, the model species, produces at least five different antibiotics. We review the regulation of antibiotic biosynthesis in S. coelicolor and other, nonmodel streptomycetes in the light of recent studies. The biosynthesis of each antibiotic is specified by a large gene cluster, usually including regulatory genes (cluster-situated regulators [CSRs]). These are the main point of connection with a plethora of generally conserved regulatory systems that monitor the organism's physiology, developmental state, population density, and environment to determine the onset and level of production of each antibiotic. Some CSRs may also be sensitive to the levels of different kinds of ligands, including products of the pathway itself, products of other antibiotic pathways in the same organism, and specialized regulatory small molecules such as gamma-butyrolactones. These interactions can result in self-reinforcing feed-forward circuitry and complex cross talk between pathways. The physiological signals and regulatory mechanisms may be of practical importance for the activation of the many cryptic secondary metabolic gene cluster pathways revealed by recent sequencing of numerous Streptomyces genomes.

Cascades and networks of regulatory genes that control antibiotic biosynthesis

Onset of the biosynthesis of bioactive secondary metabolites in batch cultures of actinomycetes occurs after the rapid growth phase, following a transition phase which involves complex metabolic changes. This transition is triggered by nutrient starvation or by other environmental stress signals. Expression of genes encoding bioactive secondary metabolites is governed by cascades of pathway specific regulators and networks of cross-talking global regulators. Pathway specific regulators such as Streptomyces antibiotic regulatory proteins, LAL-type and LysR-type regulators respond to autoregulatory proteins that act in concert with their cognate ligands (e.g. γ-butyrolactone receptor proteins and their cognate γ-butyrolactone ligands). Global regulators such as PhoR-PhoP and other two component systems and orphan response regulators, such as GlnR, control set of genes affecting primary and secondary metabolism. GlnR and, therefore, nitrogen metabolism genes are under phosphate control exerted by binding of PhoP to PHO boxes located in the promoter region of GlnR. A few pleiotropic regulatory genes, such as areB (ndgR), dmdR1 or dasR connect primary metabolism (amino acid biosynthesis, N-acetylglucosamine or iron levels) with antibiotic biosynthesis. Some atypical response regulators that require specific small ligands appear to be involved in feedback control of antibiotic production. All these mechanisms together modulate, in a coordinated manner, different aspects of Streptomyces metabolism as a real "protection net" that prevents drastic changes in metabolism that may be deleterious for cell survival.

Development of Tools for Genetic Analysis of Phenanthrene Degradation and Nanopod Production by Delftia sp. Cs1-4

The bacterium Delftia sp. Cs1-4 produces novel extracellular structures (nanopods) in conjunction with its growth on phenanthrene. While a full genome sequence is available for strain Cs1-4, genetic tools that could be applied to study phenanthrene degradation/nanopod production have not been reported. Thus, the objectives of this study were to establish such tools, and apply them for molecular analysis of nanopod formation or phenanthrene degradation. Three types of tools were developed or validated. First, we developed a new expression system based on a strong promoter controlling expression of a surface layer protein (NpdA) from Delftia sp. Cs1-4, which was ca. 2,500-fold stronger than the widely used lactose promoter. Second, the Cre-loxP system was validated for generation of markerless, in-frame, gene deletions, and for in-frame gene insertions. The gene deletion function was applied to examine potential roles in nanopod formation of three genes (omp32, lasI, and hcp), while the gene insertion function was used for reporter gene tagging of npdA. Lastly, pMiniHimar was modified to enhance gene recovery and mutant analysis in genome-wide transposon mutagenesis. Application of the latter to strain Cs1-4, revealed several new genes with potential roles in phenanthrene degradation or npdA expression. Collectively, the availability of these tools has opened new avenues of investigation in Delftia sp. Cs1-4 and other related genera/species with importance in environmental toxicology.

A truncated form of SpoT, including the ACT domain, inhibits the production of cyclic lipopeptide arthrofactin, and is associated with moderate elevation of guanosine 3',5'-bispyrophosphate level in Pseudomonas sp. MIS38

Arthrofactin is a biosurfactant produced by Pseudomonas sp. MIS38. We have reported that transposon insertion into spoT (spoT::Tn5) causes moderate accumulation of guanosine 3',5'-bispyrophosphate (ppGpp) and abrogates arthrofactin production. To analyze the linkage of SpoT function and ablation of arthrofactin production, we examined the spoT::Tn5 mutation. The results showed that spoT::Tn5 is not a null mutation, but encodes separate segments of SpoT. Deletion of the 3' region of spoT increased the level of arthrofactin production, suggesting that the C-terminal region of SpoT plays a suppressive role. We evaluated the expression of a distinct segment of SpoT. Forced expression of the C-terminal region that contains the ACT domain resulted in the accumulation of ppGpp and abrogated arthrofactin production. Expression of the C-terminal segment also reduced MIS38 swarming and resulted in extensive biofilm formation, which constitutes the phenocopy of the spoT::Tn5 mutant.

Characterisation of a γ-butyrolactone receptor of Streptomyces tacrolimicus: effect on sporulation and tacrolimus biosynthesis

Streptomyces tacrolimicus (ATCC 55098) was reported to produce the immunosuppressant tacrolimus. The wild-type strain sporulates sparsely and produces very low levels of this immunosuppressant. The lack of genetic knowledge of this strain has hampered strain improvement. In this work, we have cloned the gene encoding a γ-butyrolactone receptor protein (Gbr). The gbr gene is linked to two genes encoding two subunits of the dihydroxyacetone kinase, putatively involved in the biosynthesis of the dihydroxyacetone phosphate precursor of γ-butyrolactone but is not flanked by γ-butyrolactone synthetase genes. The Gbr protein was overexpressed in Escherichia coli and purified. Electrophoretic mobility shift assays showed that Gbr binds to a specific autoregulatory element sequence located 338 bp upstream of the gbr gene, indicating that its expression is self-regulated. The deletion mutant Δgbr showed a very early and intense sporulation in two different media. A phenotype similar to that of the wild-type strain was restored by complementation of the Δgbr mutant with a wild-type gbr allele. Duplication of the gbr gene resulted in a slower sporulation. The Δgbr mutant produced much lower amount (32%) of tacrolimus quantified by high performance liquid chromatography. This analysis, using an optimised system, allowed the resolution of tacrolimus from ascomycin and other contaminant metabolites. Our results indicate that the Gbr protein regulates negatively the sporulation and positively the production of tacrolimus.

Repression of the antifungal activity of Pseudomonas sp. strain DF41 by the stringent response

The stringent response (SR) enables bacteria to adapt to nutrient limitation through production of the nucleotides guanosine tetraphosphate and guanosine pentaphosphate, collectively known as (p)ppGpp. Two enzymes are responsible for the intracellular pools of (p)ppGpp: RelA acts as a synthetase, while SpoT can function as either a synthetase or a hydrolase. We investigated how the SR affects the ability of the biological control agent Pseudomonas sp. strain DF41 to inhibit the fungal pathogen Sclerotinia sclerotiorum (Lib.) de Bary. Strain DF41 relA and relA spoT mutants were generated and found to exhibit increased antifungal activity. Strain DF41 produces a lipopeptide (LP) molecule that is essential for Sclerotinia biocontrol. LP production and protease activity were both elevated in the relA and relA spoT mutants. Addition of relA but not spoT in trans restored the mutant phenotype to that of the parent. Next, we investigated whether an association exists between the SR and known regulators of biocontrol, including the Gac system and RpoS. A gacS mutant of strain DF41 produced less (p)ppGpp and exhibited a 1.7-fold decrease in relA expression compared to the wild type, suggesting that relA forms part of the Gac regulon. We discovered that rpoS transcription was reduced significantly in the SR mutants. Furthermore, rpoS provided in trans restored protease activity to wild-type levels but did not attenuate antifungal activity. Finally, relA expression was decreased in the mutants, indicating that the SR is required for maximum expression of relA.

The regulation of the secondary metabolism of Streptomyces: new links and experimental advances

Streptomycetes and other actinobacteria are renowned as a rich source of natural products of clinical, agricultural and biotechnological value. They are being mined with renewed vigour, supported by genome sequencing efforts, which have revealed a coding capacity for secondary metabolites in vast excess of expectations that were based on the detection of antibiotic activities under standard laboratory conditions. Here we review what is known about the control of production of so-called secondary metabolites in streptomycetes, with an emphasis on examples where details of the underlying regulatory mechanisms are known. Intriguing links between nutritional regulators, primary and secondary metabolism and morphological development are discussed, and new data are included on the carbon control of development and antibiotic production, and on aspects of the regulation of the biosynthesis of microbial hormones. Given the tide of antibiotic resistance emerging in pathogens, this review is peppered with approaches that may expand the screening of streptomycetes for new antibiotics by awakening expression of cryptic antibiotic biosynthetic genes. New technologies are also described that have potential to greatly further our understanding of gene regulation in what is an area fertile for discovery and exploitation

(p)ppGpp inhibits polynucleotide phosphorylase from streptomyces but not from Escherichia coli and increases the stability of bulk mRNA in Streptomyces coelicolor

ppGpp regulates gene expression in a variety of bacteria and in plants. We proposed previously that ppGpp or its precursor, pppGpp [referred to collectively as (p)ppGpp], or both might regulate the activity of the enzyme polynucleotide phosphorylase in Streptomyces species. We have examined the effects of (p)ppGpp on the polymerization and phosphorolysis activities of PNPase from Streptomyces coelicolor, Streptomyces antibioticus, and Escherichia coli. We have shown that (p)ppGpp inhibits the activities of both Streptomyces PNPases but not the E. coli enzyme. The inhibition kinetics for polymerization using the Streptomyces enzymes are of the mixed noncompetitive type, suggesting that (p)ppGpp binds to a region other than the active site of the enzyme. ppGpp also inhibited the phosphorolysis of a model RNA substrate derived from the rpsO-pnp operon of S. coelicolor. We have shown further that the chemical stability of mRNA increases during the stationary phase in S. coelicolor and that induction of a plasmid-borne copy of relA in a relA-null mutant increases the chemical stability of bulk mRNA as well. We speculate that the observed inhibition in vitro may reflect a role of ppGpp in the regulation of antibiotic production in vivo.

RNA degradation and the regulation of antibiotic synthesis in Streptomyces

Streptomyces are Gram-positive, soil-dwelling bacteria that are prolific producers of antibiotics. Most of the antibiotics used in clinical and veterinary medicine worldwide are produced as natural products by members of the genus Streptomyces. The regulation of antibiotic production in Streptomyces is complex and there is a hierarchy of regulatory systems that extends from the level of individual biosynthetic pathways to global regulators that, at least in some streptomycetes, control the production of all the antibiotics produced by that organism. Ribonuclease III, a double-strand specific endoribonuclease, appears to be a global regulator of antibiotic production in Streptomyces coelicolor, the model organism for the study of streptomycete biology. In this review, the enzymology of RNA degradation in Streptomyces is reviewed in comparison with what is known about the degradation pathways in Escherichia coli and other bacteria. The evidence supporting a role for RNase III as a global regulator of antibiotic production in S. coelicolor is reviewed and possible mechanisms by which this regulation is accomplished are considered.

Chapter 16. Enzymology of beta-lactam compounds with cephem structure produced by actinomycete

Cephamycins are beta-lactam antibiotics with a cephem structure produced by actinomycetes. They are synthesized by a pathway similar to that of cephalosporin C in filamentous fungi but the actinomycetes pathway contains additional enzymes for the formation of the alpha-aminoadipic acid (AAA) precursor and for the final steps specific to cephemycins. Most of the biochemical and genetic studies on cephemycins have been made on cephemycin C biosynthesis in the producer strains Streptomyces clavuligerus ATCC27064 and Amycolatopsis lactamdurans NRRL3802. Genes encoding cephamycin C biosynthetic enzymes are clustered in both actinomycetes. Ten enzymatic steps are involved in the formation of cephamycin C. The precursor alpha-AAA is formed by the sequential action of lysine-6-aminotransferase and piperideine-6-carboxylate dehydrogenase. Steps common to cephalosporin C biosynthesis include the formation of the tripeptide L-delta-alpha-aminoadipyl-L-cysteinyl-D-valine (ACV) by ACV synthetase, the cyclization of ACV to form isopenicillin N (IPN) by IPN synthase, the epimerization of IPN to penicillin N by isopenicillin N epimerase, the ring expansion of penicillin N to a six member cephem ring by deacetoxycephalosporin C synthase (DAOCS) and the hydroxylation at C-3' by deacetylcephalosporin C hydroxylase. However, in actinomycetes, the epimerization step is different from that in cephalosporin-producing fungi, and the expansion of the ring and its hydroxylation are performed by separate enzymes. Specific steps in cephamycin biosynthesis include the carbamoylation at C-3' by cephem carbamoyl transferase and the introduction of a methoxyl group at C-7 by the joint action of a C-7 cephem-hydroxylase and a methyltransferase. All the enzymes of the pathway have been purified almost to homogeneity and the DAOC synthase and 7-hydroxycephem-methyltransferase (CmcI) of S. clavuligerus have been crystallized giving insights into the mode of action of these enzymes. The cefE gene of S. clavuligerus, encoding DAOCS, has been extensively used to expand the penicillin ring in filamentous fungi in vivo using DNA recombinant technology.

Glycerol utilization gene cluster in Streptomyces clavuligerus

The Streptomyces clavuligerus ATCC 27064 glycerol cluster gylR-glpF1K1D1 is induced by glycerol but is not affected by glucose. S. clavuligerus growth and clavulanic acid production are stimulated by glycerol, but this does not occur in a glpK1-deleted mutant. Amplification of glpK1D1 results in transformants yielding larger amounts of clavulanic acid in the wild-type strain and in overproducer S. clavuligerus Gap15-7-30 or S. clavuligerus Delta relA strains.