Enzyme and Metabolic Engineering Strategies for Biosynthesis of α-Farnesene in Saccharomyces cerevisiae

α-Farnesene, a type of acyclic sesquiterpene, is an important raw material in agriculture, aircraft fuel, and the chemical industry. In this study, we constructed an efficient α-farnesene-producing yeast cell factory by combining enzyme and metabolic engineering strategies. First, we screened different plants for α-farnesene synthase (AFS) with the best activity and found that AFS from Camellia sinensis (CsAFS) exhibited the most efficient α-farnesene production in Saccharomyces cerevisiae 4741. Second, the metabolic flux of the mevalonate pathway was increased to improve the supply of the precursor farnesyl pyrophosphate. Third, inducing site-directed mutagenesis in CsAFS, the CsAFSW281C variant was obtained, which considerably increased α-farnesene production. Fourth, the N-terminal serine-lysine-isoleucine-lysine (SKIK) tag was introduced to construct the SKIK∼CsAFSW281C variant, which further increased α-farnesene production to 2.8 g/L in shake-flask cultures. Finally, the α-farnesene titer of 28.3 g/L in S. cerevisiae was obtained by fed-batch fermentation in a 5 L bioreactor.

Transcriptome Analysis and Identification of Sesquiterpene Synthases in Liverwort Jungermannia exsertifolia

The liverwort Jungermannia exsertifolia is one of the oldest terrestrial plants and rich in structurally specific sesquiterpenes. There are several sesquiterpene synthases (STSs) with non-classical conserved motifs that have been discovered in recent studies on liverworts; these motifs are rich in aspartate and bind with cofactors. However, more detailed sequence information is needed to clarify the biochemical diversity of these atypical STSs. This study mined J. exsertifolia sesquiterpene synthases (JeSTSs) through transcriptome analysis using BGISEQ-500 sequencing technology. A total of 257,133 unigenes was obtained, and the average length was 933 bp. Among them, a total of 36 unigenes participated in the biosynthesis of sesquiterpenes. In addition, the in vitro enzymatic characterization and heterologous expression in Saccharomyces cerevisiae showed that JeSTS1 and JeSTS2 produced nerolidol as the major product, while JeSTS4 could produce bicyclogermacrene and viridiflorol, suggesting a specificity of J. exsertifolia sesquiterpene profiles. Furthermore, the identified JeSTSs had a phylogenetic relationship with a new branch of plant terpene synthases, the microbial terpene synthase-like (MTPSL) STSs. This work contributes to the understanding of the metabolic mechanism for MTPSL-STSs in J. exsertifolia and could provide an efficient alternative to microbial synthesis of these bioactive sesquiterpenes.

Association of lung-intestinal microecology and lung cancer therapy

In recent years, the incidence of lung cancer is increasing. Lung cancer has become one of the most malignant tumors with the highest incidence in the world, which seriously affects people's health. The most important cause of death of lung cancer is metastasis. Therefore, it is crucial to understand the mechanism of lung cancer progression and metastasis. This review article discusses the physiological functions, pathological states and disorders of the lung and intestine based on the concepts of traditional Chinese medicine (TCM), and analyzes the etiology and mechanisms of lung cancer formation from the perspective of TCM. From the theory of "the exterior and interior of the lung and gastrointestinal tract", the theory of "the lung-intestinal axis" and the progression and metastasis of lung cancer, we proposed e "lung-gut co-treatment" therapy for lung cancer. This study provides ideas for studying the mechanism of lung cancer and the comprehensive alternative treatment for lung cancer patients.

Development of Base Editors for Simultaneously Editing Multiple Loci in Lactococcus lactis

Lactococcus lactis serves as the most extensively studied model organism and an important dairy species. Though CRISPR-Cas9 systems have been developed for robust genetic manipulations, simultaneously editing multiple endogenous loci in L. lactis is still challenging. Herein, we first report the development of a double-strand break-free, robust, multiloci editing system CRISPR-deaminase-assisted base editor (CRISPR-DBE), which comprises a cytidine (CRISPR-cDBE) and an adenosine deaminase-assisted base editor (CRISPR-aDBE). Specifically targeted by a sgRNA, CRISPR-cDBE can efficiently introduce a cytidine-to-thymidine mutation and CRISPR-aDBE can high-efficiently convert adenosine to guanosine within a 5 nt editing window. CRISPR-cDBE was validated and successfully applied to simultaneously inactivate multiple genes using a single plasmid in L. lactis strain NZ9000. Meanwhile, the temperature-sensitive plasmid of CRISPR-DBE can be cured quickly, and the continuous gene editing of L. lactis has been achieved. Furthermore, CRISPR-cDBE can also efficiently convert the targeted C to T in a nisin-producing, industrial L. lactis strain F44. Finally, we applied genome-wide bioinformatics analysis to determine the scope of gene inactivation for these base editors using different Cas9 variants and evaluated the preference of SpGn and SpRYn variants for the protospacer adjacent motif in L. lactis NZ9000. Taken together, our study provides a powerful tool for simultaneously editing multiple loci in L. lactis, which may have a wide range of industrial applications in the future.

Nisin Variants Generated by Protein Engineering and Their Properties

Nisin, a typical lantibiotic, has robust antimicrobial activity combined with limited cytotoxicity, and the development of resistance to it is slow. These properties make nisin a promising antimicrobial agent to control pathogenic microorganisms in dairy foods. However, its low solubility, poor stability and short half-life at neutral pH limit its application within the dairy industry. Protein engineering technology has revealed the potential of modifying nisin to improve its properties, and many valuable variants have emerged. This review summarizes progress in the generation of nisin variants for the dairy industry and for other purposes. These nisin variants with additional modification have improved properties and can even expand the inhibition spectrum range of nisin. Nisin, as the most thoroughly studied lantibiotic, and its variants can also guide the modification of other lantibiotics.

Molecular and Functional Evolution of the Spermatophyte Sesquiterpene Synthases

Sesquiterpenes are important defense and signal molecules for plants to adapt to the environment, cope with stress, and communicate with the outside world, and their evolutionary history is closely related to physiological functions. In this study, the information of plant sesquiterpene synthases (STSs) with identified functions were collected and sorted to form a dataset containing about 500 members. The phylogeny of spermatophyte functional STSs was constructed based on the structural comparative analysis to reveal the sequence–structure–function relationships. We propose the evolutionary history of plant sesquiterpene skeletons, from chain structure to small rings, followed by large rings for the first time and put forward a more detailed function-driven hypothesis. Then, the evolutionary origins and history of spermatophyte STSs are also discussed. In addition, three newly identified STSs CaSTS2, CaSTS3, and CaSTS4 were analyzed in this functional evolutionary system, and their germacrene D products were consistent with the functional prediction. This demonstrates an application of the structure-based phylogeny in predicting STS function. This work will help us to understand evolutionary patterns and dynamics of plant sesquiterpenes and STSs and screen or design STSs with specific product profiles as functional elements for synthetic biology application.

Characterization of trans-Nerolidol Synthase from Celastrus angulatus Maxim and Production of trans-Nerolidol in Engineered Saccharomyces cerevisiae

Volatile terpenoids are a large group of important secondary metabolites and possess many biological activities. The acyclic sesquiterpene trans-nerolidol is one of the typical representatives and widely used in cosmetics and agriculture. Here, the accumulation of volatile terpenes in different tissues of Celastrus angulatus was investigated, and two trans-nerolidol synthases, CaNES1 and CaNES2, were identified and characterized by in vitro enzymatic assays. Both genes are differentially transcribed in different tissues of C. angulatus. Next, we constructed a Saccharomyces cerevisiae cell factory to enable high-level production of trans-nerolidol. Glucose was the sole carbon source to sequentially control gene expression between the competitive squalene and trans-nerolidol pathways. Finally, the trans-nerolidol production of recombinant strain LWG003-CaNES2 was 7.01 g/L by fed-batch fermentation in a 5 L bioreactor. The results clarify volatile terpenoid biosynthesis in C. angulatus and provide a promising potential for industrial production of trans-nerolidol in S. cerevisiae.

Comparative transcriptome analysis of sesquiterpene biosynthesis and functional characterization of sesquiterpene synthases in Leonurus sibiricus L

Two sesquiterpene synthases were identified through comparative transcriptome analysis of Leonurus sibiricus. LsSqTPS2 could produce high titer of δ-cadinene in vivo which suggests the terpene specificity of L. sibiricus. Leonurus sibiricus L., a medicinal herb, is widely used in China due to its pharmacological activities. Cadinene type sesquiterpenes, one of major bioactive components mainly present in aerial parts of L. sibiricus, showed antibacterial, anti-inflammatory, antioxidant and antiproliferative properties. However, there is no report about the sesquiterpene biosynthesis in L. sibiricus. This study identified L. sibiricus sesquiterpene synthases (LsSqTPSs) through comparative transcriptome analysis of L. sibiricus leaf and root samples using the BGISEQ-500 sequencing technique. A total of 83,244 unigenes were obtained with an average length of 1025 bp. Among them, 50,356 unigenes (60.49%) acquired annotations according to the BLAST searching results. A total of 68 differentially expressed genes (DEGs) were potentially involved in the sesquiterpene biosynthesis. Furthermore, four candidate DEGs encoding LsSqTPSs were characterized. The enzymatic characterization in engineered yeast showed that LsSqTPS1 produced α-farnesene as the single product and LsSqTPS2 mainly produced 76.23 mg/L of δ-cadinene, which constituted the major component of L. sibiricus leaf essential oil. This work contributes to the investigation of sesquiterpene biosynthesis in L. sibiricus.

De novo assembly of the Mylia taylorii transcriptome and identification of sesquiterpene synthases

Mylia taylorii is an ancient nonseed land plant that accumulates various sesquiterpenes with insecticidal and antibacterial activities. Recently, microbial-type sesquiterpene synthases (STSs) with atypical aspartate-rich metal ion binding motifs have been identified in some liverworts. Here, transcriptome analysis of M. taylorii was performed to identify M. taylorii sesquiterpene synthases (MtSTSs) that are potentially involved in sesquiterpene biosynthesis and diversity. A total of 255,669 unigenes were obtained with an average length of 963 bp in the transcriptome data of M. taylorii, among which 148,093 (57.92%) unigenes had BLAST results. Forty-eight unigenes were related to the sesquiterpene backbone biosynthesis according to KEGG annotation. In addition, MtSTS1, MtSTS2 and MtSTS3 identified from putative MtSTSs display sesquiterpene catalytic activities on the basis of functional characterizations in yeast. Interestingly, MtSTSs exhibit a noncanonical metal ion binding motif and the structural composition of a single α-domain, which are features of microbial STSs instead of archetypical plant STSs. This study revealed new microbial-type STS members of nonseed plants, and functionally identified that MtSTSs may contribute to the investigation of the biosynthesis and biological role of sesquiterpenes in M. taylorii.

[Construction of five-level practical teaching system for bioengineering under Emerging Engineering Education background]

Strengthening practical teaching, together with improving innovation ability is one of the key tasks of Emerging Engineering Education. This paper is based on the revision of the training program of bioengineering in School of Chemical Engineering and Technology, Tianjin University, improved the practical teaching system and curriculum content, built a five-level teaching system for basic experiment, comprehensive experiment, course design, scientific research and practical training. In order to cultivate outstanding innovative talents with practical ability and innovative spirit, innovative teaching reform mode is proposed. Furthermore the new thought and new schemes for Emerging Engineering Education are put forward.

Co-production of Nisin and γ-Aminobutyric Acid by Engineered Lactococcus lactis for Potential Application in Food Preservation

Microbiological contamination and oxidative damage are the two main challenges in maintaining quality and improving shelf-life of foods. Here, we developed a Lactococcus lactis fermentation system that could simultaneously produce nisin, an antimicrobial peptide, and γ-aminobutyric acid (GABA), an antioxidant agent. In this system, we metabolically engineered a nisin producing strain L. lactis F44 for GABA production by expression of glutamate decarboxylase and glutamate/GABA antiporter. GABA biosynthesis could facilitate nisin production through enhancing acid resistance of the strain. By applying a two-stage pH-control fermentation strategy, the engineered strain yielded up to 9.12 g/L GABA, which was 2.2 times higher than that of pH-constant fermentation. Furthermore, we demonstrated the potential application of the freeze-dried fermentation product as a preservative to improve the storage performance of meat and fruit. These results suggested that the fermentation product of nisin-GABA co-producing strain could serve as a cost-effective, easily prepared, and high-performance food preservative.

De novo leaf and root transcriptome analysis to explore biosynthetic pathway of Celangulin V in Celastrus angulatus maxim

Background

Celastrus angulatus Maxim is a kind of crucial and traditional insecticidal plant widely distributed in the mountains of southwest China. Celangulin V is the efficient insecticidal sesquiterpenoid of C. angulatus and widely used in pest control in China, but the low yield and discontinuous supply impeded its further popularization and application. Fortunately, the development of synthetic biology provided an opportunity for sustainable supply of Celangulin V, for which understanding its biosynthetic pathway is indispensable.

Results

In this study, six cDNA libraries were prepared from leaf and root of C. angulatus before global transcriptome analyses using the BGISEQ-500 platform. A total of 104,950 unigenes were finally obtained with an average length of 1200 bp in six transcriptome databases of C. angulatus, in which 51,817 unigenes classified into 25 KOG classifications, 39,866 unigenes categorized into 55 GO functional groups, and 48,810 unigenes assigned to 135 KEGG pathways, 145 of which were putative biosynthetic genes of sesquiterpenoid and triterpenoid. 16 unigenes were speculated to be related to Celangulin V biosynthesis. De novo assembled sequences were verified by Quantitative Real-Time PCR (qRT-PCR) analysis.

Conclusions

This study is the first report on transcriptome analysis of C. angulatus, and 16 unigenes probably involved in the biosynthesis of Celangulin V were finally collected. The transcriptome data will make great contributions to research for this specific insecticidal plant and the further gene mining for biosynthesis of Celangulin V and other sesquiterpene polyol esters.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5397-z) contains supplementary material, which is available to authorized users.

The genome and transcriptome of Lactococcus lactis ssp. lactis F44 and G423: Insights into adaptation to the acidic environment

Nisin, as a common green (environmentally friendly), nontoxic antibacterial peptide secreted by Lactococcus lactis, is widely used to prevent the decomposition of meat and dairy products and maintains relatively high stability at low pH. However, the growth of Lc. lactis is frequently inhibited by high lactic acid concentrations produced during fermentation. This phenomenon has become a great challenge in enhancing the nisin yield for this strain. Here, the shuffled strain G423 that could survive on a solid plate at pH 3.7 was generated through protoplast fusion-mediated genome shuffling. The nisin titer of G423 peaked at 4,543 IU/mL, which was 59.9% higher than that of the same batch of the initial strain Lc. lactis F44. The whole genome comparisons between G423 and F44 indicated that 6 large fragments (86,725 bp) were inserted in G423 compared with that of Lc. lactis F44. Transcriptome data revealed that 4 novel noncoding transcripts, and the significantly upregulated genes were involved in multiple processes in G423. In particular, the expression of genes involved in cell wall and membrane biosynthesis was obviously perturbed under acidic stress. Quantitative real-time PCR analysis showed that the transcription of noncoding small RNA NC-1 increased by 2.35-fold at pH 3.0 compared with that of the control (pH 7.0). Overexpression assays indicated that small RNA NC-1 could significantly enhance the acid tolerance and nisin production of G423 and F44. Our work provided new insights into the sophisticated genetic mechanisms involved in Lc. lactis in an acidic environment, which might elucidate its potential application in food and dairy industries.

Engineering Saccharomyces cerevisiae for Enhanced Production of Protopanaxadiol with Cofermentation of Glucose and Xylose

Protopanaxadiol (PPD), an active triterpene compound, is the precursor of high-value ginsenosides. In this study, we report a strategy for the enhancement of PPD production in Saccharomyces cerevisiae by cofermentation of glucose and xylose. In mixed sugar fermentation, strain GW6 showed higher PPD titer and yield than that obtained from glucose cultivation. Then, engineering strategies were implemented on GW6 to enhance the PPD yields, such as adjustment of the central carbon metabolism, optimization of the mevalonate pathway, reinforcement of the xylose assimilation pathway, and regulation of cofactor balance, namely, overexpression of xPK/PTA, ERG10/ERG12/ERG13, XYL1/XYL2/TAL1, and POS5, respectively. In particular, the final obtained strain GW10, harboring overexpressed POS5, exhibited the highest PPD yield, which was 2.06 mg of PPD/g of mixed sugar. In a 5-L fermenter, PPD titer reached 152.37 mg/L. These promising results demonstrate the great advantages of mixed sugar over glucose for high-yield production of PPD.

Two-stage carbon distribution and cofactor generation for improving l-threonine production of Escherichia coli

L-Threonine, a kind of essential amino acid, has numerous applications in food, pharmaceutical, and aquaculture industries. Fermentative l-threonine production from glucose has been achieved in Escherichia coli. However, there are still several limiting factors hindering further improvement of l-threonine productivity, such as the conflict between cell growth and production, byproduct accumulation, and insufficient availability of cofactors (adenosine triphosphate, NADH, and NADPH). Here, a metabolic modification strategy of two-stage carbon distribution and cofactor generation was proposed to address the above challenges in E. coli THRD, an l-threonine producing strain. The glycolytic fluxes towards tricarboxylic acid cycle were increased in growth stage through heterologous expression of pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and citrate synthase, leading to improved glucose utilization and growth performance. In the production stage, the carbon flux was redirected into l-threonine synthetic pathway via a synthetic genetic circuit. Meanwhile, to sustain the transaminase reaction for l-threonine production, we developed an l-glutamate and NADPH generation system through overexpression of glutamate dehydrogenase, formate dehydrogenase, and pyridine nucleotide transhydrogenase. This strategy not only exhibited 2.02- and 1.21-fold increase in l-threonine production in shake flask and bioreactor fermentation, respectively, but had potential to be applied in the production of many other desired oxaloacetate derivatives, especially those involving cofactor reactions.

Enhancing nisin yield by engineering a small noncodding RNA anti41 and inhibiting the expression of glnR in Lactococcus lactis F44

OBJECTIVES

To engineer a small nonconding RNA anti41 to enhance nisin yield by inhibiting the expression of glnR in Lactococcus lactis F44.

RESULTS

We constructed a screening library to determine appropriate artificial sRNAs and obtained a sRNA anti41 that can produce approximately three fold of the inhibitory effect on GlnR. Moreover, the transcription levels of the direct inhibitory targets of GlnR (glnP, glnQ, amtB, and glnK) were dramatically upregulated in the anti41 overexpression strain (F44-anti41), thereby confirming the inhibitory effect of anti41 on GlnR. In addition, anti41 overexpression improved the survival rate of cells by approximately three fold under acid stress, promoted cell growth, and increased nisin yield by 29.83%.

CONCLUSIONS

We were able to provide a novel strategy for the construction of robust high-producing industrial strains.

Redox cofactor engineering in industrial microorganisms: strategies, recent applications and future directions

NAD and NADP, a pivotal class of cofactors, which function as essential electron donors or acceptors in all biological organisms, drive considerable catabolic and anabolic reactions. Furthermore, they play critical roles in maintaining intracellular redox homeostasis. However, many metabolic engineering efforts in industrial microorganisms towards modification or introduction of metabolic pathways, especially those involving consumption, generation or transformation of NAD/NADP, often induce fluctuations in redox state, which dramatically impede cellular metabolism, resulting in decreased growth performance and biosynthetic capacity. Here, we comprehensively review the cofactor engineering strategies for solving the problematic redox imbalance in metabolism modification, as well as their features, suitabilities and recent applications. Some representative examples of in vitro biocatalysis are also described. In addition, we briefly discuss how tools and methods from the field of synthetic biology can be applied for cofactor engineering. Finally, future directions and challenges for development of cofactor redox engineering are presented.

Improving xylose utilization of defatted rice bran for nisin production by overexpression of a xylose transcriptional regulator in Lactococcus lactis

Present investigation explores the potential of defatted rice bran (DRB) serving as sole carbon source and partial nitrogen source to support Lactococcus lactis growth and nisin production. To retain the nutrients in DRB, especially protein fractions, thermal pretreatment followed by enzymatic hydrolysis without washing step was applied for saccharification. A maximum of 45.64g reducing sugar mainly containing 30.26g glucose and 5.66g xylose from 100g DRB was attained in hydrolysates of DRB (HD). A novel strategy of xylR (xylose transcriptional regulator) overexpression followed by evolutionary engineering was proposed, which significantly increased the capacity of L. lactis to metabolize xylose. Subsequently, RT-PCR results indicated that xylR overexpression stimulated expression of xylose assimilation genes synergistically with exposure to xylose. In HD medium, the highest nisin titer of the engineered strain FEXR was 3824.53IU/mL, which was 1.37 times of that in sucrose medium by the original strain F44.

The novel sRNA s015 improves nisin yield by increasing acid tolerance of Lactococcus lactis F44

Nisin, a polycyclic antibacterial peptide produced by Lactococcus lactis, is stable at low pH. Improving the acid tolerance of L. lactis could thus enhance nisin yield. Small non-coding RNAs (sRNAs) play essential roles in acid tolerance by regulating their target mRNAs at the post-transcriptional level. In this study, a novel sRNA, s015, was identified in L. lactis F44 via the use of RNA sequencing, qRT-PCR analysis, and Northern blotting. s015 improved the acid tolerance of L. lactis and boosted nisin yield at low pH. In silico predictions enabled us to construct a library of possible s015 target mRNAs. Statistical analysis and validation suggested that s015 contains a highly conserved region (5'-GAAAAAAAC-3') that likely encompasses the regulatory core of the sRNA. atpG, busAB, cysD, ilvB, tcsR, ung, yudD, and ywdA were verified as direct targets of s015, and the interactions between s015 and its target genes were elucidated. This work provided new insight into the adaptation mechanism of L. lactis under acid stress.

Enhance nisin yield via improving acid-tolerant capability of Lactococcus lactis F44

Traditionally, nisin was produced industrially by using Lactococcus lactis in the neutral fermentation process. However, nisin showed higher activity in the acidic environment. How to balance the pH value for bacterial normal growth and nisin activity might be the key problem. In this study, 17 acid-tolerant genes and 6 lactic acid synthetic genes were introduced in L. lactis F44, respectively. Comparing to the 2810 IU/mL nisin yield of the original strain F44, the nisin titer of the engineered strains over-expressing hdeAB, ldh and murG, increased to 3850, 3979 and 4377 IU/mL, respectively. These engineered strains showed more stable intracellular pH value during the fermentation process. Improvement of lactate production could partly provide the extra energy for the expression of acid tolerance genes during growth. Co-overexpression of hdeAB, murG, and ldh(Z) in strain F44 resulted in the nisin titer of 4913 IU/mL. The engineered strain (ABGL) could grow on plates with pH 4.2, comparing to the surviving pH 4.6 of strain F44. The fed-batch fermentation showed nisin titer of the co-expression L. lactis strain could reach 5563 IU/mL with lower pH condition and longer cultivation time. This work provides a novel strategy of constructing robust strains for use in industry process.

Immobilization of Streptomyces thermotolerans 11432 on polyurethane foam to improve production of acetylisovaleryltylosin

In this study, polyurethane foam (PUF) was chemically treated to immobilize Streptomyces thermotolerans 11432 for semi-continuous production of acetylisovaleryltylosin (AIV). Based on experimental results, positive cross-linked PUF (PCPUF) was selected as the most effective carrier according to immobilized cell mass. The effect of adsorption time on immobilized mass was investigated. AIV concentration (33.54 mg/l) in batch fermentations with immobilized cells was higher than with free cells (20.34 mg/l). In repeated batch fermentations with immobilized S. thermotolerans 11432 using PCPUF cubes, high AIV concentrations and conversion rates were attained, ranging from 25.56 to 34.37 mg/l and 79.93 to 86.31 %, respectively. Significantly, this method provides a feasible strategy for efficient AIV production and offers the potential for large-scale production.

Suitable extracellular oxidoreduction potential inhibit rex regulation and effect central carbon and energy metabolism in Saccharopolyspora spinosa

Background

Polyketides, such as spinosad, are mainly synthesized in the stationary phase of the fermentation. The synthesis of these compounds requires many primary metabolites, such as acetyl-CoA, propinyl-CoA, NADPH, and succinyl-CoA. Their synthesis is also significantly influenced by NADH/NAD⁺. Rex is the sensor of NADH/NAD⁺ redox state, whose structure is under the control of NADH/NAD⁺ ratio. The structure of rex controls the expression of many NADH dehydrogenases genes and cytochrome bd genes. Intracellular redox state can be influenced by adding extracellular electron acceptor H₂O₂. The effect of extracellular oxidoreduction potential on spinosad production has not been studied. Although extracellular oxidoreduction potential is an important environment effect in polyketides production, it has always been overlooked. Thus, it is important to study the effect of extracellular oxidoreduction potential on Saccharopolyspora spinosa growth and spinosad production.

Results

During stationary phase, S. spinosa was cultured under oxidative (H₂O₂) and reductive (dithiothreitol) conditions. The results show that the yield of spinosad and pseudoaglycone increased 3.11 fold under oxidative condition. As H₂O₂ can be served as extracellular electron acceptor, the ratios of NADH/NAD⁺ were measured. We found that the ratio of NADH/NAD⁺ under oxidative condition was much lower than that in the control group. The expression of cytA and cytB in the rex mutant indicated that the expression of these two genes was controlled by rex, and it was not activated under oxidative condition. Enzyme activities of PFK, ICDH, and G6PDH and metabolites results indicated that more metabolic flux flow through spinosad synthesis.

Conclusion

The regulation function of rex was inhibited by adding extracellular electron acceptor-H₂O₂ in the stationary phase. Under this condition, many NADH dehydrogenases which were used to balance NADH/NAD⁺ by converting useful metabolites to useless metabolites and unefficient terminal oxidases (cytochrome bd) were not expressed. So lots of metabolites were not waste to balance. As a result, un-wasted metabolites related to spinosad and PSA synthesis resulted in a high production of spinosad and PSA under oxidative condition.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0098-z) contains supplementary material, which is available to authorized users.

Kinetic analysis and modeling of daptomycin batch fermentation by Streptomyces roseosporus

In this study, Streptomyces roseosporus was subjected to helium-neon (He-Ne) laser (632.8 nm) irradiation to improve the production ability of extracellular antibiotic daptomycin. Under the optimum irradiation dosage of 18 mW for 22 min, a stable positive mutant strain S. roseosporus LC-54 was obtained. The maximum A21978C (daptomycin is a semisynthetic antimicrobial substance derived from the A21978C complex) yield of this mutant strain was 296 mg/l, which was 146% higher than that of the wild strain. The mutant strain grew more quickly and utilized carbohydrate sources more efficiently than the wild strain. The batch culture kinetics was investigated in a 7 l bioreactor. The logistic equation for growth, the Luedeking-Piret equation for daptomycin production, and Luedeking-Piret-like equations for carbon substrate consumption were established. This model appeared to provide a reasonable description for each parameter during the growth phase and fitted fairly well with the experiment data.

Mutation of Candida tropicalis by irradiation with a He-Ne laser to increase its ability to degrade phenol

Candida tropicalis isolated from acclimated activated sludge was used in this study. Cell suspensions with 5 x 10(7) cells ml(-1) were irradiated by using a He-Ne laser. After mutagenesis, the irradiated cell suspension was diluted and plated on yeast extract-peptone-dextrose (YEPD) medium. Plates with approximately 20 individual colonies were selected, and all individual colonies were harvested for phenol biodegradation. The phenol biodegradation stabilities for 70 phenol biodegradation-positive mutants, mutant strains CTM 1 to 70, ranked according to their original phenol biodegradation potentials, were tested continuously during transfers. Finally, mutant strain CTM 2, which degraded 2,600 mg liter(-1) phenol within 70.5 h, was obtained on the basis of its capacity and hereditary stability for phenol biodegradation. The phenol hydroxylase gene sequences were cloned in wild and mutant strains. The results showed that four amino acids were mutated by irradiation with a laser. In order to compare the activity of phenol hydroxylase in wild and mutant strains, their genes were expressed in Escherichia coli BL21(DE3) and enzyme activities were spectrophotometrically determined. It was clear that the activity of phenol hydroxylase was promoted after irradiation with a He-Ne laser. In addition, the cell growth and intrinsic phenol biodegradation kinetics of mutant strain CTM 2 in batch cultures were also described by Haldane's kinetic equation with a wide range of initial phenol concentrations from 0 to 2,600 mg liter(-1). The specific growth and degradation rates further demonstrated that the CTM 2 mutant strain possessed a higher capacity to resist phenol toxicity than wild C. tropicalis did.

Mutant AFM 2 of Alcaligenes faecalis for phenol biodegradation using He-Ne laser irradiation

He-Ne laser technology was utilized in this study to investigate the response of Alcaligenes faecalis to laser stimulation. The irradiation experiments were conducted by the adjustment of the output power from 5 to 25 mW and the exposure time from 5 to 25 min. The results showed that the survival rate changed regularly with the variety of irradiation dose, and high positive mutation frequency was determined by both the energy density and the output power. The mutant strain AFM 2 was obtained. Phenol biodegradation assay demonstrated that AFM 2 possessed a more prominent phenol-degrading potential than its parent strain, which presumably attributed to the improvements of phenol hydroxylase and catechol 1,2-dioxygenase activities. The phenol of 2000 mgl(-1) was completely degraded by AFM 2 within 85.5h at 30 degrees C. In addition, the cell growth and phenol degradation kinetics of the mutant strain AFM 2 and its parent strain in batch cultures were also investigated at the wide initial phenol concentration ranging from 0 to 2000 mgl(-1) by Haldane model. The results of these experiments further demonstrated that the mutant strain AFM 2 possessed a higher capacity to resist phenol.