Dissecting the genome sequence of a clinical isolated Cunninghamella bertholletiae Z2 strain with rich cytochrome P450 enzymes (Article)

Mucormycosis is receiving much more attention because of its high morbidity and extremely high mortality rate in immunosuppressed populations. In this study, we isolated a Cunnignhamella bertholletiae Z2 strain from a skin lesion of a 14 year, 9 months old girl with acute lymphoblastic leukemia who die of infection from the Z2 strain. Genome sequencing was performed after isolation and amplification of the Z2 strain to reveal potential virulence factors and pathogenic mechanisms. The results showed that the genome size of the Z2 strain is 30.9 Mb with 9213 genes. Mucoral specific virulence factor genes found are ARF, CalN, and CoTH, while no gliotoxin biosynthesis gene cluster was found, which is a known virulence factor in Aspergillus fumigatus adapted to the environment. The Z2 strain was found to have 69 cytochrome P450 enzymes, which are potential drug resistant targets. Sensitivity testing of Z2 showed it was only inhibited by amphotericin B and posaconazole. Detailed genomic information of the C. bertholletiae Z2 strain may provide useful data for treatment.

A Novel Zn2Cys6 Transcription Factor, TopC, Positively Regulates Trichodin A and Asperpyridone A Biosynthesis in Tolypocladium ophioglossoides

Asperpyridone A represents an unusual class of pyridone alkaloids with demonstrated potential for hypoglycemic activity, primarily by promoting glucose consumption in HepG2 cells. Trichodin A, initially isolated from the marine fungus Trichoderma sp. strain MF106, exhibits notable antibiotic activities against Staphylococcus epidermidis. Despite their pharmacological significance, the regulatory mechanisms governing their biosynthesis have remained elusive. In this investigation, we initiated the activation of a latent gene cluster, denoted as "top", through the overexpression of the Zn2Cys6 transcription factor TopC in Tolypocladium ophioglossoides. The activation of the top cluster led to the biosynthesis of asperpyridone A, pyridoxatin, and trichodin A. Our study also elucidated that the regulator TopC exerts precise control over the biosynthesis of asperpyridone A and trichodin A through the detection of protein-nucleic acid interactions. Moreover, by complementing these findings with gene deletions involving topA and topH, we proposed a comprehensive biosynthesis pathway for asperpyridone A and trichodin A in T. ophioglossoides.

Daptomycin production enhancement by ARTP mutagenesis and fermentation optimization in Streptomyces roseosporus

AIMS

We evaluated whether the randomness of mutation breeding can be regulated through a double-reporter system. We hope that by establishing a new precursor feeding strategy, the production capacity of industrial microorganisms after pilot scale-up can be further improved.

METHODS AND RESULTS

In this study, the industrial strain Streptomyces roseosporus L2796 was used as the starter strain for daptomycin production, and a double-reporter system with the kanamycin resistance gene Neo and the chromogenic gene gusA was constructed to screen for high-yield strain L2201 through atmospheric and room temperature plasma (ARTP). Furthermore, the composition of the culture medium and the parameters of precursor replenishment were optimized, resulting in a significant enhancement of the daptomycin yield of the mutant strain L2201(752.67 mg/l).

CONCLUSIONS

This study successfully screened a high-yield strain of daptomycin through a double-reporter system combined with ARTP mutation. The expression level of two reporter genes can evaluate the strength of dptEp promoter, which can stimulate the expression level of dptE in the biosynthesis of daptomycin, thus producing more daptomycin. The developed multi-stage feeding rate strategy provides a novel way to increase daptomycin in industrial fermentation.

Production improvement of FK506 in Streptomyces tsukubaensis by metabolic engineering strategy

AIMS

Study of the effect of isoleucine on the biosynthesis of FK506 and modification of its producing strain to improve the production of FK506.

METHODS AND RESULTS

Metabolomics analysis was conducted to explore key changes in the metabolic processes of Streptomyces tsukubaensis Δ68 in medium with and without isoleucine. In-depth analysis revealed that the shikimate pathway, methylmalonyl-CoA, and pyruvate might be the rate-limiting factors in FK506 biosynthesis. Overexpression of involved gene PCCB1 in S. tsukubaensis Δ68, a high-yielding strain Δ68-PCCB1 was generated. Additionally, the amino acids supplement was further optimized to improve FK506 biosynthesis. Finally, FK506 production was increased to 929.6 mg L-1, which was 56.6% higher than that in the starter strain, when supplemented isoleucine and valine at 9 and 4 g L-1, respectively.

CONCLUSIONS

Methylmalonyl-CoA might be the key rate-limiting factors in FK506 biosynthesis and overexpression of the gene PCCB1 and further addition of isoleucine and valine could increase the yield of FK506 by 56.6%.

Activation and Characterization of Lanthomicins A-C by Promoter Engineering in Streptomyces chattanoogensis L10

The emergence of drug resistance highlights the importance of new drug discovery. Microbial secondary metabolites encoded in biosynthetic gene clusters (BGCs) are a prolific source of drugs, whereas most of these BGCs are cryptic. Thus, taking strategies to activate these cryptic BGCs is of great importance for potential drug discovery. In this work, three novel pentangular polyphenols lanthomicin A-C were identified by activating a cryptic aromatic polyketide BGC through promoter engineering combined with optimization of fermentation conditions. We further confirmed the involvement of lanthomicin (ltm) BGC in biosynthesis by CRISPR-Cpf1-assisted gene editing. Based on functional analysis of homologous genes, a putative biosynthetic pathway was proposed for the three lanthomicins. Particularly, lanthomicin A showed antiproliferative activity with IC₅₀ 0.17 μM for lung cancer cell line A-549. The discovery of lanthomicins brings new members to the pentangular polyphenol subclade of aromatic polyketide and demonstrates the potential of Streptomyces as a source for drug discovery.

Improving the Yield and Quality of Daptomycin in Streptomyces roseosporus by Multilevel Metabolic Engineering

Daptomycin is a cyclic lipopeptide antibiotic with a significant antibacterial action against antibiotic-resistant Gram-positive bacteria. Despite numerous attempts to enhance daptomycin yield throughout the years, the production remains unsatisfactory. This study reports the application of multilevel metabolic engineering strategies in Streptomyces roseosporus to reconstruct high-quality daptomycin overproducing strain L2797-VHb, including precursor engineering (i.e., refactoring kynurenine pathway), regulatory pathway reconstruction (i.e., knocking out negative regulatory genes arpA and phaR), byproduct engineering (i.e., removing pigment), multicopy biosynthetic gene cluster (BGC), and fermentation process engineering (i.e., enhancing O₂ supply). The daptomycin titer of L2797-VHb arrived at 113 mg/l with 565% higher comparing the starting strain L2790 (17 mg/l) in shake flasks and was further increased to 786 mg/l in 15 L fermenter. This multilevel metabolic engineering method not only effectively increases daptomycin production, but can also be applied to enhance antibiotic production in other industrial strains.

Redirection of acyl donor metabolic flux for lipopeptide A40926B0 biosynthesis

The metabolic flux of fatty acyl‐CoAs determines lipopeptide biosynthesis efficiency, because acyl donor competition often occurs from polyketide biosynthesis and homologous pathways. We used A40926B0 as a model to investigate this mechanism. The lipopeptide A40926B0 with a fatty acyl group is the active precursor of dalbavancin, which is considered as a new lipoglycopeptide antibiotic. The biosynthetic pathway of fatty acyl‐CoAs in the A40926B0 producer Nonomuraea gerenzanensis L70 was efficiently engineered using endogenous replicon CRISPR (erCRISPR). A polyketide pathway and straight‐chain fatty acid biosynthesis were identified as major competitors in the malonyl‐CoA pool. Therefore, we modified both pathways to concentrate acyl donors for the production of the desired compound. Combined with multiple engineering approaches, including blockage of an acetylation side reaction, overexpression of acetyl‐CoA carboxylase, duplication of the dbv gene cluster and optimization of the fermentation parameters, the final strain produced 702.4 mg l^‐1 of A40926B0, a 2.66‐fold increase, and the ratio was increased from 36.2% to 81.5%. Additionally, an efficient erCRISPR‐Cas9 editing system based on an endogenous replicon was specifically developed for L70, which increased conjugation efficiency by 660% and gene‐editing efficiency was up to 90%. Our strategy of redirecting acyl donor metabolic flux can be widely adopted for the metabolic engineering of lipopeptide biosynthesis.

Directed Evolution of a Nonheme Diiron N-oxygenase AzoC for Improving Its Catalytic Efficiency toward Nitrogen Heterocycle Substrates

The azoxy compounds with an intriguing chemical bond [-N=N⁺(-O⁻)-] are known to have broad applications in many industries. Our previous work revealed that a nonheme diiron N-oxygenase AzoC catalyzed the oxidization of amino-group to its nitroso analogue in the formation of azoxy bond in azoxymycins biosynthesis. However, except for the reported pyridine alkaloid azoxy compounds, most azoxy bonds of nitrogen heterocycles have not been biosynthesized so far, and the substrate scope of AzoC is limited to p-aminobenzene-type compounds. Therefore, it is very meaningful to use AzoC to realize the biosynthesis of azoxy nitrogen heterocycles compounds. In this work, we further studied the catalytic potential of AzoC toward nitrogen heterocycle substrates including 5-aminopyrimidine and 5-aminopyridine compounds to form new azoxy compounds through directed evolution. We constructed a double mutant L101I/Q104R via molecular engineering with improved catalytic efficiency toward 2-methoxypyrimidin-5-amine. These mutations also proved to be beneficial for N-oxygenation of methyl 5-aminopyrimidine-2-carboxylate. The structural analysis showed that relatively shorter distance between the substrate and the diiron center and amino acid residues of the active center may be responsible for the improvement of catalytic efficiency in L101I/Q104R. Our results provide a molecular basis for broadening the AzoC catalytic activity and its application in the biosynthesis of azoxy six-membered nitrogen catenation compounds.

Strategy for Producing the High-Quality Glycopeptide Antibiotic A82846B in Amycolatopsis orientalis Based on the CRISPR-Cas12a System

Oritavancin is a new-generation semisynthetic lipoglycopeptide antibiotic used to prevent the spread of vancomycin-resistant Gram-positive bacteria. The glycopeptide A82846B is the direct precursor of oritavancin. Considering the structural similarity between A82846B and vancomycin, the vancomycin producer Amycolatopsis orientalis was used as a chassis for the construction of a strain producing high-quality A82846B. To construct the A82846B synthetic pathway, we established a highly efficient CRISPR-Cas12a system by optimizing the conditions of conjugation and by screening the regulatory elements in the A. orientalis, which is difficult to be genetically manipulated. The efficiency of gene knockout was almost 100%. The glycosyltransferases module (gtfDE) and glycosyl synthesis module (vcaAEBD) in the vancomycin gene cluster were replaced with the corresponding glycosyltransferases module (gtfABC) and glycosyl synthesis module (evaAEBD) in the A82846B cluster, respectively. A82846B was successfully produced by the artificially constructed synthetic pathway. Moreover, the titer of A82846B was increased 80% by expressing the pathway-specific regulatory strR. This strategy has excellent potential for remodification of natural products to solve antibiotic resistance.

Discovery of a Potential Liver Fibrosis Inhibitor from a Mushroom Endophytic Fungus by Genome Mining of a Silent Biosynthetic Gene Cluster

Liver fibrosis has accounted for liver diseases and overall mortality, but no relevant drug has been developed. Filamentous fungi are important resources of natural products for pharmaceutical development. Calcarisporium arbuscula is a mushroom endophytic fungus, which primarily produces aurovertins. Here, in an aurovertin null-production mutant, one silent gene cluster (mca17) was activated by overexpression of a pathway-specific zinc finger transcriptional regulator, and a tetramic acid-type compound (1, MCA17-1) was identified. Along with detailed structural characterization, its biosynthesis was proposed to be produced from the core PKS-NRPS hybrid enzyme. Moreover, 1 suppressed the activation of LX-2 upon transforming growth factor-β (TGF-β) challenge and had stronger bioactivity than the positive control obeticholic acid (OCA) against liver fibrosis. Our work suggested that this engineered fungus could be a producer of 1 for promising pharmaceutical development, and alternatively, it would be developed as a mushroom ingredient in dietary therapy to prevent liver fibrosis.

Improvement of FK506 production via metabolic engineering-guided combinational strategies in Streptomyces tsukubaensis

Background

FK506, a macrolide mainly with immunosuppressive activity, can be produced by various Streptomyces strains. However, one of the major challenges in the fermentation of FK506 is its insufficient production, resulting in high fermentation costs and environmental burdens. Herein, we tried to improve its production via metabolic engineering-guided combinational strategies in Streptomyces tsukubaensis.

Results

First, basing on the genome sequencing and analysis, putative competitive pathways were deleted. A better parental strain L19-2 with increased FK506 production from 140.3 to 170.3 mg/L and a cleaner metabolic background was constructed. Subsequently, the FK506 biosynthetic gene cluster was refactored by in-situ promoter-substitution strategy basing on the regulatory circuits. This strategy enhanced transcription levels of the entire FK506 biosynthetic gene cluster in a fine-tuning manner and dramatically increased the FK506 production to 410.3 mg/mL, 1.41-fold higher than the parental strain L19-2 (170.3 mg/L). Finally, the FK506 production was further increased from 410.3 to 603 mg/L in shake-flask culture by adding L-isoleucine at a final concentration of 6 g/L. Moreover, the potential of FK506 production capacity was also evaluated in a 15-L fermenter, resulting in the FK506 production of 830.3 mg/L.

Conclusion

From the aspects of competitive pathways, refactoring of the FK506 biosynthetic gene cluster and nutrients-addition, a strategy for hyper-production and potentially industrial application of FK506 was developed and a hyper-production strain L19-9 was constructed. The strategy presented here can be generally applicable to other Streptomyces for improvement of FK506 production and streamline hyper-production of other valuable secondary metabolites.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01660-w.

Activation and discovery of tsukubarubicin from Streptomyces tsukubaensis through overexpressing SARPs

Genome sequencing has revealed that each Streptomyces contains a wide range of biosynthetic gene clusters (BGCs) and has the capability to produce more novel natural products than what is expected. However, most gene clusters for secondary metabolite biosynthesis are cryptic under normal growth conditions. In Streptomyces tsukubaensis, combining overexpression of the putative SARPs (Streptomyces antibiotic regulatory proteins) and bioactivity-guided screening, the silent gene cluster (tsu) was successfully activated and a novel bioactive anthracycline tsukubarubicin was further isolated and identified. Biological activity assays demonstrated that tsukubarubicin possessed much better antitumor bioactivities against various human cancer cell lines (especially the breast cancer cell lines) than clinically used doxorubicin. Moreover, the previously unreported gene cluster (tsu) for biosynthesis of tsukubarubicin was first characterized and detailed annotations of this gene cluster were also conducted. Our strategy presented in this work is broadly applicable in other Streptomyces and will assist in enriching the natural products for potential drug leads. KEY POINTS: • Generally scalable strategy to activate silent gene clusters by manipulating SARPs. • The novel anthracycline tsukubarubicin with potent antitumor bioactivities. • Identification and annotation of the previously uncharacterized tsu gene cluster.

A Cell Factory of a Fungicolous Fungus Calcarisporium arbuscula for Efficient Production of Natural Products

Fungal natural products are rich sources of clinical drugs. Particularly, the fungicolous fungi have a large number of biosynthetic gene clusters (BGCs) to produce numerous bioactive natural products, but most BGCs are silent in the laboratory. We have shown that a fungicolous fungus Calcarisporium arbuscula NRRL 3705 predominantly produces the highly reduced polyketide-type mycotoxins aurovertins. Here after evaluation of the aurovertin-null mutant ΔaurA as an efficient host, we further screened two strong promoters aurBp and A07068p based on RNA-Seq, and successfully activated an endogenous gene cluster from C. arbuscula as well as three additional exogenous BGCs from other fungi to produce polyketide-type natural products. Thus, we showed an efficient expression system from the fungicolous fungus C. arbuscula, which will be highly beneficial and complementary to the conventional Aspergillus and Penicillium fungal cell factories, and provides a useful toolkit for genome-wide mining of bioactive natural products from fungicolous fungi.

Genomic and transcriptomic survey of an endophytic fungus Calcarisporium arbuscula NRRL 3705 and potential overview of its secondary metabolites

BACKGROUND

Secondary metabolites as natural products from endophytic fungi are important sources of pharmaceuticals. However, there is currently little understanding of endophytic fungi at the omics levels about their potential in secondary metabolites. Calcarisporium arbuscula, an endophytic fungus from the fruit bodies of Russulaceae, produces a variety of secondary metabolites with anti-cancer, anti-nematode and antibiotic activities. A comprehensive survey of the genome and transcriptome of this endophytic fungus will help to understand its capacity to biosynthesize secondary metabolites and will lay the foundation for the development of this precious resource.

RESULTS

In this study, we reported the high-quality genome sequence of C. arbuscula NRRL 3705 based on Single Molecule Real-Time sequencing technology. The genome of this fungus is over 45 Mb in size, larger than other typical filamentous fungi, and comprises 10,001 predicted genes, encoding at least 762 secretory-proteins, 386 carbohydrate-active enzymes and 177 P450 enzymes. 398 virulence factors and 228 genes related to pathogen-host interactions were also predicted in this fungus. Moreover, 65 secondary metabolite biosynthetic gene clusters were revealed, including the gene cluster for the mycotoxin aurovertins. In addition, several gene clusters were predicted to produce mycotoxins, including aflatoxin, alternariol, destruxin, citrinin and isoflavipucine. Notably, two independent gene clusters were shown that are potentially involved in the biosynthesis of alternariol. Furthermore, RNA-Seq assays showed that only expression of the aurovertin gene cluster is much stronger than expression of the housekeeping genes under laboratory conditions, consistent with the observation that aurovertins are the predominant metabolites. Gene expression of the remaining 64 gene clusters for compound backbone biosynthesis was all lower than expression of the housekeeping genes, which partially explained poor production of other secondary metabolites in this fungus.

CONCLUSIONS

Our omics data, along with bioinformatics analysis, indicated that C. arbuscula NRRL 3705 contains a large number of biosynthetic gene clusters and has a huge potential to produce a profound number of secondary metabolites. This work also provides the basis for development of endophytic fungi as a new resource of natural products with promising biological activities.

Activation of anthrachamycin biosynthesis in Streptomyces chattanoogensis L10 by site-directed mutagenesis of rpoB

Genome sequencing projects revealed massive cryptic gene clusters encoding the undiscovered secondary metabolites in Streptomyces. To investigate the metabolic products of silent gene clusters in Streptomyces chattanoogensis L10 (CGMCC 2644), we used site-directed mutagenesis to generate ten mutants with point mutations in the highly conserved region of rpsL (encoding the ribosomal protein S12) or rpoB (encoding the RNA polymerase β-subunit). Among them, L10/RpoB (H437Y) accumulated a dark pigment on a yeast extract-malt extract-glucose (YMG) plate. This was absent in the wild type. After further investigation, a novel angucycline antibiotic named anthrachamycin was isolated and determined using nuclear magnetic resonance (NMR) spectroscopic techniques. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis and electrophoretic mobility shift assay (EMSA) were performed to investigate the mechanism underlying the activation effect on the anthrachamycin biosynthetic gene cluster. This work indicated that the rpoB-specific missense H437Y mutation had activated anthrachamycin biosynthesis in S. chattanoogensis L10. This may be helpful in the investigation of the pleiotropic regulation system in Streptomyces.

Comprehensive dissection of dispensable genomic regions in Streptomyces based on comparative analysis approach

BACKGROUND

Large-scale genome reduction has been performed to significantly improve the performance of microbial chassis. Identification of the essential or dispensable genes is pivotal for genome reduction to avoid synthetic lethality. Here, taking Streptomyces as an example, we developed a combinatorial strategy for systematic identification of large and dispensable genomic regions in Streptomyces based on multi-omics approaches.

RESULTS

Phylogenetic tree analysis revealed that the model strains including S. coelicolor A3(2), S. albus J1074 and S. avermitilis MA-4680 were preferred reference for comparative analysis of candidate genomes. Multiple genome alignment suggested that the Streptomyces genomes embodied highly conserved core region and variable sub-telomeric regions, and may present symmetric or asymmetric structure. Pan-genome and functional genome analyses showed that most conserved genes responsible for the fundamental functions of cell viability were concentrated in the core region and the vast majority of abundant genes were dispersed in the sub-telomeric regions. These results suggested that large-scale deletion can be performed in sub-telomeric regions to greatly streamline the Streptomyces genomes for developing versatile chassis.

CONCLUSIONS

The integrative approach of comparative genomics, functional genomics and pan-genomics can not only be applied to perform a multi-tiered dissection for Streptomyces genomes, but also work as a universal method for systematic analysis of removable regions in other microbial hosts in order to generate more miscellaneous and versatile chassis with minimized genome for drug discovery.

Multi-Layer Controls of Cas9 Activity Coupled With ATP Synthase Over-Expression for Efficient Genome Editing in Streptomyces

Efficient genome editing is a prerequisite of genetic engineering in synthetic biology, which has been recently achieved by the powerful CRISPR/Cas9 system. However, the toxicity of Cas9, due to its abundant intracellular expression, has impeded its extensive applications. Here we constructed a genetic cassette with triple controls of Cas9 activities at transcriptional, translational and protein levels, together with over-expression of the ATP synthase β-subunit AtpD, for the efficient genome editing in Streptomyces. By deletion of actII-ORF4 in Streptomyces coelicolor as a model, we found that constitutive expression of cas9 had about 90% editing efficiency but dramatically reduced transformation efficiency by 900-fold. However, triple controls of Cas9 under non-induction conditions to reduce its activity increased transformation efficiency over 250-fold, and had about 10% editing efficiency if combined with atpD overexpression. Overall, our strategy accounts for about 30-fold increased possibility for successful genome editing under the non-induction condition. In addition, about 80% editing efficiency was observed at the actII-ORF4 locus after simultaneous induction with thiostrepton, theophylline and blue light for Cas9 activity reconstitution. This improved straightforward efficient genome editing was also confirmed in another locus redD. Thus, we developed a new strategy for efficient genome editing, and it could be readily and widely adaptable to other Streptomyces species to improve genetic manipulation for rapid strain engineering in Streptomyces synthetic biology, due to the highly conserved genetic cassettes in this genus.

Dual regulation between the two-component system PhoRP and AdpA regulates antibiotic production in Streptomyces

Antibiotic production during secondary metabolism in Streptomyces spp. is elaborately controlled by multiple environmental signals and intracellular cascades. These include the two-component system PhoRP responding to phosphate starvation and a conserved signaling pathway mediated by the pleiotropic regulator AdpA. However, little information exists about how these two pathways work together for secondary metabolite production of Streptomyces. Herein, we report the dual regulation from the phosphate starvation-responsive regulator PhoP and AdpA on atrA promoter (atrAp) for the production of daptomycin, an antibiotic produced by Streptomyces roseosporus. We found that PhoP directly binds to atrAp, positively regulates atrA expression and thus daptomycin production. We also observed positive auto-regulation of phoRP expression during fermentation for daptomycin production. Moreover, partial overlap between PhoP- and AdpA-binding sites on atrAp was observed, which results in partial competitive binding between these two regulators. This partial overlapping and competition between PhoP and AdpA was further confirmed by mutations and binding assays. In summary, our findings have revealed dual regulation of PhoP and AdpA on the same promoter for antibiotic production in Streptomyces. This mechanism would be beneficial to further environment-responsive fermentation optimization for antibiotic production.

Rational construction of genome-reduced and high-efficient industrial Streptomyces chassis based on multiple comparative genomic approaches

BACKGROUND

Streptomyces chattanoogensis L10 is the industrial producer of natamycin and has been proved a highly efficient host for diverse natural products. It has an enormous potential to be developed as a versatile cell factory for production of heterologous secondary metabolites. Here we developed a genome-reduced industrial Streptomyces chassis by rational 'design-build-test' pipeline.

RESULTS

To identify candidate large non-essential genomic regions accurately and design large deletion rationally, we performed genome analyses of S. chattanoogensis L10 by multiple computational approaches, optimized Cre/loxP recombination system for high-efficient large deletion and constructed a series of universal suicide plasmids for rapid loxP or loxP mutant sites inserting into genome. Subsequently, two genome-streamlined mutants, designated S. chattanoogensis L320 and L321, were rationally constructed by depletion of 1.3 Mb and 0.7 Mb non-essential genomic regions, respectively. Furthermore, several biological performances like growth cycle, secondary metabolite profile, hyphae morphological engineering, intracellular energy (ATP) and reducing power (NADPH/NADP⁺) levels, transformation efficiency, genetic stability, productivity of heterologous proteins and secondary metabolite were systematically evaluated. Finally, our results revealed that L321 could serve as an efficient chassis for the production of polyketides.

CONCLUSIONS

Here we developed the combined strategy of multiple computational approaches and site-specific recombination system to rationally construct genome-reduced Streptomyces hosts with high efficiency. Moreover, a genome-reduced industrial Streptomyces chassis S. chattanoogensis L321 was rationally constructed by the strategy, and the chassis exhibited several emergent and excellent performances for heterologous expression of secondary metabolite. The strategy could be widely applied in other Streptomyces to generate miscellaneous and versatile chassis with minimized genome. These chassis can not only serve as cell factories for high-efficient production of valuable polyketides, but also will provide great support for the upgrade of microbial pharmaceutical industry and drug discovery.

Substrate Specificity of Acyltransferase Domains for Efficient Transfer of Acyl Groups

Acyltransferase domains (ATs) of polyketide synthases (PKSs) are critical for loading of acyl groups on acyl carrier protein domains (A) via self- and trans-acylation reactions, to produce structurally diverse polyketides. However, the interaction specificity between ATs and unusual acyl units is rarely documented. In Streptomycestsukubaensis YN06, we found that AT4_FkbB [an AT in the fourth module of tacrolimus (FK506) PKS] transferred both allylmalonyl (allmal) and emthylmalonyl (ethmal) units to ACPs, which was supposed responsible for the production of both FK506 and its analog FK520, respectively. Mutations of five residues in AT4_FkbB (Q119A, L185I-V186D-V187T, and F203L) caused decreased efficiency of allmal transfer, but a higher ratio of ethmal transfer, supposedly due to less nucleophilic attacks between Ser599 in the active site of AT4_FkbB and the carbonyl carbon in the allmal unit, as observed from molecular dynamics simulations. Furthermore, reverse mutations of these five residues in ethmal-specific ATs to the corresponding residues of AT4_FkbB increased its binding affinity to allmal-CoA. Among these residues, Val187 of AT4_FkbB mainly contributed to allmal recognition, and V187K mutant produced less FK520 than wild type. Our findings thus suggested that five critical residues within AT4_FkbB were important for AT functionality in polyketide extension and potentially for targeting biosynthesis by generating desirable products and eliminating undesirable analogs.

OsGLU3, a putative membrane-bound endo-1,4-beta-glucanase, is required for root cell elongation and division in rice (Oryza sativa L.)

Plant roots move through the soil by elongation. This is vital to their ability to anchor the plant and acquire water and minerals from the soil. In order to identify new genes involved in root elongation in rice, we screened an ethyl methane sulfonate (EMS)-mutagenized rice library, and isolated a short root mutant, Osglu3-1. The map-based cloning results showed that the mutant was due to a point mutation in OsGLU3, which encodes a putative membrane-bound endo-1,4-β-glucanase. Osglu3-1 displayed less crystalline cellulose content in its root cell wall, shorter root cell length, and a slightly smaller root meristem as visualized by restricted expression of OsCYCB1,1:GUS. Exogenous application of glucose can suppress both the lower root cell wall cellulose content and short root phenotypes of Osglu3-1. Consistently, OsGLU3 is ubiquitously expressed in various tissues with strong expression in root tip, lateral root, and crown root primodia. The fully functional OsGLU3-GFP was detected in plasma membrane, and FM4-64-labeled compartments in the root meristem and elongation zones. We also found that phosphate starvation, an environmental stress, altered cell wall cellulose content to modulate root elongation in a OsGLU3-dependant way.

Genetic variation in hypothalamic methyltrienolone (R1881) binding in male mice

Methyltrienolone (R1881) binding to androgen receptors (AR) from combined hypothalamic-preoptic-septal cytosol was examined in CF-1, CFW, and CD-1 male mice, strains that differ in their sensitivity to the aggression-promoting property of this hormone. Both the affinity of R1881-AR binding and the number of binding sites (Bmax) significantly differed among the genotypes. The dissociation constant (Kd) was lower in CF-1 males (6.7 nM), a behaviorally responsive strain, in comparison to the insensitive CFW and CD-1 males (3.0 nM and 2.0 nM, respectively). The number of binding sites was higher in CF-1 and CFW males (9.07 and 8.81 fmol/mg protein, respectively) than in CD-1 males (5.11 fmol). The results were basically consistent with studies of neural dihydrotestosterone (DHT) binding in these genotypes, and the implications and limits of these data for understanding the androgenic regulation of intermale aggression are discussed.