Site-specific integration of bacteriophage VWB genome into Streptomyces venezuelae and construction of a VWB-based integrative vector

How to engineer giant enzymes: A methodology for mutagenesis of polyketide synthases in native hosts

Natural products are a fascinating source of chemical diversity and their biosynthetic pathways of biological complexity. The investigation and engineering of biosynthetic pathways towards polyketides in Actinomycetes provides challenges across all steps of the mutagenesis procedure. The typically GC-rich and long genes require robust PCR protocols. The resulting amplicons, often exceeding 10 kbp in length, require equally robust cloning procedures. Finally, the genetic manipulation of Actinomycetes, especially Streptomyces spp., calls for specialized procedures, in particular when the construction of several hundred variants is needed. This chapter will detail methods for all three steps of the process and have been previously used to generate numerous polyketide synthase variants in several Actinomycete species.

Current Approaches for Genetic Manipulation of Streptomyces spp.-Key Bacteria for Biotechnology and Environment

Organisms from the genus Streptomyces feature actinobacteria with complex developmental cycles and a great ability to produce a variety of natural products. These soil bacteria produce more than 2/3 of antibiotics used in medicine, and a large variety of bioactive compounds for industrial, medical and agricultural use. Although Streptomyces spp. have been studied for decades, the engineering of these bacteria remains challenging, and the available genetic tools are rather limited. Furthermore, most biosynthetic gene clusters in these bacteria are silent and require strategies to activate them and exploit their production potential. In order to explore, understand and manipulate the capabilities of Streptomyces spp. as a key bacterial for biotechnology, synthetic biology strategies emerged as a valuable component of Streptomyces research. Recent advancements in strategies for genetic manipulation of Streptomyces involving proposals of a large variety of synthetic components for the genetic toolbox, as well as new approaches for genome mining, assembly of genetic constructs and their delivery into the cell, allowed facilitation of the turnaround time of strain engineering and efficient production of new natural products at an industrial scale, but still have strain- and design-dependent limitations. A new perspective offered recently by technical advances in DNA sequencing, analysis and editing proposed strategies to overcome strain- and construct-specific difficulties in the engineering of Streptomyces. In this review, challenges and recent developments of approaches for Streptomyces engineering are discussed, an overview of novel synthetic biology strategies is provided and examples of successful application of new technologies in molecular genetic engineering of Streptomyces are highlighted.

Biosynthesis of Arcyriaflavin F from Streptomyces venezuelae ATCC 10712

Indolocarbazoles are natural products with a broad spectrum of bioactivity. A distinct feature of indolocarbazole biosynthesis is the modification of the indole and maleimide rings by regioselective tailoring enzymes. Here, we study a new indolocarbazole variant, which is encoded by the acfXODCP genes from Streptomyces venezuelae ATCC 10712. We characterise the pathway by expressing the acfXODCP genes in Streptomyces coelicolor, which led to the production of a C-5/C-5'-dihydroxylated indolocarbazole, which we assign as arcyriaflavin F. We also show that a flavin-dependent monooxygenase AcfX catalyses the C-5/C-5' dihydroxylation of the unsubstituted arcyriaflavin A into arcyriaflavin F. Interestingly, AcfX shares homology to EspX from erdasporine A biosynthesis, which instead catalyses a single C-6 indolocarbazole hydroxylation. In summary, we report a new indolocarbazole biosynthetic pathway and a regioselective C-5 indole ring tailoring enzyme AcfX.

Cloning and Expression of Metagenomic DNA in Streptomyces lividans and Its Subsequent Fermentation for Optimized Production

The choice of an expression system for the metagenomic DNA of interest is of vital importance for the detection of any particular gene or gene cluster. Most of the screens to date have used the Gram-negative bacterium Escherichia coli as a host for metagenomic gene libraries. However, the use of E. coli introduces a potential host bias since only 40% of the enzymatic activities may be readily recovered by random cloning in E. coli. To recover some of the remaining 60%, alternative cloning hosts such as Streptomyces spp. have been used. Streptomycetes are high-GC Gram-positive bacteria belonging to the Actinomycetales and they have been studied extensively for more than 25 years as an alternative expression system. They are extremely well suited for the expression of DNA from other actinomycetes and genomes of high GC content. Furthermore, due to its high innate, extracellular secretion capacity, Streptomyces can be a better system than E. coli for the production of many extracellular proteins. In this article, an overview is given about the materials and methods for growth and successful expression and secretion of heterologous proteins from diverse origin using Streptomyces lividans as a host. More in detail, an overview is given about the protocols of transformation, type of plasmids used and of vectors useful for integration of DNA into the host chromosome, and accompanying cloning strategies. In addition, various control elements for gene expression including synthetic promoters are discussed, and methods to compare their strength are described. Stable and efficient marker-less integration of the gene of interest under the control of the promoter of choice into S. lividans chromosome via homologous recombination using pAMR23A-based system will be explained. Finally, a basic protocol for bench-top bioreactor experiments which can form the start in the production process optimization and up-scaling will be provided.

A BioBricks toolbox for metabolic engineering of the tetracenomycin pathway

BACKGROUND/GOAL/AIM

The tetracenomycins are aromatic anticancer polyketides that inhibit peptide translation via binding to the large ribosomal subunit. Here, we expressed the elloramycin biosynthetic gene cluster in the heterologous host Streptomyces coelicolor M1146 to facilitate the downstream production of tetracenomycin analogs.

MAIN METHODS AND MAJOR RESULTS

We developed a BioBricks genetic toolbox of genetic parts for substrate precursor engineering in S. coelicolor M1146::cos16F4iE. We cloned a series of integrating vectors based on the VWB, TG1, and SV1 integrase systems to interrogate gene expression in the chromosome. We genetically engineered three separate genetic constructs to modulate tetracenomycin biosynthesis: (1) the vhb hemoglobin from obligate aerobe Vitreoscilla stercoraria to improve oxygen utilization; (2) the accA2BE acetyl-CoA carboxylase to enhance condensation of malonyl-CoA; (3) lastly, the sco6196 acyltransferase, which is a "metabolic regulatory switch" responsible for mobilizing triacylglycerols to β-oxidation machinery for acetyl-CoA. In addition, we engineered the tcmO 8-O-methyltransferase and newly identified tcmD 12-O-methyltransferase from Amycolatopsis sp. A23 to generate tetracenomycins C and X. We also co-expressed the tcmO methyltransferase with oxygenase urdE to generate the analog 6-hydroxy-tetracenomycin C.

CONCLUSIONS AND IMPLICATIONS

Altogether, this system is compatible with the BioBricks [RFC 10] cloning standard for the co-expression of multiple gene sets for metabolic engineering of Streptomyces coelicolor M1146::cos16F4iE. This production platform improves access to potent analogs, such as tetracenomycin X, and sets the stage for the production of new tetracenomycins via combinatorial biosynthesis.

Genetic approaches to improve clorobiocin production in Streptomyces roseochromogenes NRRL 3504

Streptomyces roseochromogenes NRRL 3504 is best known as a producer of clorobiocin, a DNA replication inhibitor from the aminocoumarin family of antibiotics. This natural product currently draws attention as a promising adjuvant for co-application with other antibiotics against Gram-negative multidrug-resistant pathogens. Herein, we expand the genetic toolkit for NRRL 3504 by showing that a set of integrative and replicative vectors, not tested previously for this strain, could be conjugally transferred at high frequency from Escherichia coli to NRRL 3504. Using this approach, we leverage a cumate-inducible expression of cluster-situated regulatory gene novG to increase clorobiocin titers by 30-fold (up to approximately 200 mg/L). To our best knowledge, this is the highest level of clorobiocin production reported so far. Our findings set a working ground for further improvement of clorobiocin production as well as for the application of genetic methods to illuminate the cryptic secondary metabolome of NRRL 3504. Key Points • Efficient system for conjugative transfer of plasmids into NRRL 3504 was developed. • Expression of regulatory genes in NRRL 3504 led to increase in clorobiocin titer. • Secondary metabolome of NRRL 3504 becomes an accessible target for genetic manipulations using the expanded vector set and improved intergeneric conjugation protocol.

Streptomyces as Microbial Chassis for Heterologous Protein Expression

Heterologous production of recombinant proteins is gaining increasing interest in biotechnology with respect to productivity, scalability, and wide applicability. The members of genus Streptomyces have been proposed as remarkable hosts for heterologous production due to their versatile nature of expressing various secondary metabolite biosynthetic gene clusters and secretory enzymes. However, there are several issues that limit their use, including low yield, difficulty in genetic manipulation, and their complex cellular features. In this review, we summarize rational engineering approaches to optimizing the heterologous production of secondary metabolites and recombinant proteins in Streptomyces species in terms of genetic tool development and chassis construction. Further perspectives on the development of optimal Streptomyces chassis by the design-build-test-learn cycle in systems are suggested, which may increase the availability of secondary metabolites and recombinant proteins.

An Efficient Markerless Deletion System Suitable for the Industrial Strains of Streptomyces

The genus Streptomyces is intensively studied due to its excellent ability to produce secondary metabolites with diverse bioactivities. In particular, adequate precursors of secondary metabolites as well as sophisticated post modification systems make some high-yield industrial strains of Streptomyces the promising chassis for the heterologous production of natural products. However, lack of efficient genetic tools for the manipulation of industrial strains, especially the episomal vector independent tools suitable for large DNA fragment deletion, makes it difficult to remold the metabolic pathways and streamline the genomes in these strains. In this respect, we developed an efficient deletion system independent of the episomal vector for large DNA fragment deletion. Based on this system, four large segments of DNA, ranging in length from 10 kb to 200 kb, were knocked out successfully from three industrial Streptomyces strains without any marker left. Notably, compared to the classical deletion system used in Streptomyces, this deletion system takes about 25% less time in our cases. This work provides a very effective tool for further genetic engineering of the industrial Streptomyces.

Streptomyces: host for refactoring of diverse bioactive secondary metabolites

Microbial secondary metabolites are intensively explored due to their demands in pharmaceutical, agricultural and food industries. Streptomyces are one of the largest sources of secondary metabolites having diverse applications. In particular, the abundance of secondary metabolites encoding biosynthetic gene clusters and presence of wobble position in Streptomyces strains make it potential candidate as a native or heterologous host for secondary metabolite production including several cryptic gene clusters expression. Here, we have discussed the developments in Streptomyces strains genome mining, its exploration as a suitable host and application of synthetic biology for refactoring genetic systems for developing chassis for enhanced as well as novel secondary metabolites with reduced genome and cleaned background.

The Design-Build-Test-Learn cycle for metabolic engineering of Streptomycetes

Streptomycetes are producers of a wide range of specialized metabolites of great medicinal and industrial importance, such as antibiotics, antifungals, or pesticides. Having been the drivers of the golden age of antibiotics in the 1950s and 1960s, technological advancements over the last two decades have revealed that very little of their biosynthetic potential has been exploited so far. Given the great need for new antibiotics due to the emerging antimicrobial resistance crisis, as well as the urgent need for sustainable biobased production of complex molecules, there is a great renewed interest in exploring and engineering the biosynthetic potential of streptomycetes. Here, we describe the Design-Build-Test-Learn (DBTL) cycle for metabolic engineering experiments in streptomycetes and how it can be used for the discovery and production of novel specialized metabolites.

An Update on Molecular Tools for Genetic Engineering of Actinomycetes-The Source of Important Antibiotics and Other Valuable Compounds

The first antibiotic-producing actinomycete (Streptomyces antibioticus) was described by Waksman and Woodruff in 1940. This discovery initiated the "actinomycetes era", in which several species were identified and demonstrated to be a great source of bioactive compounds. However, the remarkable group of microorganisms and their potential for the production of bioactive agents were only partially exploited. This is caused by the fact that the growth of many actinomycetes cannot be reproduced on artificial media at laboratory conditions. In addition, sequencing, genome mining and bioactivity screening disclosed that numerous biosynthetic gene clusters (BGCs), encoded in actinomycetes genomes are not expressed and thus, the respective potential products remain uncharacterized. Therefore, a lot of effort was put into the development of technologies that facilitate the access to actinomycetes genomes and activation of their biosynthetic pathways. In this review, we mainly focus on molecular tools and methods for genetic engineering of actinomycetes that have emerged in the field in the past five years (2015-2020). In addition, we highlight examples of successful application of the recently developed technologies in genetic engineering of actinomycetes for activation and/or improvement of the biosynthesis of secondary metabolites.

Enhanced Triacylglycerol Metabolism Contributes to Efficient Oil Utilization and High-Level Production of Salinomycin in Streptomyces albus ZD11

Streptomyces is well known for biosynthesis of secondary metabolites with diverse bioactivities. Although oils have been employed as carbon sources to produce polyketide antibiotics for several industrial Streptomyces strains, the intrinsic correlation between oil utilization and high production of antibiotics still remains unclear. In this study, we investigated the correlation between oil metabolism and salinomycin biosynthesis in Streptomyces albus ZD11, which employs soybean oil as the main carbon source. Comparative genomic analysis revealed the enrichment of genes related to triacylglycerol (TAG) metabolism in S. albus ZD11. Transcriptomic profiling further confirmed the enhancement of TAG metabolism and acyl coenzyme A biosynthesis in S. albus ZD11. Multiple secreted lipases, which catalyze TAG hydrolysis, were seen to be working in a synergistic and complementary manner in aiding the efficient and stable hydrolyzation of TAGs. Together, our results suggest that enhanced TAG hydrolysis and fatty acid degradation contribute to the high efficiency of oil utilization in S. albus ZD11 in order to provide abundant carbon precursors for cell growth and salinomycin biosynthesis.IMPORTANCE In order to obtain high-level production of antibiotics, oils have been used as the main carbon source for some Streptomyces strains. Based on multiomics analysis, this study provides insight into the relationship between triacylglycerol (TAG) metabolism and antibiotic biosynthesis in S. albus ZD11, an oil-preferring industrial Streptomyces strain. Our investigation into TAG hydrolysis yielded further evidence that this strain utilizes complicated strategies enabling an efficient TAG metabolism. In addition, a novel secreted lipase was identified that exhibited highly hydrolytic activity for medium- and long-chain TAGs. Our findings represent a good start toward clarifying the complicated relationship between TAG catabolism and high-level antibiotic production in the industrial strains.

Development of a gene expression system for the uncommon actinomycete Actinoplanes rectilineatus NRRL B-16090

The urgent need for discovering new bioactive metabolites prompts exploring novel actinobacterial taxa by developing appropriate tools for their genome mining and rational genetic engineering. One promising source of new bioactive natural products is the genus Actinoplanes, a home to filamentous sporangia-forming actinobacteria producing many important specialized metabolites such as teicoplanin, ramoplanin, and acarbose. Here we describe the development of a gene expression system for a new Actinoplanes species, A. rectilineatus (NRRL B-16090), which is a potential producer of moenomycin-like antibiotics. We have determined the optimal conditions for spore formation in A. rectilineatus and a plasmid transfer procedure for its engineering via intergeneric E. coli-A. rectilineatus conjugation. The φC31- and pSG5-based vectors were successfully transferred into A. rectilineatus, but φBT1- and VWB-based vectors were not transferable. Finally, using the glucuronidase reporter system, we assessed the strength of several heterologous promoters for gene expression in A. rectilineatus.

Challenges and advances in genetic manipulation of filamentous actinomycetes - the remarkable producers of specialized metabolites

Covering: up to February 2019Actinomycetes are Gram positive bacteria of the phylum Actinobacteria. These organisms are one of the most important sources of structurally diverse, clinically used antibiotics and other valuable bioactive products, as well as biotechnologically relevant enzymes. Most strains were discovered by their ability to produce a given molecule and were often poorly characterized, physiologically and genetically. The development of genetic methods for Streptomyces and related filamentous actinomycetes has led to the successful manipulation of antibiotic biosynthesis to attain structural modification of microbial metabolites that would have been inaccessible by chemical means and improved production yields. Moreover, genome mining reveals that actinomycete genomes contain multiple biosynthetic gene clusters (BGCs), however only a few of them are expressed under standard laboratory conditions, leading to the production of the respective compound(s). Thus, to access and activate the so-called "silent" BGCs, to improve their biosynthetic potential and to discover novel natural products methodologies for genetic manipulation are required. Although different methods have been applied for many actinomycete strains, genetic engineering is still remaining very challenging for some "underexplored" and poorly characterized actinomycetes. This review summarizes the strategies developed to overcome the obstacles to genetic manipulation of actinomycetes and allowing thereby rational genetic engineering of this industrially relevant group of microorganisms. At the end of this review we give some tips to researchers with limited or no previous experience in genetic manipulation of actinomycetes. The article covers the most relevant literature published until February 2019.

Modular and Integrative Vectors for Synthetic Biology Applications in Streptomyces spp

With the development of synthetic biology in the field of (actinobacterial) specialized metabolism, new tools are needed for the design or refactoring of biosynthetic gene clusters. If libraries of synthetic parts (such as promoters or ribosome binding sites) and DNA cloning methods have been developed, to our knowledge, not many vectors designed for the flexible cloning of biosynthetic gene clusters have been constructed. We report here the construction of a set of 12 standardized and modular vectors designed to afford the construction or the refactoring of biosynthetic gene clusters in Streptomyces species, using a large panel of cloning methods. Three different resistance cassettes and four orthogonal integration systems are proposed. In addition, FLP recombination target sites were incorporated to allow the recycling of antibiotic markers and to limit the risks of unwanted homologous recombination in Streptomyces strains when several vectors are used. The functionality and proper integration of the vectors in three commonly used Streptomyces strains, as well as the functionality of the Flp-catalyzed excision, were all confirmed. To illustrate some possible uses of our vectors, we refactored the albonoursin gene cluster from Streptomyces noursei using the BioBrick assembly method. We also used the seamless ligase chain reaction cloning method to assemble a transcription unit in one of the vectors and genetically complement a mutant strain.IMPORTANCE One of the strategies employed today to obtain new bioactive molecules with potential applications for human health (for example, antimicrobial or anticancer agents) is synthetic biology. Synthetic biology is used to biosynthesize new unnatural specialized metabolites or to force the expression of otherwise silent natural biosynthetic gene clusters. To assist the development of synthetic biology in the field of specialized metabolism, we constructed and are offering to the community a set of vectors that were intended to facilitate DNA assembly and integration in actinobacterial chromosomes. These vectors are compatible with various DNA cloning and assembling methods. They are standardized and modular, allowing the easy exchange of a module by another one of the same nature. Although designed for the assembly or the refactoring of specialized metabolite gene clusters, they have a broader potential utility, for example, for protein production or genetic complementation.

Monitoring Protein Secretion in Streptomyces Using Fluorescent Proteins

Fluorescent proteins are a major cell biology tool to analyze protein sub-cellular topology. Here we have applied this technology to study protein secretion in the Gram-positive bacterium Streptomyces lividans TK24, a widely used host for heterologous protein secretion biotechnology. Green and monomeric red fluorescent proteins were fused behind Sec (SP^Sec) or Tat (SP^Tat) signal peptides to direct them through the respective export pathway. Significant secretion of fluorescent eGFP and mRFP was observed exclusively through the Tat and Sec pathways, respectively. Plasmid over-expression was compared to a chromosomally integrated sp^Sec-mRFP gene to allow monitoring secretion under high and low level synthesis in various media. Fluorimetric detection of SP^Sec-mRFP recorded folded states, while immuno-staining detected even non-folded topological intermediates. Secretion of SP^Sec-mRFP is unexpectedly complex, is regulated independently of cell growth phase and is influenced by the growth regime. At low level synthesis, highly efficient secretion occurs until it is turned off and secretory preforms accumulate. At high level synthesis, the secretory pathway overflows and proteins are driven to folding and subsequent degradation. High-level synthesis of heterologous secretory proteins, whether secretion competent or not, has a drastic effect on the endogenous secretome, depending on their secretion efficiency. These findings lay the foundations of dissecting how protein targeting and secretion are regulated by the interplay between the metabolome, secretion factors and stress responses in the S. lividans model.

aMSGE: advanced multiplex site-specific genome engineering with orthogonal modular recombinases in actinomycetes

Chromosomal integration of genes and pathways is of particular importance for large-scale and long-term fermentation in industrial biotechnology. However, stable, multi-copy integration of long DNA segments (e.g., large gene clusters) remains challenging. Here, we describe a plug-and-play toolkit that allows for high-efficiency, single-step, multi-locus integration of natural product (NP) biosynthetic gene clusters (BGCs) in actinomycetes, based on the innovative concept of "multiple integrases-multiple attB sites". This toolkit consists of 27 synthetic modular plasmids, which contain single- or multi-integration modules (from two to four) derived from five orthogonal site-specific recombination (SSR) systems. The multi-integration modules can be readily ligated into plasmids containing large BGCs by Gibson assembly, which can be simultaneously inserted into multiple native attB sites in a single step. We demonstrated the applicability of this toolkit by performing stabilized amplification of acetyl-CoA carboxylase genes to facilitate actinorhodin biosynthesis in Streptomyces coelicolor. Furthermore, using this toolkit, we achieved a 185.6% increase in 5-oxomilbemycin titers (from 2.23 to 6.37 g/L) in Streptomyces hygroscopicus via the multi-locus integration of the entire 5-oxomilbemycin BGC (72 kb) (up to four copies). Compared with previously reported methods, the advanced multiplex site-specific genome engineering (aMSGE) method does not require the introduction of any modifications into host genomes before the amplification of target genes or BGCs, which will drastically simplify and accelerate efforts to improve NP production. Considering that SSR systems are widely distributed in a variety of industrial microbes, this novel technique also promises to be a valuable tool for the enhanced biosynthesis of other high-value bioproducts.

Flexible enzymatic activation of artificial polyketide extender units by Streptomyces cinnamonensis into the monensin biosynthetic pathway

Streptomyces cinnamonensis A495 is a variant of the monensin producer which instead of the native polyether antibiotic gives rise to antibiotic and anti-tumour shunt-product premonensin. Through the supplementation of the fermentation medium with suitable precursors, premonensin can be derivatized via the incorporation of new-to-nature extender units into the biosynthetic machinery. Polyketide extender units require activation, typically in form of coenzyme A-thioesters. These are membrane impermeable and thus in the past an artificial mimic was employed. Here, we show the use and preliminary characterization of a highly substrate promiscuous new enzyme for the endogenous thioester formation in a Streptomyces strain. These intracellularly activated alternative extender units are significantly better incorporated into premonensin than the synthetically activated counterparts.

SIGNIFICANCE AND IMPACT OF THE STUDY

Polyketide natural products are of enormous relevance in medicine. The hit-rate in finding active compounds for the potential treatment of various diseases among this substance family of microbial origin is high. However, most polyketides require derivatization to render them suitable for the application. Of relevance in this field is the incorporation of artificial substances into the biogenesis of polyketides, hampered by both the microbial metabolism and the complexity of the enzymes involved. This manuscript describes the straightforward and selective biosynthetic incorporation of synthetic substances into a reduced polyketide and showcases a promising new enzyme to aid this purpose.

Development of Next Generation Synthetic Biology Tools for Use in Streptomyces venezuelae

Streptomyces have a rich history as producers of important natural products and this genus of bacteria has recently garnered attention for its potential applications in the broader context of synthetic biology. However, the dearth of genetic tools available to control and monitor protein production precludes rapid and predictable metabolic engineering that is possible in hosts such as Escherichia coli or Saccharomyces cerevisiae. In an effort to improve genetic tools for Streptomyces venezuelae, we developed a suite of standardized, orthogonal integration vectors and an improved method to monitor protein production in this host. These tools were applied to characterize heterologous promoters and various attB chromosomal integration sites. A final study leveraged the characterized toolset to demonstrate its use in producing the biofuel precursor bisabolene using a chromosomally integrated expression system. These tools advance S. venezuelae to be a practical host for future metabolic engineering efforts.

Multiple transporters are involved in natamycin efflux in Streptomyces chattanoogensis L10

Antibiotic-producing microorganisms have evolved several self-resistance mechanisms to prevent auto-toxicity. Overexpression of specific transporters to improve the efflux of toxic antibiotics has been found one of the most important and intrinsic resistance strategies used by many Streptomyces strains. In this work, two ATP-binding cassette (ABC) transporter-encoding genes located in the natamycin biosynthetic gene cluster, scnA and scnB, were identified as the primary exporter genes for natamycin efflux in Streptomyces chattanoogensis L10. Two other transporters located outside the cluster, a major facilitator superfamily transporter Mfs1 and an ABC transporter NepI/II were found to play a complementary role in natamycin efflux. ScnA/ScnB and Mfs1 also participate in exporting the immediate precursor of natamycin, 4,5-de-epoxynatamycin, which is more toxic to S. chattanoogensis L10 than natamycin. As the major complementary exporter for natamycin efflux, Mfs1 is up-regulated in response to intracellular accumulation of natamycin and 4,5-de-epoxynatamycin, suggesting a key role in the stress response for self-resistance. This article discusses a novel antibiotic-related efflux and response system in Streptomyces, as well as a self-resistance mechanism in antibiotic-producing strains.

Cloning and Expression of Metagenomic DNA in Streptomyces lividans and Subsequent Fermentation for Optimized Production

The choice of an expression system for the metagenomic DNA of interest is of vital importance for the detection of any particular gene or gene cluster. Most of the screens to date have used the gram-negative bacterium Escherichia coli as a host for metagenomic gene libraries. However, the use of E. coli introduces a potential host bias since only 40 % of the enzymatic activities may be readily recovered by random cloning in E. coli. To recover some of the remaining 60 %, alternative cloning hosts such as Streptomyces spp. have been used. Streptomycetes are high-GC gram-positive bacteria belonging to the Actinomycetales and they have been studied extensively for more than 15 years as an alternative expression system. They are extremely well suited for the expression of DNA from other actinomycetes and genomes of high GC content. Furthermore, due to its high innate, extracellular secretion capacity, Streptomyces can be a better system than E. coli for the production of many extracellular proteins. In this article an overview is given about the materials and methods for growth and successful expression and secretion of heterologous proteins from diverse origin using Streptomyces lividans has a host. More in detail, an overview is given about the protocols of transformation, type of plasmids used and of vectors useful for integration of DNA into the host chromosome, and accompanying cloning strategies. In addition, various control elements for gene expression including synthetic promoters are discussed, and methods to compare their strength are described. Integration of the gene of interest under the control of the promoter of choice into S. lividans chromosome via homologous recombination using pAMR4-based system is explained. Finally a basic protocol for benchtop bioreactor experiments which can form the start in the production process optimization and upscaling is provided.

Evolutionary relationships among actinophages and a putative adaptation for growth in Streptomyces spp

The genome sequences of eight Streptomyces phages are presented, four of which were isolated for this study. Phages R4, TG1, Hau3, and SV1 were isolated previously and have been exploited as tools for understanding and genetically manipulating Streptomyces spp. We also extracted five apparently intact prophages from recent Streptomyces spp. genome projects and, together with six phage genomes in the database, we analyzed all 19 Streptomyces phage genomes with a view to understanding their relationships to each other and to other actinophages, particularly the mycobacteriophages. Fifteen of the Streptomyces phages group into four clusters of related genomes. Although the R4-like phages do not share nucleotide sequence similarity with other phages, they clearly have common ancestry with cluster A mycobacteriophages, sharing many protein homologues, common gene syntenies, and similar repressor-stoperator regulatory systems. The R4-like phage Hau3 and the prophage StrepC.1 (from Streptomyces sp. strain C) appear to have hijacked a unique adaptation of the streptomycetes, i.e., use of the rare UUA codon, to control translation of the essential phage protein, the terminase. The Streptomyces venezuelae generalized transducing phage SV1 was used to predict the presence of other generalized transducing phages for different Streptomyces species.

Genome engineering in actinomycetes using site-specific recombinases

The rational modification of the actinomycetes genomes has a variety of applications in research, medicine, and biotechnology. The use of site-specific recombinases allows generation of multiple mutations, large DNA deletions, integrations, and inversions and may lead to significant progress in all of these fields. Despite their huge potential, site-specific recombinase-based technologies have primarily been used for simple marker removal from a chromosome. In this review, we summarise the site-specific recombination approaches for genome engineering in various actinomycetes.

Improvement of FK506 production in Streptomyces tsukubaensis by genetic enhancement of the supply of unusual polyketide extender units via utilization of two distinct site-specific recombination systems

FK506 is a potent immunosuppressant that has a wide range of clinical applications. Its 23-member macrocyclic scaffold, mainly with a polyketide origin, features two methoxy groups at C-13 and C-15 and one allyl side chain at C-21, due to the region-specific incorporation of two unusual extender units derived from methoxymalonyl-acyl carrier protein (ACP) and allylmalonyl-coenzyme A (CoA), respectively. Whether their intracellular formations can be a bottleneck for FK506 production remains elusive. In this study, we report the improvement of FK506 yield in the producing strain Streptomyces tsukubaensis by the duplication of two sets of pathway-specific genes individually encoding the biosyntheses of these two extender units, thereby providing a promising approach to generate high-FK506-producing strains via genetic manipulation. Taking advantage of the fact that S. tsukubaensis is amenable to two actinophage (ΦC31 and VWB) integrase-mediated recombination systems, we genetically enhanced the biosyntheses of methoxymalonyl-ACP and allylmalonyl-CoA, as indicated by transcriptional analysis. Together with the optimization of glucose supplementation, the maximal FK506 titer eventually increased by approximately 150% in comparison with that of the original strain. The strategy of engineering the biosynthesis of unusual extender units described here may be applicable to improving the production of other polyketide or nonribosomal peptide natural products that contain pathway-specific building blocks.

Streptomyces temperate bacteriophage integration systems for stable genetic engineering of actinomycetes (and other organisms)

ϕC31, ϕBT1, R4, and TG1 are temperate bacteriophages with broad host specificity for species of the genus Streptomyces. They form lysogens by integrating site-specifically into diverse attB sites located within individual structural genes that map to the conserved core region of streptomycete linear chromosomes. The target genes containing the ϕC31, ϕBT1, R4, and TG1 attB sites encode a pirin-like protein, an integral membrane protein, an acyl-CoA synthetase, and an aminotransferase, respectively. These genes are highly conserved within the genus Streptomyces, and somewhat conserved within other actinomycetes. In each case, integration is mediated by a large serine recombinase that catalyzes unidirectional recombination between the bacteriophage attP and chromosomal attB sites. The unidirectional nature of the integration mechanism has been exploited in genetic engineering to produce stable recombinants of streptomycetes, other actinomycetes, eucaryotes, and archaea. The ϕC31 attachment/integration (Att/Int) system has been the most widely used, and it has been coupled with the ϕBT1 Att/Int system to facilitate combinatorial biosynthesis of novel lipopeptide antibiotics in Streptomyces fradiae.

Genetic manipulation of Staphylococci-breaking through the barrier

Most strains of Staphylococcus aureus and Staphylococcus epidermidis possess a strong restriction barrier that hinders exchange of DNA. Recently, major advances have been made in identifying and characterizing the restriction-modification (RM) systems involved. In particular a novel type IV restriction enzyme that recognizes cytosine methylated DNA has been shown to be the major barrier to transfer of plasmid DNA from Escherichia coli into S. aureus and S. epidermidis. While the conserved type I RM system provides a further barrier. Here we review the recent advances in understanding of restriction systems in staphylococci and highlight how this has been exploited to improve our ability to manipulate genetically previously untransformable strains.

Genome comparison and context analysis reveals putative mobile forms of restriction-modification systems and related rearrangements

The mobility of restriction–modification (RM) gene complexes and their association with genome rearrangements is a subject of active investigation. Here we conducted systematic genome comparisons and genome context analysis on fully sequenced prokaryotic genomes to detect RM-linked genome rearrangements. RM genes were frequently found to be linked to mobility-related genes such as integrase and transposase homologs. They were flanked by direct and inverted repeats at a significantly high frequency. Insertion by long target duplication was observed for I, II, III and IV restriction types. We found several RM genes flanked by long inverted repeats, some of which had apparently inserted into a genome with a short target duplication. In some cases, only a portion of an apparently complete RM system was flanked by inverted repeats. We also found a unit composed of RM genes and an integrase homolog that integrated into a tRNA gene. An allelic substitution of a Type III system with a linked Type I and IV system pair, and allelic diversity in the putative target recognition domain of Type IIG systems were observed. This study revealed the possible mobility of all types of RM systems, and the diversity in their mobility-related organization.

Identification and characterization of Streptomyces ghanaensis ATCC14672 integration sites for three actinophage-based plasmids

Streptomyces ghanaensis produces the antibiotic moenomycin A, which is the only known direct inhibitor of bacterial peptidoglycan glycosyltransferases (transglycosylases). Recent progress in understanding moenomycin biosynthesis opens the door to the generation of novel moenomycins via biocombinatorial approaches. To realize the promise of such an approach, one needs better knowledge of the S. ghanaensis genome and diverse genetic tools for stable expression of recombinant constructs in this strain. In this respect, we report the intergeneric Escherichia coli-S. ghanaensis conjugal transfer of plasmids pRT801 and pSOK804 based on the actinophage BT1 and VWB integrase systems, respectively. The attB sites for these two plasmids and for pSET152 were characterized. In particular, sequencing revealed that a putative Arg-tRNA gene serves as an integration site for both phage VWB and pSAM2-like actinomycete integrative and conjugative element recently suggested to be widespread and functional in actinomycetes. The stability of the studied plasmids and their neutrality with respect to antibiotic production warrant their use for manipulations of S. ghanaensis genome.

Isolation and sequencing of a temperate transducing phage for Pasteurella multocida

A temperate bacteriophage (F108) has been isolated through mitomycin C induction of a Pasteurella multocida serogroup A strain. F108 has a typical morphology of the family Myoviridae, presenting a hexagonal head and a long contractile tail. F108 is able to infect all P. multocida serogroup A strains tested but not those belonging to other serotypes. Bacteriophage F108, the first P. multocida phage sequenced so far, presents a 30,505-bp double-stranded DNA genome with cohesive ends (CTTCCTCCCC cos site). The F108 genome shows the highest homology with those of Haemophilus influenzae HP1 and HP2 phages. Furthermore, an F108 prophage attachment site in the P. multocida chromosome has been established to be inside a gene encoding tRNA(Leu). By using several chromosomal markers that are spread along the P. multocida chromosome, it has been demonstrated that F108 is able to perform generalized transduction. This fact, together with the absence of pathogenic genes in the F108 genome, makes this bacteriophage a valuable tool for P. multocida genetic manipulation.

An efficient gene transfer system for the pimaricin producerStreptomyces natalensis

Streptomyces natalensis produces the antifungal polyene macrolide pimaricin. Genetic manipulation of its biosynthetic genes has been hampered by the lack of efficient gene transfer systems. We have developed a gene transfer system based on intergeneric conjugation from Escherichia coli. Using this approach, we managed to attain transformation efficiencies of 1 x 10(-4) exconjugants per recipient when using self-replicating vectors such as pHZ1358. The use of integrative vectors such as pSET152 or pSOK804 resulted in significantly lower efficiencies. Site-specific integration or the use of self-replicating plasmids did not affect pimaricin production or the essential functions of S. natalensis. Use of DNA methylation proficient E. coli donor strains resulted in no transformants, indicating the presence of methyl-specific restriction systems in S. natalensis. This methodology will enable easier manipulation of the genes responsible for pimaricin biosynthesis, and could prove valuable for the generation of new designer polyene macrolides with better antifungal activity and pharmacological properties. As an example of the validity of the method, we describe the introduction of Supercos-1-derived cosmid vectors into S. natalensis in order to promote gene replacements by double crossover recombination.

The Structural Basis for Docking in Modular Polyketide Biosynthesis

Polyketide natural products such as erythromycin and rapamycin are assembled on polyketide synthases (PKSs), which consist of modular sets of catalytic activities distributed across multiple protein subunits. Correct protein-protein interactions among the PKS subunits which are critical to the fidelity of biosynthesis are mediated in part by "docking domains" at the termini of the proteins. The NMR solution structure of a representative docking domain complex from the erythromycin PKS (DEBS) was recently solved, and on this basis it has been proposed that PKS docking is mediated by the formation of an intermolecular four-alpha-helix bundle. Herein, we report the genetic engineering of such a docking domain complex by replacement of specific helical segments and analysis of triketide synthesis by mutant PKSs in vivo. The results of these helix swaps are fully consistent with the model and highlight residues in the docking domains that may be targeted to alter the efficiency or specificity of subunit-subunit docking in hybrid PKSs.

Characterization of the Micromonospora rosaria pMR2 plasmid and development of a high G+C codon optimized integrase for site-specific integration

pMR2, an 11.1 kb plasmid was isolated from Micromonospora rosaria SCC2095, NRRL3718, and its complete nucleotide sequence determined. Analysis revealed 13 ORFs including homologs of a KorSA regulatory protein and TraB plasmid transfer protein found on other actinomycete plasmids. pMR2 contains att/int functions consisting of an integrase, an excisionase, and a putative plasmid attachment site (attP). The integrase gene contained a high frequency of codons rarely used in high G+C actinomycete coding regions. The gene was codon optimized for actinomycete codon usage to create the synthetic gene int-OPT. pSPRX740, containing an rpsL promoter and the att/int-OPT region, was introduced into Micromonospora halophytica var. nigra ATCC33088. Analysis of DNA flanking the pSPRX740 integration site confirmed site-specific integration into a tRNA(Phe) gene in the M. halopytica var. nigra chromosome. The pMR2 attP element and chromosomal attachment (attB) site contain a 63 bp region of sequence identity overlapping the 3' end of the tRNA(Phe) gene. Plasmids comprising the site-specific att/int-OPT functions of pMR2 can be used to integrate genes into the chromosome of actinomycetes with an appropriate tRNA gene. The development of an integrative system for Micromonospora will expand our ability to study antibiotic biosynthesis in this important actinomycete genus.

Complete genomic nucleotide sequence and analysis of the temperate bacteriophage VWB

The entire double-stranded DNA genome of the Streptomyces venezuelae bacteriophage VWB was sequenced and analyzed. Its size is 49,220 bp with an overall molar G + C content of 71.2 mol%. Sixty-one potential open reading frames were identified and annotated using several complementary bioinformatics tools. Clusters of functionally related putative genes were defined, supporting a refined version of the modular theory of phage evolution.

In vivo analysis of the regulatory genes in the nystatin biosynthetic gene cluster of Streptomyces noursei ATCC 11455 reveals their differential control over antibiotic biosynthesis

Six putative regulatory genes are located at the flank of the nystatin biosynthetic gene cluster in Streptomyces noursei ATCC 11455. Gene inactivation and complementation experiments revealed that nysRI, nysRII, nysRIII, and nysRIV are necessary for efficient nystatin production, whereas no significant roles could be demonstrated for the other two regulatory genes. To determine the in vivo targets for the NysR regulators, chromosomal integration vectors with the xylE reporter gene under the control of seven putative promoter regions upstream of the nystatin structural and regulatory genes were constructed. Expression analyses of the resulting vectors in the S. noursei wild-type strain and regulatory mutants revealed that the four regulators differentially affect certain promoters. According to these analyses, genes responsible for initiation of nystatin biosynthesis and antibiotic transport were the major targets for regulation. Data from cross-complementation experiments showed that nysR genes could in some cases substitute for each other, suggesting a functional hierarchy of the regulators and implying a cascade-like mechanism of regulation of nystatin biosynthesis.

Development of the Micromonospora carbonacea var. africana ATCC 39149 bacteriophage pMLP1 integrase for site-specific integration in Micromonospora spp

Micromonospora carbonacea var. africana ATCC 39149 contains a temperate bacteriophage, pMLP1, that is present both as a replicative element and integrated into the chromosome. Sequence analysis of a 4.4 kb KpnI fragment revealed pMLP1 att/int functions consisting of an integrase, an excisionase and the phage attachment site (attP). Plasmids pSPRH840 and pSPRH910, containing the pMLP1 att/int region, were introduced into Micromonospora spp. by conjugation from Escherichia coli. Sequence analysis of DNA flanking the integration site confirmed site-specific integration into a tRNAHis gene in the chromosome. The pMLP1 attP element and chromosomal bacterial attachment (attB) site contain a 24 bp region of sequence identity located at the 3' end of the tRNA. Integration of pMLP1-based plasmids in M. carbonacea var. africana caused a loss of the pMLP1 phage. Placement of an additional attB site into the chromosome allowed integration of pSPRH840 into the alternate attB site. Plasmids containing the site-specific att/int functions of pMLP1 can be used to integrate genes into the chromosome.

The gene cluster for the biosynthesis of the glycopeptide antibiotic A40926 by nonomuraea species

The glycopeptide A40926 is the precursor of dalbavancin, a second-generation glycopeptide currently under clinical development. The dbv gene cluster, devoted to A40926 biosynthesis, was isolated and characterized from the actinomycete Nonomuraea species ATCC39727. From sequence analysis, 37 open reading frames (ORFs) participate in A40926 biosynthesis, regulation, resistance, and export. Of these, 27 ORFs find a match in at least one of the previously characterized glycopeptide gene clusters, while 10 ORFs are, so far, unique to the dbv cluster. Putative genes could be identified responsible for some of the tailoring steps (attachment of glucosamine, sugar oxidation, and mannosylation) expected during A40926 biosynthesis. After constructing a Nonomuraea mutant by deleting dbv ORFs 8 to 10, the novel compound dechloromannosyl-A40926 aglycone was isolated.

Genetic engineering of aminodeoxyhexose biosynthesis in Streptomyces fradiae

The antibacterial properties of macrolide antibiotics (such as erythromycin, tylosin, and narbomycin) depend ultimately on the glycosylation of otherwise inactive polyketide lactones. Among the sugars commonly found in such macrolides are various 6-deoxyhexoses including the 3-dimethylamino sugars mycaminose and desosamine (4-deoxymycaminose). Some macrolides (such as tylosin) possess multiple sugar moieties, whereas others (such as narbomycin) have only single sugar substituents. As patterns of glycosylation markedly influence a macrolide's drug activity, there is considerable interest in the possibility of using combinatorial biosynthesis to generate new pairings of polyketide lactones with sugars, especially 6-deoxyhexoses. Here, we report a successful attempt to alter the aminodeoxyhexose-biosynthetic capacity of Streptomyces fradiae (a producer of tylosin) by importing genes from the narbomycin producer Streptomyces narbonensis. This engineered S. fradiae produced substantial amounts of two potentially useful macrolides that had not previously been obtained by fermentation.

Integration sites for genetic elements in prokaryotic tRNA and tmRNA genes: sublocation preference of integrase subfamilies

Most classical integrases of prokaryotic genetic elements specify integration into tRNA or tmRNA genes. Sequences shared between element and host integration sites suggest that crossover can occur at any of three sublocations within a tRNA gene, two with flanking symmetry (anticodon-loop and T-loop tDNA) and the third at the asymmetric 3' end of the gene. Integrase phylogeny matches this classification: integrase subfamilies use exclusively either the symmetric sublocations or the asymmetric sublocation, although tRNA genes of several different aminoacylation identities may be used within any subfamily. These two familial sublocation preferences imply two modes by which new integration site usage evolves. The tmRNA gene has been adopted as an integration site in both modes, and its distinctive structure imposes some constraints on proposed evolutionary mechanisms.

Site-specific integrative elements of rhizobiophage 16-3 can integrate into proline tRNA (CGG) genes in different bacterial genera

The integrase protein of the Rhizobium meliloti 41 phage 16-3 has been classified as a member of the Int family of tyrosine recombinases. The site-specific recombination system of the phage belongs to the group in which the target site of integration (attB) is within a tRNA gene. Since tRNA genes are conserved, we expected that the target sequence of the site-specific recombination system of the 16-3 phage could occur in other species and integration could take place if the required putative host factors were also provided by the targeted cells. Here we report that a plasmid (pSEM167) carrying the attP element and the integrase gene (int) of the phage can integrate into the chromosomes of R. meliloti 1021 and eight other species. In all cases integration occurred at so-far-unidentified, putative proline tRNA (CGG) genes, indicating the possibility of their common origin. Multiple alignment of the sequences suggested that the location of the att core was different from that expected previously. The minimal attB was identified as a 23-bp sequence corresponding to the anticodon arm of the tRNA.

Monitoring genome evolution ex vivo: reversible chromosomal integration of a 106 kb plasmid at two tRNA(Lys) gene loci in sequential Pseudomonas aeruginosa airway isolates

The genome rearrangements in sequential Pseudomonas aeruginosa clone K isolates from the airways of a patient with cystic fibrosis were determined by an integrated approach of mapping, sequencing and bioinformatics. Restriction mapping uncovered an 8.9 kb deletion of PAO sequence between phnAB and oprL in clone K, and two 106 kb insertions either adjacent to this deletion or several hundred kilobases away, close to the pilA locus. These 106 kb blocks of extra DNA also co-existed as the circular plasmid pKLK106 in several clone K isolates and were found to be closely related to plasmid pKLC102 in P. aeruginosa clone C isolates. The breakpoints of the deletion in clone K and the attB-attP sequences for the reversible integration of the plasmid in clones C and K were located within the 3' end of the lysine tRNA structural genes (att site). pKLK106 sequentially recombined with either of the two tRNA(Lys) genes in clone K isolates. The att site of the pilA hypervariable region has been utilized by clone C to target its plasmid pKLC102 into the chromosome; the att site of the phnAB-oprL region has been employed by strain PAO to incorporate a DNA block encoding pyocin, transposases and IS elements. The use of typical phage attachment sites by conjugative genetic elements could be one of the major mechanisms used by P. aeruginosa to generate the mosaic genome structure of blocks of species-, clone- and strain-specific DNA. The example described here demonstrates the potential impact of systematic genome analysis of sequential isolates from the same habitat on our understanding of the evolution of microbial genomes.