Genes required for aerial growth, cell division, and chromosome segregation are targets of WhiA before sporulation in Streptomyces venezuelae

Deletion of fbiC in Streptomyces venezuelae removes autofluorescence in the excitation-emission range of cyan fluorescent protein

Autofluorescence poses an impediment to fluorescence microscopy of biological samples. In the Gram-positive, soil-dwelling bacteria of the genus Streptomyces, sources of autofluorescence have not been examined systematically to date. Here, we show that the model organism for the genus, Streptomyces venezuelae, shows autofluorescence in two of the commonly used fluorescence channels for visualizing cyan and green/yellow fluorescent proteins. We identify the source of autofluorescence in the cyan fluorescence channel as redox cofactor factor 420 (F420) and target its synthesis to remove it. By deleting the vnz15170 (fbiC) gene, which is a key biosynthetic gene for the production of F420, we were able to create an autofluorescence-free strain in the cyan range of fluorescence excitation-emission. We demonstrate the usefulness of this strain by imaging the mTurquoise-tagged polar growth-related protein DivIVA and the cell division-related protein FtsZ in the fbiC deletion background. Using live-cell imaging to follow the dynamics of DivIVA and FtsZ, we demonstrate an improved signal-to-noise ratio in the mutant strain. We show that this strain can be a suitable tool for visualizing the localization of proteins in Streptomyces spp. and can facilitate the utilization of multi-colour imaging and fluorescence resonance energy transfer-based imaging.

A dispensable SepIVA orthologue in Streptomyces venezuelae is associated with polar growth and not cell division

BACKGROUND

SepIVA has been reported to be an essential septation factor in Mycolicibacterium smegmatis and Mycobacterium tuberculosis. It is a coiled-coil protein with similarity to DivIVA, a protein necessary for polar growth in members of the phylum Actinomycetota. Orthologues of SepIVA are broadly distributed among actinomycetes, including in Streptomyces spp.

RESULTS

To clarify the role of SepIVA and its potential involvement in cell division in streptomycetes, we generated sepIVA deletion mutants in Streptomyces venezuelae and found that sepIVA is dispensable for growth, cell division and sporulation. Further, mNeonGreen-SepIVA fusion protein did not localize at division septa, and we found no evidence of involvement of SepIVA in cell division. Instead, mNeonGreen-SepIVA was accumulated at the tips of growing vegetative hyphae in ways reminiscent of the apical localization of polarisome components like DivIVA. Bacterial two-hybrid system analyses revealed an interaction between SepIVA and DivIVA. The results indicate that SepIVA is associated with polar growth. However, no phenotypic effects of sepIVA deletion could be detected, and no evidence was observed of redundancy with the other DivIVA-like coiled-coil proteins Scy and FilP that are also associated with apical growth in streptomycetes.

CONCLUSIONS

We conclude that S. venezuelae SepIVA, in contrast to the situation in mycobacteria, is dispensable for growth and viability. The results suggest that it is associated with polar growth rather than septum formation.

WhiB-like proteins: Diversity of structure, function and mechanism

The WhiB-Like (Wbl) proteins are a large family of iron-sulfur (Fe-S) cluster-containing transcription factors exclusively found in the phylum Actinobacteria, including the notable genera like Mycobacteria, Streptomycetes and Corynebacteria. These proteins play pivotal roles in diverse biological processes, such as cell development, redox stress response and antibiotic resistance. Members of the Wbl family exhibit remarkable diversity in their sequences, structures and functions, attracting great attention since their first discovery. This review highlights the most recent breakthroughs in understanding the structural and mechanistic aspects of Wbl-dependent transcriptional regulation.

The activity of CobB1 protein deacetylase contributes to nucleoid compaction in Streptomyces venezuelae spores by increasing HupS affinity for DNA

Streptomyces are soil bacteria with complex life cycle. During sporulation Streptomyces linear chromosomes become highly compacted so that the genetic material fits within limited spore volume. The key players in this process are nucleoid-associated proteins (NAPs). Among them, HU (heat unstable) proteins are the most abundant NAPs in the cell and the most conserved in bacteria. HupS, one of the two HU homologues encoded by the Streptomyces genome, is the best-studied spore-associated NAP. In contrast to other HU homologues, HupS contains a long, C-terminal domain that is extremely rich in lysine repeats (LR domain) similar to eukaryotic histone H2B and mycobacterial HupB protein. Here, we have investigated, whether lysine residues in HupS are posttranslationally modified by reversible lysine acetylation. We have confirmed that Streptomyces venezuelae HupS is acetylated in vivo. We showed that HupS binding to DNA in vitro is controlled by the acetylation. Moreover, we identified that CobB1, one of two Sir2 homologues in Streptomyces, controls HupS acetylation levels in vivo. We demonstrate that the elimination of CobB1 increases HupS mobility, reduces chromosome compaction in spores, and affects spores maturation. Thus, our studies indicate that HupS acetylation affects its function by diminishing DNA binding and disturbing chromosome organization.

SVEN_5003 is a Major Developmental Regulator in Streptomyces venezuelae

Many regulatory genes that affect cellular development in Streptomyces, such as the canonical bld genes, have already been identified. However, in this study, we identified sven_5003 in Streptomyces venezuelae as a major new developmental regulatory gene, the deletion of which leads to a bald phenotype, typical of bld mutants, under multiple growth conditions. Our data indicated that disruption of sven_5003 also has a differential impact on the production of the two antibiotics jadomycin and chloramphenicol. Enhanced production of jadomycin but reduced production of chloramphenicol were detected in our sven_5003 mutant strain (S. venezuelae D5003). RNA-Seq analysis indicated that SVEN_5003 impacts expression of hundreds of genes, including genes involved in development, primary and secondary metabolism, and genes of unknown function, a finding confirmed by real-time PCR analysis. Transcriptional analysis indicated that sven_5003 is an auto-regulatory gene, repressing its own expression. Despite the evidence indicating that SVEN_5003 is a regulatory factor, a putative DNA-binding domain was not predicted from its primary amino acid sequence, implying an unknown regulatory mechanism by SVEN_5003. Our findings revealed that SVEN_5003 is a pleiotropic regulator with a critical role in morphological development in S. venezuelae.

Co-PATHOgenex web application for assessing complex stress responses in pathogenic bacteria

IMPORTANCE

Unveiling gene co-expression networks in bacterial pathogens has the potential for gaining insights into their adaptive strategies within the host environment. Here, we developed Co-PATHOgenex, an interactive and user-friendly web application that enables users to construct networks from gene co-expressions using custom-defined thresholds (https://avicanlab.shinyapps.io/copathogenex/). The incorporated search functions and visualizations within the tool simplify the usage and facilitate the interpretation of the analysis output. Co-PATHOgenex also includes stress stimulons for various bacterial species, which can help identify gene products not previously associated with a particular stress condition.

Harnessing regulatory networks in Actinobacteria for natural product discovery

Microbes typically live in complex habitats where they need to rapidly adapt to continuously changing growth conditions. To do so, they produce an astonishing array of natural products with diverse structures and functions. Actinobacteria stand out for their prolific production of bioactive molecules, including antibiotics, anticancer agents, antifungals, and immunosuppressants. Attention has been directed especially towards the identification of the compounds they produce and the mining of the large diversity of biosynthetic gene clusters (BGCs) in their genomes. However, the current return on investment in random screening for bioactive compounds is low, while it is hard to predict which of the millions of BGCs should be prioritized. Moreover, many of the BGCs for yet undiscovered natural products are silent or cryptic under laboratory growth conditions. To identify ways to prioritize and activate these BGCs, knowledge regarding the way their expression is controlled is crucial. Intricate regulatory networks control global gene expression in Actinobacteria, governed by a staggering number of up to 1000 transcription factors per strain. This review highlights recent advances in experimental and computational methods for characterizing and predicting transcription factor binding sites and their applications to guide natural product discovery. We propose that regulation-guided genome mining approaches will open new avenues toward eliciting the expression of BGCs, as well as prioritizing subsets of BGCs for expression using synthetic biology approaches.

ONE-SENTENCE SUMMARY

This review provides insights into advances in experimental and computational methods aimed at predicting transcription factor binding sites and their applications to guide natural product discovery.

Metabolic and chromosomal changes in a Bacillus subtilis whiA mutant

IMPORTANCE

WhiA is a conserved DNA-binding protein that influences cell division in many Gram-positive bacteria and, in B. subtilis, also chromosome segregation. How WhiA works in Bacillus subtilis is unknown. Here, we tested three hypothetical mechanisms using metabolomics, fatty acid analysis, and chromosome confirmation capture experiments. This revealed that WhiA does not influence cell division and chromosome segregation by modulating either central carbon metabolism or fatty acid composition. However, the inactivation of WhiA reduces short-range chromosome interactions. These findings provide new avenues to study the molecular mechanism of WhiA in the future.

The Best of Both Worlds-Streptomyces coelicolor and Streptomyces venezuelae as Model Species for Studying Antibiotic Production and Bacterial Multicellular Development

Streptomyces bacteria have been studied for more than 80 years thanks to their ability to produce an incredible array of antibiotics and other specialized metabolites and their unusual fungal-like development. Their antibiotic production capabilities have ensured continual interest from both academic and industrial sectors, while their developmental life cycle has provided investigators with unique opportunities to address fundamental questions relating to bacterial multicellular growth. Much of our understanding of the biology and metabolism of these fascinating bacteria, and many of the tools we use to manipulate these organisms, have stemmed from investigations using the model species Streptomyces coelicolor and Streptomyces venezuelae. Here, we explore the pioneering work in S. coelicolor that established foundational genetic principles relating to specialized metabolism and development, alongside the genomic and cell biology developments that led to the emergence of S. venezuelae as a new model system. We highlight key discoveries that have stemmed from studies of these two systems and discuss opportunities for future investigations that leverage the power and understanding provided by S. coelicolor and S. venezuelae.

Cellular differentiation into hyphae and spores in halophilic archaea

Several groups of bacteria have complex life cycles involving cellular differentiation and multicellular structures. For example, actinobacteria of the genus Streptomyces form multicellular vegetative hyphae, aerial hyphae, and spores. However, similar life cycles have not yet been described for archaea. Here, we show that several haloarchaea of the family Halobacteriaceae display a life cycle resembling that of Streptomyces bacteria. Strain YIM 93972 (isolated from a salt marsh) undergoes cellular differentiation into mycelia and spores. Other closely related strains are also able to form mycelia, and comparative genomic analyses point to gene signatures (apparent gain or loss of certain genes) that are shared by members of this clade within the Halobacteriaceae. Genomic, transcriptomic and proteomic analyses of non-differentiating mutants suggest that a Cdc48-family ATPase might be involved in cellular differentiation in strain YIM 93972. Additionally, a gene encoding a putative oligopeptide transporter from YIM 93972 can restore the ability to form hyphae in a Streptomyces coelicolor mutant that carries a deletion in a homologous gene cluster (bldKA-bldKE), suggesting functional equivalence. We propose strain YIM 93972 as representative of a new species in a new genus within the family Halobacteriaceae, for which the name Actinoarchaeum halophilum gen. nov., sp. nov. is herewith proposed. Our demonstration of a complex life cycle in a group of haloarchaea adds a new dimension to our understanding of the biological diversity and environmental adaptation of archaea.

Structural basis of dual activation of cell division by the actinobacterial transcription factors WhiA and WhiB

Studies of transcriptional initiation in different bacterial clades reveal diverse molecular mechanisms regulating this first step in gene expression. The WhiA and WhiB factors are both required to express cell division genes in Actinobacteria and are essential in notable pathogens such as Mycobacterium tuberculosis. The WhiA/B regulons and binding sites have been elucidated in Streptomyces venezuelae (Sven), where they coordinate to activate sporulation septation. However, how these factors cooperate at the molecular level is not understood. Here we present cryoelectron microscopy structures of Sven transcriptional regulatory complexes comprising RNA polymerase (RNAP) σA-holoenzyme and WhiA and WhiB, in complex with the WhiA/B target promoter sepX. These structures reveal that WhiB binds to domain 4 of σA (σA4) of the σA-holoenzyme, bridging an interaction with WhiA while making non-specific contacts with the DNA upstream of the -35 core promoter element. The N-terminal homing endonuclease-like domain of WhiA interacts with WhiB, while the WhiA C-terminal domain (WhiA-CTD) makes base-specific contacts with the conserved WhiA GACAC motif. Notably, the structure of the WhiA-CTD and its interactions with the WhiA motif are strikingly similar to those observed between σA4 housekeeping σ-factors and the -35 promoter element, suggesting an evolutionary relationship. Structure-guided mutagenesis designed to disrupt these protein-DNA interactions reduces or abolishes developmental cell division in Sven, confirming their significance. Finally, we compare the architecture of the WhiA/B σA-holoenzyme promoter complex with the unrelated but model CAP Class I and Class II complexes, showing that WhiA/WhiB represent a new mechanism in bacterial transcriptional activation.

Cytoplasmic contractile injection systems mediate cell death in Streptomyces

Contractile injection systems (CIS) are bacteriophage tail-like structures that mediate bacterial cell-cell interactions. While CIS are highly abundant across diverse bacterial phyla, representative gene clusters in Gram-positive organisms remain poorly studied. Here we characterize a CIS in the Gram-positive multicellular model organism Streptomyces coelicolor and show that, in contrast to most other CIS, S. coelicolor CIS (CIS^Sc) mediate cell death in response to stress and impact cellular development. CIS^Sc are expressed in the cytoplasm of vegetative hyphae and are not released into the medium. Our cryo-electron microscopy structure enabled the engineering of non-contractile and fluorescently tagged CIS^Sc assemblies. Cryo-electron tomography showed that CIS^Sc contraction is linked to reduced cellular integrity. Fluorescence light microscopy furthermore revealed that functional CIS^Sc mediate cell death upon encountering different types of stress. The absence of functional CIS^Sc had an impact on hyphal differentiation and secondary metabolite production. Finally, we identified three putative effector proteins, which when absent, phenocopied other CIS^Sc mutants. Our results provide new functional insights into CIS in Gram-positive organisms and a framework for studying novel intracellular roles, including regulated cell death and life-cycle progression in multicellular bacteria.

ParA's Impact beyond Chromosome Segregation in Caulobacter crescentus

Maintaining proper chromosome inheritance after the completion of each cell cycle is paramount for bacterial survival. Mechanistic details remain incomplete for how bacteria manage to retain complete chromosomes after each cell cycle. In this study, we examined the potential roles of the partitioning protein ParA on chromosomal maintenance that go beyond triggering the onset of chromosome segregation in Caulobacter crescentus. Our data revealed that increasing the levels of ParA result in cells with multiple origins of replication in a DnaA-ATP-dependent manner. This ori supernumerary is retained even when expressing variants of ParA that are deficient in promoting chromosome segregation. Our data suggest that in Caulobacter ParA's impact on replication initiation is likely indirect, possibly through the effect of other cell cycle events. Overall, our data provide new insights into the highly interconnected network that drives the forward progression of the bacterial cell cycle. IMPORTANCE The successful generation of a daughter cell containing a complete copy of the chromosome requires the exquisite coordination of major cell cycle events. Any mistake in this coordination can be lethal, making these processes ideal targets for novel antibiotics. In this study, we focused on the coordination between the onset of chromosome replication, and the partitioning protein ParA. We demonstrate that altering the cellular levels of ParA causes cells to accumulate multiple origins of replication in Caulobacter crescentus. Our work provides important insights into the complex regulation involved in the coordination of the bacterial cell cycle.

Tuberculosis treatment failure associated with evolution of antibiotic resilience

The widespread use of antibiotics has placed bacterial pathogens under intense pressure to evolve new survival mechanisms. Genomic analysis of 51,229 Mycobacterium tuberculosis (Mtb)clinical isolates has identified an essential transcriptional regulator, Rv1830, herein called resR for resilience regulator, as a frequent target of positive (adaptive) selection. resR mutants do not show canonical drug resistance or drug tolerance but instead shorten the post-antibiotic effect, meaning that they enable Mtb to resume growth after drug exposure substantially faster than wild-type strains. We refer to this phenotype as antibiotic resilience. ResR acts in a regulatory cascade with other transcription factors controlling cell growth and division, which are also under positive selection in clinical isolates of Mtb. Mutations of these genes are associated with treatment failure and the acquisition of canonical drug resistance.

Biotechnological application of Streptomyces for the production of clinical drugs and other bioactive molecules

Streptomyces is one of the most relevant genera in biotechnology, and its rich secondary metabolism is responsible for the biosynthesis of a plethora of bioactive compounds, including several clinically relevant drugs. The use of Streptomyces species for the manufacture of natural products has been established for more than half a century; however, the tremendous advances observed in recent years in genetic engineering and molecular biology have revolutionised the optimisation of Streptomyces as cell factories and drastically expanded the biotechnological potential of these bacteria. Here, we illustrate the most exciting advances reported in the past few years, with a particular focus on the approaches significantly improving the biotechnological capacity of Streptomyces to produce clinical drugs and other valuable secondary metabolites.

Genome-Wide Identification of the LexA-Mediated DNA Damage Response in Streptomyces venezuelae

DNA damage triggers a widely conserved stress response in bacteria called the SOS response, which involves two key regulators, the activator RecA and the transcriptional repressor LexA. Despite the wide conservation of the SOS response, the number of genes controlled by LexA varies considerably between different organisms. The filamentous soil-dwelling bacteria of the genus Streptomyces contain LexA and RecA homologs, but their roles in Streptomyces have not been systematically studied. Here, we demonstrate that RecA and LexA are required for the survival of Streptomyces venezuelae during DNA-damaging conditions and for normal development during unperturbed growth. Monitoring the activity of a fluorescent recA promoter fusion and LexA protein levels revealed that the activation of the SOS response is delayed in S. venezuelae. By combining global transcriptional profiling and chromatin immunoprecipitation sequencing (ChIP-seq) analysis, we determined the LexA regulon and defined the core set of DNA damage repair genes that are expressed in response to treatment with the DNA-alkylating agent mitomycin C. Our results show that DNA damage-induced degradation of LexA results in the differential regulation of LexA target genes. Using surface plasmon resonance, we further confirmed the LexA DNA binding motif (SOS box) and demonstrated that LexA displays tight but distinct binding affinities to its target promoters, indicating a graded response to DNA damage.

IMPORTANCE The transcriptional regulator LexA functions as a repressor of the bacterial SOS response, which is induced under DNA-damaging conditions. This results in the expression of genes important for survival and adaptation. Here, we report the regulatory network controlled by LexA in the filamentous antibiotic-producing Streptomyces bacteria and establish the existence of the SOS response in Streptomyces. Collectively, our work reveals significant insights into the DNA damage response in Streptomyces that will promote further studies to understand how these important bacteria adapt to their environment.

ActinoBase: tools and protocols for researchers working on Streptomyces and other filamentous actinobacteria

Actinobacteria is an ancient phylum of Gram-positive bacteria with a characteristic high GC content to their DNA. The ActinoBase Wiki is focused on the filamentous actinobacteria, such as Streptomyces species, and the techniques and growth conditions used to study them. These organisms are studied because of their complex developmental life cycles and diverse specialised metabolism which produces many of the antibiotics currently used in the clinic. ActinoBase is a community effort that provides valuable and freely accessible resources, including protocols and practical information about filamentous actinobacteria. It is aimed at enabling knowledge exchange between members of the international research community working with these fascinating bacteria. ActinoBase is an anchor platform that underpins worldwide efforts to understand the ecology, biology and metabolic potential of these organisms. There are two key differences that set ActinoBase apart from other Wiki-based platforms: [1] ActinoBase is specifically aimed at researchers working on filamentous actinobacteria and is tailored to help users overcome challenges working with these bacteria and [2] it provides a freely accessible resource with global networking opportunities for researchers with a broad range of experience in this field.

DNA damage-induced block of sporulation in Streptomyces venezuelae involves downregulation of ssgB

DNA damage often causes an arrest of the cell cycle that provides time for genome integrity to be restored. In bacteria, the classical SOS DNA damage response leads to an inhibition of cell division resulting in temporarily filamentous growth. This raises the question as to whether such a response mechanism might similarly function in naturally filamentous bacteria such as Streptomyces. Streptomyces exhibit two functionally distinct forms of cell division: cross-wall formation in vegetative hyphae and sporulation septation in aerial hyphae. Here, we show that the genotoxic agent mitomycin C confers a block in sporulation septation in Streptomyces venezuelae in a mechanism that involves, at least in part, the downregulation of ssgB. Notably, this DNA damage response does not appear to block cross-wall formation and may be independent of canonical SOS and developmental regulators. We also show that the mitomycin C-induced block in sporulation can be partially bypassed by the constitutive expression of ssgB, though this appears to be largely limited to mitomycin C treatment and the resultant spore-like cells have reduced viability.

The regulatory gene wblA is a target of the orphan response regulator OrrA in Streptomyces coelicolor

Our previous study using transposon mutagenesis indicated that disruption of the putative response regulator gene orrA impacted antibiotic production in Streptomyces coelicolor. In this study, the role of OrrA was further characterized by comparing the phenotypes and transcriptomic profiles of the wild-type S. coelicolor strain M145 and ΔorrA, a strain with an inactivated orrA gene. Chromatin immunoprecipitation using a strain expressing OrrA fused with FLAG showed that OrrA binds the promoter of wblA, whose expression was downregulated in ΔorrA. The interaction of OrrA with the wblA promoter was further validated by a pull-down assay. Similar to ΔorrA, the deletion mutant of wblA (ΔwblA) was defective in development, and developmental genes were expressed at similar levels in ΔorrA and ΔwblA. Although both OrrA and WblA downregulated actinorhodin and undecylprodigiosin, their roles in regulation of the calcium-dependent antibiotic and yellow-pigmented type I polyketide differed. sco1375, a gene of unknown function, was identified as another OrrA target, and overexpression of either sco1375 or wblA in ΔorrA partially restored the wild-type phenotype, indicating that these genes mediate some of the effects of OrrA. This study revealed targets of OrrA and provided more insights into the role of the orphan response regulator OrrA in Streptomyces. This article is protected by copyright. All rights reserved.

How Streptomyces thrive: Advancing our understanding of classical development and uncovering new behaviors

Streptomyces are soil- and marine-dwelling microbes that need to survive dramatic fluctuations in nutrient levels and environmental conditions. Here, we explore the advances made in understanding how Streptomyces bacteria can thrive in their natural environments. We examine their classical developmental cycle, and the intricate regulatory cascades that govern it. We discuss alternative growth strategies and behaviors, like the rapid expansion and colonization properties associated with exploratory growth, the release of membrane vesicles and S-cells from hyphal tips, and the acquisition of exogenous DNA along the lateral walls. We further investigate Streptomyces interactions with other organisms through the release of volatile compounds that impact nutrient levels, microbial growth, and insect behavior. Finally, we explore the increasingly diverse strategies employed by Streptomyces species in escaping and thwarting phage infections.

Effects of dual deletion of glnR and mtrA on expression of nitrogen metabolism genes in Streptomyces venezuelae

GlnR activates nitrogen metabolism genes under nitrogen‐limited conditions, whereas MtrA represses these genes under nutrient‐rich conditions in Streptomyces. In this study, we compared the transcription patterns of nitrogen metabolism genes in a double deletion mutant (ΔmtrA‐glnR) lacking both mtrA and glnR and in mutants lacking either mtrA (ΔmtrA) or glnR (ΔglnR). The nitrogen metabolism genes were expressed similarly in ΔmtrA‐glnR and ΔglnR under both nitrogen‐limited and nutrient‐rich conditions, with patterns distinctly different from that of ΔmtrA, suggesting a decisive role for GlnR in the control of nitrogen metabolism genes and further suggesting that regulation of these genes by MtrA is GlnR‐dependent. MtrA and GlnR utilize the same binding sites upstream of nitrogen metabolism genes, and we showed stronger in vivo binding of MtrA to these sites under nutrient‐rich conditions and of GlnR under nitrogen‐limited conditions, consistent with the higher levels of MtrA or GlnR under those respective conditions. In addition, we showed that both mtrA and glnR are self‐regulated. Our study provides new insights into the regulation of nitrogen metabolism genes in Streptomyces.

Hyphal compartmentalization and sporulation in Streptomyces require the conserved cell division protein SepX

Filamentous actinobacteria such as Streptomyces undergo two distinct modes of cell division, leading to partitioning of growing hyphae into multicellular compartments via cross-walls, and to septation and release of unicellular spores. Specific determinants for cross-wall formation and the importance of hyphal compartmentalization for Streptomyces development are largely unknown. Here we show that SepX, an actinobacterial-specific protein, is crucial for both cell division modes in Streptomyces venezuelae. Importantly, we find that sepX-deficient mutants grow without cross-walls and that this substantially impairs the fitness of colonies and the coordinated progression through the developmental life cycle. Protein interaction studies and live-cell imaging suggest that SepX contributes to the stabilization of the divisome, a mechanism that also requires the dynamin-like protein DynB. Thus, our work identifies an important determinant for cell division in Streptomyces that is required for cellular development and sporulation.

Streptomyces bacteria undergo two modes of cell division: formation of cross-walls in hyphae, leading to multicellular compartments, and septation for release of unicellular spores. Here, Bush et al. identify a protein that is important for both cell division modes in Streptomyces, likely by contributing to stabilization of the divisome.

AdpA Positively Regulates Morphological Differentiation and Chloramphenicol Biosynthesis in Streptomyces venezuelae

In members of genus Streptomyces, AdpA is a master transcriptional regulator that controls the expression of hundreds of genes involved in morphological differentiation, secondary metabolite biosynthesis, chromosome replication, etc. However, the function of AdpASv, an AdpA ortholog of Streptomyces venezuelae, is unknown. This bacterial species is a natural producer of chloramphenicol and has recently become a model organism for studies on Streptomyces. Here, we demonstrate that AdpASv is essential for differentiation and antibiotic biosynthesis in S. venezuelae and provide evidence suggesting that AdpASv positively regulates its own gene expression. We speculate that the different modes of AdpA-dependent transcriptional autoregulation observed in S. venezuelae and other Streptomyces species reflect the arrangement of AdpA binding sites in relation to the transcription start site. Lastly, we present preliminary data suggesting that AdpA may undergo a proteolytic processing and we speculate that this may potentially constitute a novel regulatory mechanism controlling cellular abundance of AdpA in Streptomyces. IMPORTANCEStreptomyces are well-known producers of valuable secondary metabolites which include a large variety of antibiotics and important model organisms for developmental studies in multicellular bacteria. The conserved transcriptional regulator AdpA of Streptomyces exerts a pleiotropic effect on cellular processes, including the morphological differentiation and biosynthesis of secondary metabolites. Despite extensive studies, the function of AdpA in these processes remains elusive. This work provides insights into the role of a yet unstudied AdpA ortholog of Streptomyces venezuelae, now considered a novel model organism. We found that AdpA plays essential role in morphological differentiation and biosynthesis of chloramphenicol, a broad-spectrum antibiotic. We also propose that AdpA may undergo a proteolytic processing that presumably constitutes a novel mechanism regulating cellular abundance of this master regulator.

A High-Yield Streptomyces Transcription-Translation Toolkit for Synthetic Biology and Natural Product Applications

Streptomyces spp. are a major source of clinical antibiotics and industrial chemicals. Streptomyces venezuelae ATCC 10712 is a fast-growing strain and a natural producer of chloramphenicol, jadomycin, and pikromycin, which makes it an attractive candidate as a next-generation synthetic biology chassis. Therefore, genetic tools that accelerate the development of S. venezuelae ATCC 10712, as well as other Streptomyces spp. models, are highly desirable for natural product engineering and discovery. To this end, a dedicated S. venezuelae ATCC 10712 cell-free system is provided in this protocol to enable high-yield heterologous expression of high G+C (%) genes. This protocol is suitable for small-scale (10-100 μL) batch reactions in either 96-well or 384-well plate format, while reactions are potentially scalable. The cell-free system is robust and can achieve high yields (~5-10 μM) for a range of recombinant proteins in a minimal setup. This work also incorporates a broad plasmid toolset for real-time measurement of mRNA and protein synthesis, as well as in-gel fluorescence staining of tagged proteins. This protocol can also be integrated with high-throughput gene expression characterization workflows or the study of enzyme pathways from high G+C (%) genes present in Actinomycetes genomes.

Application of Streptomyces Antimicrobial Compounds for the Control of Phytopathogens

One of the relevant problems in today's agriculture is related to phytopathogenic microorganisms that cause between 30–40% of crop losses. Synthetic chemical pesticides and antibiotics have brought human and environmental health problems and microbial resistance to these treatments. So, the search for natural alternatives is necessary. The genus Streptomyces have broad biotechnological potential, being a promising candidate for the biocontrol of phytopathogenic microorganisms. The efficacy of some species of this genus in plant protection and their continued presence in the intensely competitive rhizosphere is due to its great potential to produce a wide variety of soluble bioactive secondary metabolites and volatile organic compounds. However, more attention is still needed to develop novel formulations that could increase the shelf life of streptomycetes, ensuring their efficacy as a microbial pesticide. In this sense, encapsulation offers an advantageous and environmentally friendly option. The present review aims to describe some phytopathogenic microorganisms with economic importance that require biological control. In addition, it focuses mainly on the Streptomyces genus as a great producer of secondary metabolites that act on other microorganisms and plants, exercising its role as biological control. The review also covers some strategies and products based on Streptomyces and the problems of its application in the field.

Spatial rearrangement of the Streptomyces venezuelae linear chromosome during sporogenic development

Bacteria of the genus Streptomyces have a linear chromosome, with a core region and two ‘arms’. During their complex life cycle, these bacteria develop multi-genomic hyphae that differentiate into chains of exospores that carry a single copy of the genome. Sporulation-associated cell division requires chromosome segregation and compaction. Here, we show that the arms of Streptomyces venezuelae chromosomes are spatially separated at entry to sporulation, but during sporogenic cell division they are closely aligned with the core region. Arm proximity is imposed by segregation protein ParB and condensin SMC. Moreover, the chromosomal terminal regions are organized into distinct domains by the Streptomyces-specific HU-family protein HupS. Thus, as seen in eukaryotes, there is substantial chromosomal remodelling during the Streptomyces life cycle, with the chromosome undergoing rearrangements from an ‘open’ to a ‘closed’ conformation.

Streptomyces bacteria have a linear chromosome and a complex life cycle, including development of multi-genomic hyphae that differentiate into mono-genomic exospores. Here, Szafran et al. show that the chromosome of Streptomyces venezuelae undergoes substantial remodelling during sporulation, from an ‘open’ to a ‘closed’ conformation.

Interplay between Nucleoid-Associated Proteins and Transcription Factors in Controlling Specialized Metabolism in Streptomyces

Lsr2 is a small nucleoid-associated protein found throughout the actinobacteria. Lsr2 functions similarly to the well-studied H-NS, in that it preferentially binds AT-rich sequences and represses gene expression. In Streptomyces venezuelae, Lsr2 represses the expression of many specialized metabolic clusters, including the chloramphenicol antibiotic biosynthetic gene cluster, and deleting lsr2 leads to significant upregulation of chloramphenicol cluster expression. We show here that Lsr2 likely exerts its repressive effects on the chloramphenicol cluster by polymerizing along the chromosome and by bridging sites within and adjacent to the chloramphenicol cluster. CmlR is a known activator of the chloramphenicol cluster, but expression of its associated gene is not upregulated in an lsr2 mutant strain. We demonstrate that CmlR is essential for chloramphenicol production, and further reveal that CmlR functions to “countersilence” Lsr2’s repressive effects by recruiting RNA polymerase and enhancing transcription, with RNA polymerase effectively clearing bound Lsr2 from the chloramphenicol cluster DNA. Our results provide insight into the interplay between opposing regulatory proteins that govern antibiotic production in S. venezuelae, which could be exploited to maximize the production of bioactive natural products in other systems.

Streptomyces venezuelae NRRL B-65442: genome sequence of a model strain used to study morphological differentiation in filamentous actinobacteria

For over a decade, Streptomyces venezuelae has been used to study the molecular mechanisms that control morphological development in streptomycetes and it is now a well-established model strain. Its rapid growth and ability to sporulate in a near-synchronised manner in liquid culture, unusual among streptomycetes, greatly facilitates the application of modern molecular techniques such as ChIP-seq and RNA-seq, as well as fluorescence time-lapse imaging of the complete Streptomyces life cycle. Here we describe a high-quality genome sequence of our isolate of the strain (NRRL B-65442) consisting of an 8.2 Mb chromosome and a 158 kb plasmid, pSVJI1, which had not been reported previously. Surprisingly, while NRRL B-65442 yields green spores on MYM agar, the ATCC type strain 10712 (from which NRRL B-65442 was derived) produces grey spores. While comparison of the genome sequences of the two isolates revealed almost total identity, it did reveal a single nucleotide substitution in a gene, vnz_33525, likely to be involved in spore pigment biosynthesis. Replacement of the vnz_33525 allele of ATCC 10712 with that of NRRL B-65442 resulted in green spores, explaining the discrepancy in spore pigmentation. We also applied CRISPR-Cas9 to delete the essential parB of pSVJI1 to cure the plasmid from the strain without obvious phenotypic consequences.

A conserved cell division protein directly regulates FtsZ dynamics in filamentous and unicellular actinobacteria

Bacterial cell division is driven by the polymerization of the GTPase FtsZ into a contractile structure, the so-called Z-ring. This essential process involves proteins that modulate FtsZ dynamics and hence the overall Z-ring architecture. Actinobacteria like Streptomyces and Mycobacterium lack known key FtsZ-regulators. Here we report the identification of SepH, a conserved actinobacterial protein that directly regulates FtsZ dynamics. We show that SepH is crucially involved in cell division in Streptomyces venezuelae and that it binds FtsZ via a conserved helix-turn-helix motif, stimulating the assembly of FtsZ protofilaments. Comparative in vitro studies using the SepH homolog from Mycobacterium smegmatis further reveal that SepH can also bundle FtsZ protofilaments, indicating an additional Z-ring stabilizing function in vivo. We propose that SepH plays a crucial role at the onset of cytokinesis in actinobacteria by promoting the assembly of FtsZ filaments into division-competent Z-rings that can go on to mediate septum synthesis.

Compaction and control-the role of chromosome-organizing proteins in Streptomyces

Chromosomes are dynamic entities, whose organization and structure depend on the concerted activity of DNA-binding proteins and DNA-processing enzymes. In bacteria, chromosome replication, segregation, compaction and transcription are all occurring simultaneously, and to ensure that these processes are appropriately coordinated, all bacteria employ a mix of well-conserved and species-specific proteins. Unusually, Streptomyces bacteria have large, linear chromosomes and life cycle stages that include multigenomic filamentous hyphae and unigenomic spores. Moreover, their prolific secondary metabolism yields a wealth of bioactive natural products. These different life cycle stages are associated with profound changes in nucleoid structure and chromosome compaction, and require distinct repertoires of architectural-and regulatory-proteins. To date, chromosome organization is best understood during Streptomyces sporulation, when chromosome segregation and condensation are most evident, and these processes are coordinated with synchronous rounds of cell division. Advances are, however, now being made in understanding how chromosome organization is achieved in multigenomic hyphal compartments, in defining the functional and regulatory interplay between different architectural elements, and in appreciating the transcriptional control exerted by these 'structural' proteins.

Identification and Characterization of Four c-di-GMP-Metabolizing Enzymes from Streptomyces ghanaensis ATCC14672 Involved in the Regulation of Morphogenesis and Moenomycin A Biosynthesis

Diguanylate cyclases (DGCs) and phosphodiesterases (PDEs) are essential enzymes deputed to maintain the intracellular homeostasis of the second messenger cyclic dimeric (3'→5') GMP (c-di-GMP). Recently, c-di-GMP has emerged as a crucial molecule for the streptomycetes life cycle, governing both morphogenesis and secondary metabolite production. Indeed, in Streptomyces ghanaensis ATCC14672 c-di-GMP was shown to be involved in the regulatory cascade of the peptidoglycan glycosytransferases inhibitor moenomycin A (MmA) biosynthesis. Here, we report the role of four c-di-GMP-metabolizing enzymes on MmA biosynthesis as well as morphological progression in S. ghanaensis. Functional characterization revealed that RmdA_gh and CdgA_gh are two active PDEs, while CdgE_gh is a DGC. In vivo, overexpression of rmdA_gh and cdgA_gh led to precocious sporulation, whereas overexpression of cdgE_gh and cdgD_gh (encoding a predicted DGC) caused an arrest of morphological development. Furthermore, we demonstrated that individual deletion of rmdA_gh, cdgA_gh, and cdgD_gh enhances MmA accumulation, whereas deletion of cdgE_gh has no impact on antibiotic production. Conversely, an individual deletion of each studied gene does not affect morphogenesis. Altogether, our results show that manipulation of c-di-GMP-metabolizing enzymes represent a useful approach to improving MmA production titers in S. ghanaensis.

Identification of a TetR family regulator and a polyketide synthase gene cluster involved in growth development and butenyl-spinosyn biosynthesis of Saccharopolyspora pogona

Butenyl-spinosyn produced by Saccharopolyspora pogona exhibits strong insecticidal activity and broad pesticidal spectrum. However, its synthetic level was low in the wild-type strain. At present, important functional genes involved in butenyl-spinosyn biosynthesis remain unknown, which leads to difficulty in efficiently editing its genome to improve the butenyl-spinosyn yield. To accelerate the genetic modification of S. pogona, we conducted comparative proteomics analysis to screen differentially expressed proteins related to butenyl-spinosyn biosynthesis. A TetR family regulatory protein was selected from the 289 differentially expressed proteins, and its encoding gene (SP_1288) was successfully deleted by CRISPR/Cas9 system. We further deleted a 32-kb polyketide synthase gene cluster (cluster 28) to reduce the competition for precursors. Phenotypic analysis revealed that the deletion of the SP_1288 and cluster 28 resulted in a 3.10-fold increase and a 35.4% decrease in the butenyl-spinosyn levels compared with the wild-type strain, respectively. The deletion of cluster 28 affected the cell growth, glucose consumption, mycelium morphology, and sporulation by controlling the expression of ptsH, ptsI, amfC, and other genes related to sporulation, whereas SP_1288 did not. These findings confirmed not only that the CRISPR/Cas9 system can be applied to the S. pogona genome editing but also that SP_1288 and cluster 28 are closely related to the butenyl-spinosyn biosynthesis and growth development of S. pogona. The strategy reported here will be useful to reveal the regulatory mechanism of butenyl-spinosyn and improve antibiotic production in other actinomycetes. KEY POINTS: • SP_1288 deletion can significantly promote the butenyl-spinosyn biosynthesis. • Cluster 28 deletion showed pleiotropic effects on S. pogona. • SP_1288 and cluster 28 were deleted by CRISPR/Cas9 system in S. pogona.

Influence of core divisome proteins on cell division in Streptomyces venezuelae ATCC 10712

The sporulating, filamentous soil bacterium Streptomyces venezuelae ATCC 10712 differentiates under submerged and surface growth conditions. In order to lay a solid foundation for the study of development-associated division for this organism, a congenic set of mutants was isolated, individually deleted for a gene encoding either a cytoplasmic (i.e. ftsZ) or core inner membrane (i.e. divIC, ftsL, ftsI, ftsQ, ftsW) component of the divisome. While ftsZ mutants are completely blocked for division, single mutants in the other core divisome genes resulted in partial, yet similar, blocks in sporulation septum formation. Double and triple mutants for core divisome membrane components displayed phenotypes that were similar to those of the single mutants, demonstrating that the phenotypes were not synergistic. Division in this organism is still partially functional without multiple core divisome proteins, suggesting that perhaps other unknown lineage-specific proteins perform redundant functions. In addition, by isolating an ftsZ2p mutant with an altered -10 region, the conserved developmentally controlled promoter was also shown to be required for sporulation-associated division. Finally, microscopic observation of FtsZ-YFP dynamics in the different mutant backgrounds led to the conclusion that the initial assembly of regular Z rings does not per se require the tested divisome membrane proteins, but the stability of Z rings is dependent on the divisome membrane components tested. The observation is consistent with the interpretation that Z ring instability likely results from and further contributes to the observed defects in sporulation septation in mutants lacking core divisome proteins.

Systems and synthetic biology to elucidate secondary metabolite biosynthetic gene clusters encoded in Streptomyces genomes

Covering: 2010 to 2020 Over the last few decades, Streptomyces have been extensively investigated for their ability to produce diverse bioactive secondary metabolites. Recent advances in Streptomyces research have been largely supported by improvements in high-throughput technology 'omics'. From genomics, numerous secondary metabolite biosynthetic gene clusters were predicted, increasing their genomic potential for novel bioactive compound discovery. Additional omics, including transcriptomics, translatomics, interactomics, proteomics and metabolomics, have been applied to obtain a system-level understanding spanning entire bioprocesses of Streptomyces, revealing highly interconnected and multi-layered regulatory networks for secondary metabolism. The comprehensive understanding derived from this systematic information accelerates the rational engineering of Streptomyces to enhance secondary metabolite production, integrated with the exploitation of the highly efficient 'Design-Build-Test-Learn' cycle in synthetic biology. In this review, we describe the current status of omics applications in Streptomyces research to better understand the organism and exploit its genetic potential for higher production of valuable secondary metabolites and novel secondary metabolite discovery.

Arrayed CRISPRi and quantitative imaging describe the morphotypic landscape of essential mycobacterial genes

Mycobacterium tuberculosis possesses a large number of genes of unknown or predicted function, undermining fundamental understanding of pathogenicity and drug susceptibility. To address this challenge, we developed a high-throughput functional genomics approach combining inducible CRISPR-interference and image-based analyses of morphological features and sub-cellular chromosomal localizations in the related non-pathogen, M. smegmatis. Applying automated imaging and analysis to 263 essential gene knockdown mutants in an arrayed library, we derive robust, quantitative descriptions of bacillary morphologies consequent on gene silencing. Leveraging statistical-learning, we demonstrate that functionally related genes cluster by morphotypic similarity and that this information can be used to inform investigations of gene function. Exploiting this observation, we infer the existence of a mycobacterial restriction-modification system, and identify filamentation as a defining mycobacterial response to histidine starvation. Our results support the application of large-scale image-based analyses for mycobacterial functional genomics, simultaneously establishing the utility of this approach for drug mechanism-of-action studies.

System-level understanding of gene expression and regulation for engineering secondary metabolite production in Streptomyces

The gram-positive bacterium, Streptomyces, is noticed for its ability to produce a wide array of pharmaceutically active compounds through secondary metabolism. To discover novel bioactive secondary metabolites and increase the production, Streptomyces species have been extensively studied for the past decades. Among the cellular components, RNA molecules play important roles as the messengers for gene expression and diverse regulations taking place at the RNA level. Thus, the analysis of RNA-level regulation is critical to understanding the regulation of Streptomyces’ metabolism and secondary metabolite production. A dramatic advance in Streptomyces research was made recently, by exploiting high-throughput technology to systematically understand RNA levels. In this review, we describe the current status of the system-wide investigation of Streptomyces in terms of RNA, toward expansion of its genetic potential for secondary metabolite synthesis.

SufR, a [4Fe-4S] Cluster-Containing Transcription Factor, Represses the sufRBDCSU Operon in Streptomyces avermitilis Iron-Sulfur Cluster Assembly

Iron-sulfur (Fe-S) clusters are ubiquitous and versatile inorganic cofactors that are crucial for many fundamental bioprocesses in nearly all organisms. How cells maintain Fe-S cluster homeostasis is not well understood in Gram-positive bacteria. Genomic analysis showed that the Suf system, which is encoded by the sufRBDCSU operon, is the sole Fe-S cluster assembly system in the genus Streptomyces Streptomyces avermitilis is the industrial producer of avermectins, which are widely used as agricultural pesticides and antiparasitic agents. sufR (SAV6324) encodes a putative ArsR-family transcriptional regulator, which was characterized as a repressor of the sufRBDCSU operon in this investigation. Spectroscopy and mass spectrometry demonstrated that anaerobically isolated SufR contained an oxidation-sensitive [4Fe-4S] cluster and existed as a homodimer. Electrophoretic mobility shift assays (EMSAs) and DNase I footprinting analyses revealed that [4Fe-4S]-SufR bound specifically and tightly to a 14-bp palindromic sequence (CAAC-N6-GTTG) in the promoter region of the sufR operon, repressing expression of the sufRBDCSU operon. The presence of the [4Fe-4S] cluster is critical for the DNA-binding activity of SufR. Cys¹⁸², Cys¹⁹⁵, and Cys²²³ in the C-terminal region of SufR are essential for [4Fe-4S] cluster coordination, but Cys¹⁷⁸ is not. The fourth non-Cys ligand in coordination of the [4Fe-4S] cluster for SufR remains to be identified. The findings clarify the transcriptional control of the suf operon by [4Fe-4S] SufR to satisfy the various Fe-S cluster demands. SufR senses the intracellular Fe-S cluster status and modulates the expression of the sole Fe-S cluster assembly system via its Fe-S cluster occupancy.IMPORTANCE Fe-S clusters function as cofactors of proteins controlling diverse biological processes, such as respiration, photosynthesis, nitrogen fixation, DNA replication, and gene regulation. The mechanism of how Actinobacteria regulate the expression of the sole Fe-S cluster assembly system in response to the various Fe-S cluster demands remains to be elucidated. In this study, we showed that SufR functions as a transcriptional repressor of the sole Fe-S cluster assembly system in the avermectin producer S. avermitilis [4Fe-4S]-SufR binds to the promoter region of the suf operon and represses its expression. When Fe-S cluster levels are insufficient, SufR loses its [4Fe-4S] cluster and DNA-binding activity. Apo-SufR dissociates from the promoter region of suf operon, and the expression of the suf system is strongly increased by derepression to promote the synthesis of Fe-S clusters. The study clarifies how Streptomyces maintains its Fe-S cluster homeostasis through the activity of SufR to modulate the various Fe-S cluster demands.

Specialized and shared functions of diguanylate cyclases and phosphodiesterases in Streptomyces development

The second messenger bis-3,5-cyclic di-guanosine monophosphate (c-di-GMP) determines when Streptomyces initiate sporulation. c-di-GMP signals are integrated into the genetic differentiation network by the regulator BldD and the sigma factor σ^WhiG . However, functions of the development-specific diguanylate cyclases (DGCs) CdgB and CdgC, and the c-di-GMP phosphodiesterases (PDEs) RmdA and RmdB, are poorly understood. Here, we provide biochemical evidence that the GGDEF-EAL domain protein RmdB from S. venezuelae is a monofunctional PDE that hydrolyzes c-di-GMP to 5'pGpG. Despite having an equivalent GGDEF-EAL domain arrangement, RmdA cleaves c-di-GMP to GMP and exhibits residual DGC activity. We show that an intact EAL motif is crucial for the in vivo function of both enzymes since strains expressing protein variants with an AAA motif instead of EAL are delayed in development, similar to null mutants. Transcriptome analysis of ∆cdgB, ∆cdgC, ∆rmdA, and ∆rmdB strains revealed that the c-di-GMP specified by these enzymes has a global regulatory role, with about 20% of all S. venezuelae genes being differentially expressed in the cdgC mutant. Our data suggest that the major c-di-GMP-controlled targets determining the timing and mode of sporulation are genes involved in cell division and the production of the hydrophobic sheath that covers Streptomyces aerial hyphae and spores.

Thiol-specific oxidant diamide downregulates whiA gene of Corynebacterium glutamicum, thereby suppressing cell division and metabolism

The whiA (NCgl1527) gene from Corynebacterium glutamicum plays a crucial role during cell growth, and WhiA is recognized as the transcription factor for genes involved in cell division. In this study, we assessed the regulatory role of the gene in cell physiology. Transcription of the gene was specifically downregulated by the thiol-specific oxidant, diamide, and by heat stress. Cells exposed to diamide showed decreased transcription of genes involved in cell division and these effects were more profound in ΔwhiA cells. In addition, the ΔwhiA cells showed sensitivity to thiol-specific oxidants, DNA-damaging agents, and high temperature. Further, downregulation of sigH (NCgl0733), the central regulator in stress responses, along with master regulatory genes in cell metabolism, was observed in the ΔwhiA strain. Moreover, the amount of cAMP in the ΔwhiA cells in the early stationary phase was only at 30% level of that for the wild-type strain. Collectively, our data indicate that the role of whiA is to downregulate genes associated with cell division in response to heat or thiol-specific oxidative stress, and may suggest a role for the gene in downshifting cell metabolism by downregulating global regulatory genes when growth condition is not optimal for cells.

Construction of Inducible Genetic Switch for the Global Regulator WblA To Sustain Both Overproduction of Tiancimycins and On-Demand Sporulation in Streptomyces sp. CB03234

The complex life cycle of streptomycetes is closely related to their secondary metabolisms, all controlled by cascade regulations. Tiancimycins (TNMs) are ten-membered enediynes possessing great potential for antitumor drug development. However, their low yields in Streptomyces sp. CB03234 have greatly limited subsequent studies. Through transcriptome analysis and genetic characterization, we proved that WblA is one pivotal global regulator to repress the biosynthesis of TNMs. The deletion of wblA could significantly enhance the production of TNMs, but also abolish the sporulation in CB03234. By constructing the NitR-ε-caprolactam inducible genetic switch, the expression of wblA was governed in CB03234-NRW, thereby sustaining the overproduction of TNMs and recovering the normal sporulation upon induction, which were practical for the scaled-up production of TNMs. Considering the prevalence and conserved regulatory roles of WblA in streptomycetes, our developed strategy shall provide an effective and practical approach to facilitate titer improvement and discovery of natural products.

Chromosome Segregation Proteins as Coordinators of Cell Cycle in Response to Environmental Conditions

Chromosome segregation is a crucial stage of the cell cycle. In general, proteins involved in this process are DNA-binding proteins, and in most bacteria, ParA and ParB are the main players; however, some bacteria manage this process by employing other proteins, such as condensins. The dynamic interaction between ParA and ParB drives movement and exerts positioning of the chromosomal origin of replication (oriC) within the cell. In addition, both ParA and ParB were shown to interact with the other proteins, including those involved in cell division or cell elongation. The significance of these interactions for the progression of the cell cycle is currently under investigation. Remarkably, DNA binding by ParA and ParB as well as their interactions with protein partners conceivably may be modulated by intra- and extracellular conditions. This notion provokes the question of whether chromosome segregation can be regarded as a regulatory stage of the cell cycle. To address this question, we discuss how environmental conditions affect chromosome segregation and how segregation proteins influence other cell cycle processes.

Nucleotide second messengers in Streptomyces

Antibiotic producing Streptomyces sense and respond to environmental signals by using nucleotide second messengers, including (p)ppGpp, cAMP, c-di-GMP and c-di-AMP. As summarized in this review, these molecules are important message carriers that coordinate the complex Streptomyces morphological transition from filamentous growth to sporulation along with the secondary metabolite production. Here, we provide an overview of the enzymes that make and break these second messengers and suggest candidates for (p)ppGpp and cAMP enzymes to be studied. We highlight the target molecules that bind these signalling molecules and elaborate individual functions that they control in the context of Streptomyces development. Finally, we discuss open questions in the field, which may guide future studies in this exciting research area.

Hierarchical interactions between Fnr orthologs allows fine-tuning of transcription in response to oxygen in Herbaspirillum seropedicae

Bacteria adjust the composition of their electron transport chain (ETC) to efficiently adapt to oxygen gradients. This involves differential expression of various ETC components to optimize energy generation. In Herbaspirillum seropedicae, reprogramming of gene expression in response to oxygen availability is controlled at the transcriptional level by three Fnr orthologs. Here, we characterised Fnr regulons using a combination of RNA-Seq and ChIP-Seq analysis. We found that Fnr1 and Fnr3 directly regulate discrete groups of promoters (Groups I and II, respectively), and that a third group (Group III) is co-regulated by both transcription factors. Comparison of DNA binding motifs between the three promoter groups suggests Group III promoters are potentially co-activated by Fnr3–Fnr1 heterodimers. Specific interaction between Fnr1 and Fnr3, detected in two-hybrid assays, was dependent on conserved residues in their dimerization interfaces, indicative of heterodimer formation in vivo. The requirements for co-activation of the fnr1 promoter, belonging to Group III, suggest either sequential activation by Fnr3 and Fnr1 homodimers or the involvement of Fnr3–Fnr1 heterodimers. Analysis of Fnr proteins with swapped activation domains provides evidence that co-activation by Fnr1 and Fnr3 at Group III promoters optimises interactions with RNA polymerase to fine-tune transcription in response to prevailing oxygen concentrations.

Transcriptional Response of Streptomyces coelicolor to Rapid Chromosome Relaxation or Long-Term Supercoiling Imbalance

Negative DNA supercoiling allows chromosome condensation and facilitates DNA unwinding, which is required for the occurrence of DNA transaction processes, i.e., DNA replication, transcription and recombination. In bacteria, changes in chromosome supercoiling impact global gene expression; however, the limited studies on the global transcriptional response have focused mostly on pathogenic species and have reported various fractions of affected genes. Furthermore, the transcriptional response to long-term supercoiling imbalance is still poorly understood. Here, we address the transcriptional response to both novobiocin-induced rapid chromosome relaxation or long-term topological imbalance, both increased and decreased supercoiling, in environmental antibiotic-producing bacteria belonging to the Streptomyces genus. During the Streptomyces complex developmental cycle, multiple copies of GC-rich linear chromosomes present in hyphal cells undergo profound topological changes, from being loosely condensed in vegetative hyphae, to being highly compacted in spores. Moreover, changes in chromosomal supercoiling have been suggested to be associated with the control of antibiotic production and environmental stress response. Remarkably, in S. coelicolor, a model Streptomyces species, topoisomerase I (TopA) is solely responsible for the removal of negative DNA supercoils. Using a S. coelicolor strain in which topA transcription is under the control of an inducible promoter, we identified genes involved in the transcriptional response to long-term supercoiling imbalance. The affected genes are preferentially organized in several clusters, and a supercoiling-hypersensitive cluster (SHC) was found to be located in the core of the S. coelicolor chromosome. The transcripts affected by long-term topological imbalance encompassed genes encoding nucleoid-associated proteins, DNA repair proteins and transcriptional regulators, including multiple developmental regulators. Moreover, using a gyrase inhibitor, we identified those genes that were directly affected by novobiocin, and found this was correlated with increased AT content in their promoter regions. In contrast to the genes affected by long-term supercoiling changes, among the novobiocin-sensitive genes, a significant fraction encoded for proteins associated with membrane transport or secondary metabolite synthesis. Collectively, our results show that long-term supercoiling imbalance globally regulates gene transcription and has the potential to impact development, secondary metabolism and DNA repair, amongst others.

Silencing cryptic specialized metabolism in Streptomyces by the nucleoid-associated protein Lsr2

Lsr2 is a nucleoid-associated protein conserved throughout the actinobacteria, including the antibiotic-producing Streptomyces. Streptomyces species encode paralogous Lsr2 proteins (Lsr2 and Lsr2-like, or LsrL), and we show here that of the two, Lsr2 has greater functional significance. We found that Lsr2 binds AT-rich sequences throughout the chromosome, and broadly represses gene expression. Strikingly, specialized metabolic clusters were over-represented amongst its targets, and the cryptic nature of many of these clusters appears to stem from Lsr2-mediated repression. Manipulating Lsr2 activity in model species and uncharacterized isolates resulted in the production of new metabolites not seen in wild type strains. Our results suggest that the transcriptional silencing of biosynthetic clusters by Lsr2 may protect Streptomyces from the inappropriate expression of specialized metabolites, and provide global control over Streptomyces’ arsenal of signaling and antagonistic compounds.

Specific amino acid substitutions in β strand S2 of FtsZ cause spiraling septation and impair assembly cooperativity in Streptomyces spp

Bacterial cell division is orchestrated by the Z ring, which is formed by single-stranded treadmilling protofilaments of FtsZ. In Streptomyces, during sporulation, multiple Z rings are assembled and lead to formation of septa that divide a filamentous hyphal cell into tens of prespore compartments. We describe here mutant alleles of ftsZ in Streptomyces coelicolor and Streptomyces venezuelae that perturb cell division in such a way that constriction is initiated along irregular spiral-shaped paths rather than as regular septa perpendicular to the cell length axis. This conspicuous phenotype is caused by amino acid substitutions F37I and F37R in β strand S2 of FtsZ. The F37I mutation leads, instead of regular Z rings, to formation of relatively stable spiral-shaped FtsZ structures that are capable of initiating cell constriction. Further, we show that the F37 mutations affect the polymerization properties and impair the cooperativity of FtsZ assembly in vitro. The results suggest that specific residues in β strand S2 of FtsZ affect the conformational switch in FtsZ that underlies assembly cooperativity and enable treadmilling of protofilaments, and that these features are required for formation of regular Z rings. However, the data also indicate FtsZ-directed cell constriction is not dependent on assembly cooperativity.

Light Modulates the Physiology of Nonphototrophic Actinobacteria

Light is a source of energy and an environmental cue that is available in excess in most surface environments. In prokaryotic systems, conversion of light to energy by photoautotrophs and photoheterotrophs is well understood, but the conversion of light to information and the cellular response to that information have been characterized in only a few species. Our goal was to explore the response of freshwater Actinobacteria, which are ubiquitous in illuminated aquatic environments, to light. We found that Actinobacteria without functional photosystems grow faster in the light, likely because sugar transport and metabolism are upregulated in the light. Based on the action spectrum of the growth effect and comparisons of the genomes of three Actinobacteria with this growth rate phenotype, we propose that the photosensor in these strains is a putative CryB-type cryptochrome. The ability to sense light and upregulate carbohydrate transport during the day could allow these cells to coordinate their time of maximum organic carbon uptake with the time of maximum organic carbon release by primary producers.IMPORTANCE Sunlight provides information about both place and time. In sunlit aquatic environments, primary producers release organic carbon and nitrogen along with other growth factors during the day. The ability of Actinobacteria to coordinate organic carbon uptake and utilization with production of photosynthate enables them to grow more efficiently in the daytime, and it potentially gives them a competitive advantage over heterotrophs that constitutively produce carbohydrate transporters, which is energetically costly, or produce transporters only after detection of the substrate(s), which delays their response. Understanding how light cues the transport of organic carbon and its conversion to biomass is key to understanding biochemical mechanisms within the carbon cycle, the fluxes through it, and the variety of mechanisms by which light enhances growth.

Defining the regulon of genes controlled by σE , a key regulator of the cell envelope stress response in Streptomyces coelicolor

The extracytoplasmic function (ECF) σ factor, σ^E , is a key regulator of the cell envelope stress response in Streptomyces coelicolor. Although its role in maintaining cell wall integrity has been known for over a decade, a comprehensive analysis of the genes under its control has not been undertaken. Here, using a combination of chromatin immunoprecipitation-sequencing (ChIP-seq), microarray transcriptional profiling and bioinformatic analysis, we attempt to define the σ^E regulon. Approximately half of the genes identified encode proteins implicated in cell envelope function. Seventeen novel targets were validated by S1 nuclease mapping or in vitro transcription, establishing a σ^E -binding consensus. Subsequently, we used bioinformatic analysis to look for conservation of the σ^E target promoters identified in S. coelicolor across 19 Streptomyces species. Key proteins under σ^E control across the genus include the actin homolog MreB, three penicillin-binding proteins, two L,D-transpeptidases, a LytR-CpsA-Psr-family protein predicted to be involved in cell wall teichoic acid deposition and a predicted MprF protein, which adds lysyl groups to phosphatidylglycerol to neutralize membrane surface charge. Taken together, these analyses provide biological insight into the σ^E -mediated cell envelope stress response in the genus Streptomyces.