The versatile TolC-like Slr1270 in the cyanobacteriumSynechocystissp. PCC 6803

Triple-Gene Overexpression of the AcrA-AcrB-TolC Transporter System in Synechocystis sp. PCC 6803 Contributes to a Higher Secretion of Free Fatty Acids in Response to Nitrogen Shortage and Salt Stress

One important aspect of cyanobacterial homoeostasis is reducing the toxicity of excess free fatty acids (FFAs) generated in the cells by means of both secreting these into the medium and recycling them toward membrane lipid synthesis. In this study, the cyanobacterium Synechocystis sp. PCC 6803 served to implement the overexpression of native genes of the transportation system. Specifically, we worked with the Sll0180-Slr2131-Slr1270 homologs of Escherichia coli AcrA-AcrB-TolC, respectively, to create single- and triple-overexpressing strains of OA, OB, OC, and OABC. Remarkably, the OABC strain that triply overexpressed the sll0180_slr2131_slr1270 genes acquired a significant amount of intracellular lipids, up to 23.5% of dry cell weight, under the normal condition. Nitrogen-deficient stress undoubtedly raised extracellular FFAs and intracellular lipids in overexpressing strains, especially in the OABC strain, which exhibited 33.9% and 41.5% of dry cell weight, respectively. During the first 5 days of treatment, salt stress at 256 mM significantly increased the FFA efflux, notably for the OB strain, but had no effect on intracellular lipids. It is noteworthy that the OA and OABC strains outperformed all other strains in terms of growth throughout the 16 days of nitrogen shortage. Furthermore, in comparison to the wild-type control, all the overexpressing strains exhibited a considerable increase in carotenoid accumulation. Thus, our results point to the effective role of the sll0180_slr2131_slr1270 transportation system in facilitating FFA secretion, especially in response to environmental stressors.

EXCRETE workflow enables deep proteomics of the microbial extracellular environment

Extracellular proteins play a significant role in shaping microbial communities which, in turn, can impact ecosystem function, human health, and biotechnological processes. Yet, for many ubiquitous microbes, there is limited knowledge regarding the identity and function of secreted proteins. Here, we introduce EXCRETE (enhanced exoproteome characterization by mass spectrometry), a workflow that enables comprehensive description of microbial exoproteomes from minimal starting material. Using cyanobacteria as a case study, we benchmark EXCRETE and show a significant increase over current methods in the identification of extracellular proteins. Subsequently, we show that EXCRETE can be miniaturized and adapted to a 96-well high-throughput format. Application of EXCRETE to cyanobacteria from different habitats (Synechocystis sp. PCC 6803, Synechococcus sp. PCC 11901, and Nostoc punctiforme PCC 73102), and in different cultivation conditions, identified up to 85% of all potentially secreted proteins. Finally, functional analysis reveals that cell envelope maintenance and nutrient acquisition are central functions of the predicted cyanobacterial secretome. Collectively, these findings challenge the general belief that cyanobacteria lack secretory proteins and suggest that multiple functions of the secretome are conserved across freshwater, marine, and terrestrial species.

Novel protein carrier system based on cyanobacterial nano-sized extracellular vesicles for application in fish

Aquaculture has been one of the fastest-growing food industry sectors, expanding at the pace of consumers' demands. To promote safe and effective fish growth performance strategies, and to stimulate environmentally friendly solutions to protect fish against disease outbreaks, new approaches are needed to safeguard fish welfare, as well as farmers and consumers interests. Here, we tested the use of cyanobacterial extracellular vesicles (EVs) as a novel nanocarrier system of heterologous proteins for applications in fish. We started by incubating zebrafish larvae with Synechocystis sp. PCC6803 EVs, isolated from selected mutant strains with different cell envelope characteristics. Results show that Synechocystis EVs are biocompatible with fish larvae, regardless of their structural composition, as EVs neither induced fish mortality nor triggered significant inflammatory responses. We establish also that cyanobacteria are amenable to engineering heterologous protein expression and loading into EVs, for which we used the reporter sfGFP. Moreover, upon immersion treatment, we successfully demonstrate that sfGFP-loaded Synechocystis EVs accumulate in the gastrointestinal tract of zebrafish larvae. This work opens the possibility of using cyanobacterial EVs as a novel biotechnological tool in fish, with prospective applications in carrying proteins/enzymes, for example for modulating their nutritional status or stimulating specific adaptive immune responses.

Antibiotic-Induced Changes in Pigment Accumulation, Photosystem II, and Membrane Permeability in a Model Cyanobacterium

Fremyella diplosiphon is a well-studied a model cyanobacterium for photosynthesis due to its efficient light absorption potential and pigment accumulation. In the present study, the impact of ampicillin, tetracycline, kanamycin, and cefotaxime on pigment fluorescence and photosynthetic capacity in Fremyella diplosiphon strains B481-WT and B481-SD was investigated. Our results indicated that both strains exposed to kanamycin from 0.2 to 3.2 mg/L and tetracycline from 0.8 to 12.8 mg/L enhanced growth and pigment accumulation. Additionally, B481-SD treated with 0.2-51.2 mg/L ampicillin resulted in a significant enhancement of pigment fluorescence. A detrimental effect on growth and pigmentation in both the strains exposed to 6.4-102.5 mg/L kanamycin and 0.8-102.5 mg/L cefotaxime was observed. Detection of reactive oxygen species revealed highest levels of oxidative stress at 51.2 and 102.5 mg/L kanamycin for B481-SD and 102.5 mg/L for B481-WT. Membrane permeability detected by lactate dehydrogenase assay indicated maximal activity at 0.8 mg/L ampicillin, kanamycin, and tetracycline treatments on day 6. Abundant vacuolation, pyrophosphate, and cyanophycin granule formation were observed in treated cells as a response to antibiotic stress. These findings on the hormetic effect of antibiotics on F. diplosiphon indicate that optimal antibiotic concentrations induce cellular growth while high concentrations severely impact cellular functionality. Future studies will be aimed to enhance cellular lipid productivity at optimal antibiotic concentrations to disintegrate the cell wall, thus paving the way for clean bioenergy applications.

Extracellular vesicles as an alternative copper-secretion mechanism in bacteria

Metal homeostasis is fundamental for optimal performance of cell metabolic pathways. Over the course of evolution, several systems emerged to warrant an intracellular metal equilibrium. When exposed to growth-challenging copper concentrations, Gram-negative bacteria quickly activate copper-detoxification mechanisms, dependent on transmembrane-protein complexes and metallochaperones that mediate metal efflux. Here, we show that vesiculation is also a common bacterial response mechanism to high copper concentrations, and that extracellular vesicles (EVs) play a role in transporting copper. We present evidence that bacteria from different ecological niches release copious amounts of EVs when exposed to copper. Along with the activation of the classical detoxification systems, we demonstrate that copper-stressed cells of the cyanobacterium Synechocystis sp. PCC6803 release EVs loaded with the copper-binding metallochaperone CopM. Under standard growth conditions, CopM-loaded EVs could also be isolated from a Synechocystis strain lacking a functional TolC-protein, which we characterize here as exhibiting a copper-sensitive phenotype. Analyses of Synechocystis tolC-mutant's EVs isolated from cells cultivated under standard conditions indicated the presence of copper therein, in significantly higher levels as compared to those from the wild-type. Altogether, these results suggest that release of EVs in bacteria represent a novel copper-secretion mechanism, shedding light into alternative mechanisms of bacterial metal resistance.

Linear Six-Carbon Sugar Alcohols Induce Lysis of Microcystis aeruginosa NIES-298 Cells

Cyanobacterial blooms are a global concern due to their adverse effects on water quality and human health. Therefore, we examined the effects of various compounds on Microcystis aeruginosa growth. We found that Microcystis aeruginosa NIES-298 cells were lysed rapidly by linear six-carbon sugar alcohols including mannitol, galactitol, iditol, fucitol, and sorbitol, but not by other sugar alcohols. Microscopic observations revealed that mannitol treatment induced crumpled inner membrane, an increase in periplasmic space, uneven cell surface with outer membrane vesicles, disruption of membrane structures, release of intracellular matter including chlorophylls, and eventual cell lysis in strain NIES-298, which differed from the previously proposed cell death modes. Mannitol metabolism, antioxidant-mediated protection of mannitol-induced cell lysis by, and caspase-3 induction in strain NIES-298 were not observed, suggesting that mannitol may not cause organic matter accumulation, oxidative stress, and programmed cell death in M. aeruginosa. No significant transcriptional expression was induced in strain NIES-298 by mannitol treatment, indicating that cell lysis is not induced through transcriptional responses. Mannitol-induced cell lysis may be specific to strain NIES-298 and target a specific component of strain NIES-298. This study will provide a basis for controlling M. aeruginosa growth specifically by non-toxic substances.

Extracellular Vesicle-Mediated Secretion of Protochlorophyllide in the Cyanobacterium Leptolyngbya boryana

Protochlorophyllide (Pchlide) reduction in the late stage of chlorophyll a (Chl) biosynthesis is catalyzed by two enzymes: light-dependent Pchlide oxidoreductase (LPOR) and dark-operative Pchlide oxidoreductase (DPOR). The differential operation of LPOR and DPOR enables a stable supply of Chl in response to changes in light conditions and environmental oxygen levels. When a DPOR-deficient mutant (YFC2) of the cyanobacterium Leptolyngbya boryana is grown heterotrophically in the dark, Pchlide accumulates in the cells and is secreted into the culture medium. In this study, we demonstrated the extracellular vesicle-mediated secretion of Pchlide. Pchlide fractions were isolated from the culture medium using sucrose density gradient centrifugation. Mass spectrometry analysis revealed that the Pchlide fractions contained porin isoforms, TolC, and FG-GAP repeat-containing protein, which are localized in the outer membrane. Transmission electron microscopy revealed extracellular vesicle-like structures in the vicinity of YFC2 cells and the Pchlide fractions. These findings suggested that the Pchlide secretion is mediated by extracellular vesicles in dark-grown YFC2 cells.

The Role of Outer Membrane Protein(s) Harboring SLH/OprB-Domains in Extracellular Vesicles’ Production in Synechocystis sp. PCC 6803

Cyanobacteria are a group of photosynthetic prokaryotes that contribute to primary production on a global scale. These microorganisms release vesicles to the extracellular environment, spherical nanosized structures, derived essentially from the outer membrane. Even though earlier works in model Gram-negative bacteria have hypothesized that outer membrane stability is crucial in vesicle formation, the mechanisms determining vesicle biogenesis in cyanobacteria remain unknown. Here, we report on the identification of six candidate genes encoding outer membrane proteins harboring SLH/OprB-domains in the genome of the model cyanobacterium Synechocystis sp. PCC 6803. Using a genetics-based approach, one gene was found to encode an essential protein (Slr1841), while the remaining five are not essential for growth under standard conditions. Vesicle production was monitored, and it was found that a mutant in the gene encoding the second most abundant SLH/OprB protein in Synechocystis sp. PCC 6803 outer membrane (Slr1908) produces more vesicles than any of the other tested strains. Moreover, the Slr1908-protein was also found to be important for iron uptake. Altogether, our results suggest that proteins containing the SLH/OprB-domains may have dual biological role, related to micronutrient uptake and to outer membrane stability, which, together or alone, seem to be involved in cyanobacterial vesicle biogenesis.

Improved Bioproduction of 1-Octanol Using Engineered Synechocystis sp. PCC 6803

1-Octanol has gained interest as a chemical precursor for both high and low value commodities including fuel, solvents, surfactants, and fragrances. By harnessing the power from sunlight and CO₂ as carbon source, cyanobacteria has recently been engineered for renewable production of 1-octanol. The productivity, however, remained low. In the present work, we report efforts to further improve the 1-octanol productivity. Different N-terminal truncations were evaluated on three thioesterases from different plant species, resulting in several candidate thioesterases with improved activity and selectivity toward octanoyl-ACP. The structure/function trials suggest that current knowledge and/or state-of-the art computational tools are insufficient to determine the most appropriate cleavage site for thioesterases in Synechocystis. Additionally, by tuning the inducer concentration and light intensity, we further improved the 1-octanol productivity, reaching up to 35% (w/w) carbon partitioning and a titer of 526 ± 5 mg/L 1-octanol in 12 days. Long-term cultivation experiments demonstrated that the improved strain can be stably maintained for at least 30 days and/or over ten times serial dilution. Surprisingly, the improved strain was genetically stable in contrast to earlier strains having lower productivity (and hence a reduced chance of reaching toxic product concentrations). Altogether, improved enzymes and environmental conditions (e.g., inducer concentration and light intensity) substantially increased the 1-octanol productivity. When cultured under continuous conditions, the bioproduction system reached an accumulative titer of >3.5 g/L 1-octanol over close to 180 days.

Special roles for efflux systems in iron homeostasis of non-siderophore-producing cyanobacteria

In oligotrophic oceans, low bioavailability of Fe is a key factor limiting primary productivity. However, excessive Fe in cells leads to the Fenton reaction, which is toxic to cells. Cyanobacteria must strictly maintain intracellular Fe homeostasis. Here, we knocked out a series of genes encoding efflux systems in Synechocystis sp. PCC 6803, and found eight genes that are required for high Fe detoxification. Unexpectedly, the HlyBD-TolC efflux system plays an important role in the adaptation of Synechocystis under Fe-deficient conditions. Mutants of HlyD and TolC grew worse than the wild-type strain under low-Fe conditions and showed significantly lower intracellular Fe contents than the wild-type strain. We excluded the possibility that the low Fe sensitivity of the HlyBD-TolC mutants was caused by a loss of the S-layer, the main extracellular protein secreted via this efflux system. Inactivation of the HlyD protein influenced type IV pili formation and direct inactivation of type IV pili related genes affected the adaptation to low-Fe conditions. HlyBD-TolC system is likely involved in the formation of type IV pili and indirectly influenced Fe acquisition. Our findings suggest that efflux system in non-siderophore-producing cyanobacteria can facilitate Fe uptake and help cells adapt to Fe-deficient conditions via novel pathways.

A Cyanobacterial Component Required for Pilus Biogenesis Affects the Exoproteome

Protein secretion as well as the assembly of bacterial motility appendages are central processes that substantially contribute to fitness and survival. This study highlights distinctive features of the mechanism that serves these functions in cyanobacteria, which are globally prevalent photosynthetic prokaryotes that significantly contribute to primary production. Our studies of biofilm development in the cyanobacterium Synechococcus elongatus uncovered a novel component required for the biofilm self-suppression mechanism that operates in this organism. This protein, which is annotated as "hypothetical," is denoted EbsA (essential for biofilm self-suppression A) here. EbsA homologs are highly conserved and widespread in diverse cyanobacteria but are not found outside this clade. We revealed a tripartite complex of EbsA, Hfq, and the ATPase homolog PilB (formerly called T2SE) and demonstrated that each of these components is required for the assembly of the hairlike type IV pili (T4P) appendages, for DNA competence, and affects the exoproteome in addition to its role in biofilm self-suppression. These data are consistent with bioinformatics analyses that reveal only a single set of genes in S. elongatus to serve pilus assembly or protein secretion; we suggest that a single complex is involved in both processes. A phenotype resulting from the impairment of the EbsA homolog in the cyanobacterium Synechocystis sp. strain PCC 6803 implies that this feature is a general cyanobacterial trait. Moreover, comparative exoproteome analyses of wild-type and mutant strains of S. elongatus suggest that EbsA and Hfq affect the exoproteome via a process that is independent of PilB, in addition to their involvement in a T4P/secretion machinery.IMPORTANCE Cyanobacteria, environmentally prevalent photosynthetic prokaryotes, contribute ∼25% of global primary production. Cyanobacterial biofilms elicit biofouling, thus leading to substantial economic losses; however, these microbial assemblages can also be beneficial, e.g., in wastewater purification processes and for biofuel production. Mechanistic aspects of cyanobacterial biofilm development were long overlooked, and genetic and molecular information emerged only in recent years. The importance of this study is 2-fold. First, it identifies novel components of cyanobacterial biofilm regulation, thus contributing to the knowledge of these processes and paving the way for inhibiting detrimental biofilms or promoting beneficial ones. Second, the data suggest that cyanobacteria may employ the same complex for the assembly of the motility appendages, type 4 pili, and protein secretion. A shared pathway was previously shown in only a few cases of heterotrophic bacteria, whereas numerous studies demonstrated distinct systems for these functions. Thus, our study broadens the understanding of pilus assembly/secretion in diverse bacteria and furthers the aim of controlling the formation of cyanobacterial biofilms.

Proximity-based proteomics reveals the thylakoid lumen proteome in the cyanobacterium Synechococcus sp. PCC 7002

Cyanobacteria possess unique intracellular organization. Many proteomic studies have examined different features of cyanobacteria to learn about the intracellular structures and their respective functions. While these studies have made great progress in understanding cyanobacterial physiology, the conventional fractionation methods used to purify cellular structures have limitations; specifically, certain regions of cells cannot be purified with existing fractionation methods. Proximity-based proteomics techniques were developed to overcome the limitations of biochemical fractionation for proteomics. Proximity-based proteomics relies on spatiotemporal protein labeling followed by mass spectrometry of the labeled proteins to determine the proteome of the region of interest. We performed proximity-based proteomics in the cyanobacterium Synechococcus sp. PCC 7002 with the APEX2 enzyme, an engineered ascorbate peroxidase. We determined the proteome of the thylakoid lumen, a region of the cell that has remained challenging to study with existing methods, using a translational fusion between APEX2 and PsbU, a lumenal subunit of photosystem II. Our results demonstrate the power of APEX2 as a tool to study the cell biology of intracellular features and processes, including photosystem II assembly in cyanobacteria, with enhanced spatiotemporal resolution.

Absence of KpsM (Slr0977) Impairs the Secretion of Extracellular Polymeric Substances (EPS) and Impacts Carbon Fluxes in Synechocystis sp. PCC 6803

Many cyanobacteria produce extracellular polymeric substances (EPS), composed mainly of heteropolysaccharides, that play a variety of physiological roles, being crucial for cell protection, motility, and biofilm formation. However, due to their complexity, the EPS biosynthetic pathways as well as their assembly and export mechanisms are still far from being fully understood. Here, we show that the absence of a putative EPS-related protein, KpsM (Slr0977), has a pleiotropic effect on Synechocystis sp. strain PCC 6803 physiology, with a strong impact on the export of EPS and carbon fluxes. The kpsM mutant exhibits a significant reduction of released polysaccharides and a smaller decrease of capsular polysaccharides, but it accumulates more polyhydroxybutyrate (PHB) than the wild type. In addition, this strain shows a light/cell density-dependent clumping phenotype and exhibits an altered protein secretion capacity. Furthermore, the most important structural component of pili, the protein PilA, was found to have a modified glycosylation pattern in the mutant compared to the wild type. Proteomic and transcriptomic analyses revealed significant changes in the mechanisms of energy production and conversion, namely, photosynthesis, oxidative phosphorylation, and carbon metabolism, in response to the inactivation of slr0977 Overall, this work shows for the first time that cells with impaired EPS secretion undergo transcriptomic and proteomic adjustments, highlighting the importance of EPS as a major carbon sink in cyanobacteria. The accumulation of PHB in cells of the mutant, without affecting significantly its fitness/growth rate, points to its possible use as a chassis for the production of compounds of interest.IMPORTANCE Most cyanobacteria produce extracellular polymeric substances (EPS) that fulfill different biological roles depending on the strain/environmental conditions. The interest in the cyanobacterial EPS synthesis/export pathways has been increasing, not only to optimize EPS production but also to efficiently redirect carbon flux toward the production of other compounds, allowing the implementation of industrial systems based on cyanobacterial cell factories. Here, we show that a Synechocystis kpsM (slr0977) mutant secretes less EPS than the wild type, accumulating more carbon intracellularly, as polyhydroxybutyrate. Further characterization showed a light/cell density-dependent clumping phenotype, altered protein secretion, and modified glycosylation of PilA. The proteome and transcriptome of the mutant revealed significant changes, namely, in photosynthesis and carbon metabolism. Altogether, this work provides a comprehensive overview of the impact of kpsM disruption on Synechocystis physiology, highlighting the importance of EPS as a carbon sink and showing how cells adapt when their secretion is impaired, and the redirection of the carbon fluxes.

The Two TpsB-Like Proteins in Anabaena sp. Strain PCC 7120 Are Involved in Secretion of Selected Substrates

The outer membrane of Gram-negative bacteria acts as an initial diffusion barrier that shields the cell from the environment. It contains many membrane-embedded proteins required for functionality of this system. These proteins serve as solute and lipid transporters or as machines for membrane insertion or secretion of proteins. The genome of Anabaena sp. strain PCC 7120 codes for two outer membrane transporters termed TpsB1 and TpsB2. They belong to the family of the two-partner secretion system proteins which are characteristic of pathogenic bacteria. Because pathogenicity of Anabaena sp. strain PCC 7120 has not been reported, the function of these two cyanobacterial TpsB proteins was analyzed. TpsB1 is encoded by alr1659, while TpsB2 is encoded by all5116 The latter is part of a genomic region containing 11 genes encoding TpsA-like proteins. However, tpsB2 is transcribed independently of a tpsA gene cluster. Bioinformatics analysis revealed the presence of at least 22 genes in Anabaena sp. strain PCC 7120 putatively coding for substrates of the TpsB system, suggesting a rather global function of the two TpsB proteins. Insertion of a plasmid into each of the two genes resulted in altered outer membrane integrity and antibiotic resistance. In addition, the expression of genes coding for the Clp and Deg proteases is dysregulated in these mutants. Moreover, for two of the putative substrates, a dependence of the secretion on functional TpsB proteins could be confirmed. We confirm the existence of a two-partner secretion system in Anabaena sp. strain PCC 7120 and predict a large pool of putative substrates.IMPORTANCE Cyanobacteria are important organisms for the ecosystem, considering their contribution to carbon fixation and oxygen production, while at the same time some species produce compounds that are toxic to their environment. As a consequence, cyanobacterial overpopulation might negatively impact the diversity of natural communities. Thus, a detailed understanding of cyanobacterial interaction with the environment, including other organisms, is required to define their impact on ecosystems. While two-partner secretion systems in pathogenic bacteria are well known, we provide a first description of the cyanobacterial two-partner secretion system.

Untargeted Lipidomics Analysis of the Cyanobacterium Synechocystis sp. PCC 6803: Lipid Composition Variation in Response to Alternative Cultivation Setups and to Gene Deletion

Cyanobacteria play an important role in several ecological environments, and they are widely accepted to be the ancestors of chloroplasts in modern plants and green algae. Cyanobacteria have become attractive models for metabolic engineering, with the goal of exploring them as microbial cell factories. However, the study of cyanobacterial lipids’ composition and variation, and the assessment of the lipids’ functional and structural roles have been largely overlooked. Here, we aimed at expanding the cyanobacterial lipidomic analytical pipeline by using an untargeted lipidomics approach. Thus, the lipid composition variation of the model cyanobacterium Synechocystis sp. PCC 6803 was investigated in response to both alternative cultivation setups and gene deletion. This approach allowed for detecting differences in total lipid content, alterations in fatty-acid unsaturation level, and adjustments of specific lipid species among the identified lipid classes. The employed method also revealed that the cultivation setup tested in this work induced a deeper alteration of the cyanobacterial cell lipidome than the deletion of a gene that results in a dramatic increase in the release of lipid-rich outer membrane vesicles. This study further highlights how growth conditions must be carefully selected when cyanobacteria are to be engineered and/or scaled-up for lipid or fatty acids production.

Genomic insights into cyanobacterial protein translocation systems

Cyanobacteria are ubiquitous oxygenic photosynthetic bacteria with a versatile metabolism that is highly dependent on effective protein targeting. Protein sorting in diderm bacteria is not trivial and, in cyanobacteria, even less so due to the presence of a complex membrane system: the outer membrane, the plasma membrane and the thylakoid membrane. In cyanobacteria, protein import into the thylakoids is essential for photosynthesis, export to the periplasm fulfills a multifunctional role in maintaining cell homeostasis, and secretion mediates motility, DNA uptake and environmental interactions. Intriguingly, only one set of genes for the general secretory and the twin-arginine translocation pathways seem to be present. However, these systems have to operate in both plasma and thylakoid membranes. This raises the question of how substrates are recognized and targeted to their correct, final destination. Additional complexities arise when a protein has to be secreted across the outer membrane, where very little is known regarding the mechanisms involved. Given their ecological importance and biotechnological interest, a better understanding of protein targeting in cyanobacteria is of great value. This review will provide insights into the known knowns of protein targeting, propose hypotheses based on available genomic sequences and discuss future directions.

Extracellular Vesicles: An Overlooked Secretion System in Cyanobacteria

In bacteria, the active transport of material from the interior to the exterior of the cell, or secretion, represents a very important mechanism of adaptation to the surrounding environment. The secretion of various types of biomolecules is mediated by a series of multiprotein complexes that cross the bacterial membrane(s), each complex dedicated to the secretion of specific substrates. In addition, biological material may also be released from the bacterial cell in the form of vesicles. Extracellular vesicles (EVs) are bilayered, nanoscale structures, derived from the bacterial cell envelope, which contain membrane components as well as soluble products. In cyanobacteria, the knowledge regarding EVs is lagging far behind compared to what is known about, for example, other Gram-negative bacteria. Here, we present a summary of the most important findings regarding EVs in Gram-negative bacteria, discussing aspects of their composition, formation processes and biological roles, and highlighting a number of technological applications tested. This lays the groundwork to raise awareness that the release of EVs by cyanobacteria likely represents an important, and yet highly disregarded, survival strategy. Furthermore, we hope to motivate future studies that can further elucidate the role of EVs in cyanobacterial cell biology and physiology.

Production of glutamate and stereospecific flavors, (S)-linalool and (+)-valencene, by Synechocystis sp. PCC6803

Cyanobacteria can grow photoautotrophically, producing a range of substances by absorbing sunlight and utilizing carbon dioxide, and can potentially be used as industrial microbes that have minimal sugar requirements. To evaluate this potential, we explored the possibility of l-glutamate production using the Synechocystis sp. PCC6803. The ybjL gene encoding the putative l-glutamate exporter from Escherichia coli was introduced, and l-glutamate production reached 2.3 g/L in 143 h (34°C, 100 μmol m^-2 s^-1). Then, we attempted to produce two flavor substances, (S)-linalool, a monoterpene alcohol, and the sesquiterpene (+)-valencene. The Synechocystis sp. PCC6803 strain in which the linalool synthase gene (LINS) from Actinidia arguta (AaLINS) was expressed under control of the tac promoter (GT0846K-P_tac-AaLINS) produced 11.4 mg/L (S)-linalool in 160 h (30°C, 50 μmol m^-2 s^-1). The strain in which AaLINS2 and the mutated farnesyl diphosphate synthase gene ispA∗ (S80F) from E. coli (GT0846K-P_psbA2-AaLINS-ispA∗) were expressed from the P_psbA2 promoter accumulated 11.6 mg/L (S)-linalool in 160 h. Genome analysis revealed that both strains had mutations in slr1270, suggesting that loss of Slr1270 function was necessary for high linalool accumulation. For sesquiterpene production, the valencene synthase gene from Callitropsis nootkatensis and the fernesyl diphosphate synthase (ispA) gene from E. coli were introduced, and the resultant strain produced 9.6 mg/L of (+)-valencene in 166 h (30°C, 50 μmol m^-2 s^-1). This study highlights the production efficiency of engineered cyanobacteria, providing insight into potential industrial applications.

The cyanobacterial phytochrome 2 regulates the expression of motility-related genes through the second messenger cyclic di-GMP

The cyanobacterial phytochrome Cph2 is a light-dependent diguanylate cyclase of the cyanobacterium Synechocystis 6803. Under blue light, Cph2-dependent increase in the cellular c-di-GMP concentration leads to inhibition of surface motility and enhanced flocculation of cells in liquid culture. However, the targets of second messenger signalling in this cyanobacterium and its mechanism of action remained unclear. Here, we determined the cellular concentrations of cAMP and c-di-GMP in wild-type and Δcph2 cells after exposure to blue and green light. Inactivation of cph2 completely abolished the blue-light dependent increase in c-di-GMP content. Therefore, a microarray analysis with blue-light grown wild-type and Δcph2 mutant cells was used to identify c-di-GMP dependent alterations in transcript accumulation. The increase in the c-di-GMP content alters expression of genes encoding putative cell appendages, minor pilins and components of chemotaxis systems. The mRNA encoding the minor pilins pilA5-pilA6 was negatively affected by high c-di-GMP content under blue light, whereas the minor pilin encoding operon pilA9-slr2019 accumulates under these conditions, suggesting opposing functions of the respective gene sets. Artificial overproduction of c-di-GMP leads to similar changes in minor pilin gene expression and supports previous findings that c-di-GMP is important for flocculation via the function of minor pilins. Mutational and gene expression analysis further suggest that SyCRP2, a CRP-like transcription factor, is involved in regulation of minor pilin and putative chaperone usher pili gene expression.

A requirement of TolC1 for effective survival, colonization and pathogenicity of Actinobacillus pleuropneumoniae

To establish infection in the host, pathogens have evolved sophisticated systems to cope with environmental conditions and to protect cells against host immunity. TolC is the outer membrane channel component of type 1 secretion systems and multidrug efflux pumps that plays critical roles during the infection process in many pathogens. However, little is known about the exact roles of TolC1 in the pathogenicity of A. pleuropneumoniae, an etiological agent of the porcine contagious pleuropneumoniae that causes severe respiratory disease. In this study, deletion of tolC1 causes apparent ultrastructural defects in A. pleuropneumoniae cell examined by transmission electron microscopy. The tolC1 mutant is hypersensitivity to oxidative, osmotic and acid challenges by in vitro stress assays. Analysis on secreted proteins shows that the excretion of ApxIIA and an ApxIVA-like protein, ApxIVA-S, is abolished in the absence of TolC1. This result confirms the essential role of TolC1 in the secretion of Apx toxins and this is the first identification of an ApxIVA-like protein in in vitro culture of A. pleuropneumoniae. Besides, disruption of TolC1 leads to a significant attenuation of virulence in mice by an intraperitoneal route of A. pleuropneumoniae. The basis for the attenuation is further investigated using a mouse intranasal infection model, which reveals an impaired ability to colonize and induce lesions in the lungs for the loss of TolC1 of A. pleuropneumoniae. In conclusion, our findings demonstrate significant roles of TolC1 in facilitating bacterial survival in hostile conditions, maximum colonization as well as pathogenicity during the infection of A. pleuropneumoniae.

A microcin processing peptidase-like protein of the cyanobacterium Synechococcus elongatus is essential for secretion of biofilm-promoting proteins

Small secreted compounds, e.g. microcins, are characterized by a double-glycine (GG) secretion motif that is cleaved off upon maturation. Genomic analysis suggests that small proteins that possess a GG motif are widespread in cyanobacteria; however, the roles of these proteins are largely unknown. Using a biofilm-proficient mutant of the cyanobacterium Synechococcus elongatus PCC 7942 in which the constitutive biofilm self-suppression mechanism is inactivated, we previously demonstrated that four small proteins, Enable biofilm formation with a GG motif (EbfG1-4), each with a GG motif, enable biofilm formation. Furthermore, a peptidase belonging to the C39 family, Peptidase transporter enabling Biofilm (PteB), is required for secretion of these proteins. Here, we show that the microcin processing peptidase-like protein encoded by gene Synpcc7942_1127 is also required for biofilm development - inactivation of this gene in the biofilm-proficient mutant abrogates biofilm development. Additionally, this peptidase-like protein (denoted EbfE - enables biofilm formation peptidase) is required for secretion of the EbfG biofilm-promoting small proteins. Given their protein-domain characteristics, we suggest that PteB and EbfE take part in a maturation-secretion system, with PteB being located to the cell membrane while EbfE is directed to the periplasmic space via its secretion signal.

Characterization of ABC transporter genes, sll1180, sll1181, and slr1270, involved in acid stress tolerance of Synechocystis sp. PCC 6803

Over 50 ATP-binding cassette (ABC) transporter-related genes are detected in the Synechocystis sp. PCC 6803 genome by genome sequence analysis. Deletion mutants of other substrate-unknown ABC transporter genes were screened for their acid stress sensitivities in a low-pH medium to identify ABC transporters involved in acid resistance. We found that a mutant of sll1180 encoding proteins with homology to HlyB in Escherichia coli (E.coli) is more sensitive to acid stress than wild-type (WT) cells and analyzed the abundance of expression of the genes in WT cells under acid stress condition by quantitative real-time reverse transcriptase-polymerase chain reaction. sll1180 expression increased in the WT cells after acid stress treatment. Immunofluorescence revealed that Sll1180 localized in the plasma membrane. These results suggest that Sll1180 has an important role in the growth of Synechocystis sp. PCC 6803 under acid stress conditions. HlyB, HlyD, and TolC complex transport HlyA in E.coli; therefore, we searched for genes corresponding to these in Synechocystis sp. PCC 6803. A BlastP search suggested that HlyA, HlyD, and TolC proteins had homology to Sll1951, Sll1181, and Slr1270. Therefore, we constructed deletion mutant of these genes. sll1181 and slr1270 mutant cells revealed acid stress sensitivity. The bacterial two-hybrid analysis showed that Sll1180 interacted with Sll1181 and Sll1951. Dot blot analysis of Sll1951-His revealed that the sll1180 and sll1181 mutant cells did not transport Sll1951-His from the cytoplasm to the extracellular matrix. These results suggest that Sll1180 and Sll1181 transport Sll1951 and that Sll1951-outside of the cells-might be a key factor in acid stress tolerance.

Characterizing active transportation mechanisms for free fatty acids and antibiotics in Synechocystis sp. PCC 6803

BACKGROUND

Synechocystis sp. PCC 6803 is a photosynthetic bacterium that has been genetically modified to produce industrially relevant chemicals, yet efflux mechanisms have not been well elucidated. These photosynthetic organisms live in environments that are often nutrient limited; therefore, the genome of these organisms encodes far fewer proteins used for efflux of chemicals when compared to members of the Enterobacteriaceae family. Understanding efflux mechanisms can lead to a greater efficiency of chemical production within the cyanobacterial cell.

RESULTS

Both sll0180 and slr2131 genes that encode the Sll0180 and Slr2131 proteins, respectively, were removed from Synechocystis sp. PCC 6803 and SD277, a high fatty acid-producing Synechocystis-based strain, to test the hypothesis that Sll0180 and Slr2131 contribute to the efflux of chemicals out of Synechocystis sp. PCC 6803 and SD277. The mutant Synechocystis sp. PCC 6803 and SD277 strains with either sll0180 or slr2131 removed from the chromosome had significantly decreased half maximal inhibitory concentrations to various antibiotics. The free fatty acid (FFA) concentration of the SD277 mutant strains increased intracellularly yet decreased extracellularly indicating that Sll0180 and Slr2131 have a role in FFA efflux. E. coli wild-type gene acrA (a homolog to sll0180) was added on a plasmid to the respective mutant strains lacking the sll0180 gene. Similarly, the E. coli wild-type gene acrB (a homolog to slr2131) was added to the respective mutant strains lacking the slr2131 gene. The tolerance to chloramphenicol of each mutant strain containing the wild-type E. coli gene was restored when compared to the parent stains. The extracellular FFA concentration of SD277 Δslr2131 with E. coli acrB increased significantly compared to both SD277 and SD277 Δslr2131.

CONCLUSIONS

Two proteins involved in the transportation of antibiotics and FFAs out of the Synechocystis sp. PCC 6803 cell were identified. In an effort to alleviate costs associated with mechanically or chemically separating the cells from the FFAs, the combination of genome editing of SD277 and the addition of exogenous transport gene increased extracellular concentrations of FFAs. This understanding of active transportation is critical to improving the production efficiency for all industrially relevant chemicals produced in Synechocystis sp. PCC 6803.

Surface Display of Small Affinity Proteins on Synechocystis sp. Strain PCC 6803 Mediated by Fusion to the Major Type IV Pilin PilA1

Functional surface display of small affinity proteins, namely, affibodies (6.5 kDa), was evaluated for the model cyanobacterium Synechocystis sp. strain PCC 6803 through anchoring to native surface structures. These structures included confirmed or putative subunits of the type IV pili, the S-layer protein, and the heterologous Escherichia coli autotransporter antigen 43 system. The most stable display system was determined to be through C-terminal fusion to PilA1, the major type IV pilus subunit in Synechocystis, in a strain unable to retract these pili (ΔpilT1). Type IV pilus synthesis was upheld, albeit reduced, when fusion proteins were incorporated. However, pilus-mediated functions, such as motility and transformational competency, were negatively affected. Display of affibodies on Synechocystis and the complementary anti-idiotypic affibodies on E. coli or Staphylococcus carnosus was able to mediate interspecies cell-cell binding by affibody complex formation. The same strategy, however, was not able to drive cell-cell binding and aggregation of Synechocystis-only mixtures. Successful affibody tagging of the putative minor pilin PilA4 showed that it locates to the type IV pili in Synechocystis and that its extracellular availability depends on PilA1. In addition, affibody tagging of the S-layer protein indicated that the domains responsible for the anchoring and secretion of this protein are located at the N and C termini, respectively. This study can serve as a basis for future surface display of proteins on Synechocystis for biotechnological applications.IMPORTANCE Cyanobacteria are gaining interest for their potential as autotrophic cell factories. Development of efficient surface display strategies could improve their suitability for large-scale applications by providing options for designed microbial consortia, cell immobilization, and biomass harvesting. Here, surface display of small affinity proteins was realized by fusing them to the major subunit of the native type IV pili in Synechocystis sp. strain PCC 6803. The display of complementary affinity proteins allowed specific cell-cell binding between Synechocystis and Escherichia coli or Staphylococcus carnosus Additionally, successful tagging of the putative pilin PilA4 helped determine its localization to the type IV pili. Analogous tagging of the S-layer protein shed light on the regions involved in its secretion and surface anchoring.

The Cyanobacterium Cylindrospermopsis raciborskii (CYRF-01) Responds to Environmental Stresses with Increased Vesiculation Detected at Single-Cell Resolution

Secretion of membrane-limited vesicles, collectively termed extracellular vesicles (EVs), is an important biological process of both eukaryotic and prokaryotic cells. This process has been observed in bacteria, but remains to be better characterized at high resolution in cyanobacteria. In the present work, we address the release of EVs by Cylindrospermopsis raciborskii (CYRF-01), a filamentous bloom-forming cyanobacterium, exposed to environmental stressors. First, non-axenic cultures of C. raciborskii (CYRF-01) were exposed to ultraviolet radiation (UVA + UVB) over a 6 h period, which is known to induce structural damage to this species. Second, C. raciborskii was co-cultured in interaction with another cyanobacterium species, Microcystis aeruginosa (MIRF-01), over a 24 h period. After the incubation times, cell density and viability were analyzed, and samples were processed for transmission electron microscopy (TEM). Our ultrastructural analyses revealed that C. raciborskii constitutively releases EVs from the outer membrane during its normal growth and amplifies such ability in response to environmental stressors. Both situations induced significant formation of outer membrane vesicles (OMVs) by C. raciborskii compared to control cells. Quantitative TEM revealed an increase of 48% (UV) and 60% (interaction) in the OMV numbers compared to control groups. Considering all groups, the OMVs ranged in size from 20 to 300 nm in diameter, with most OMVs showing diameters between 20 and 140 nm. Additionally, we detected that OMV formation is accompanied by phosphatidylserine exposure, a molecular event also observed in EV-secreting eukaryotic cells. Altogether, we identified for the first time that C. raciborskii has the competence to secrete OMVs and that under different stress situations the genesis of these vesicles is increased. The amplified ability of cyanobacteria to release OMVs may be associated with adaptive responses to changes in environmental conditions and interspecies cell communication.

Outer Membrane Permeability of Cyanobacterium Synechocystis sp. Strain PCC 6803: Studies of Passive Diffusion of Small Organic Nutrients Reveal the Absence of Classical Porins and Intrinsically Low Permeability

The outer membrane of heterotrophic Gram-negative bacteria plays the role of a selective permeability barrier that prevents the influx of toxic compounds while allowing the nonspecific passage of small hydrophilic nutrients through porin channels. Compared with heterotrophic Gram-negative bacteria, the outer membrane properties of cyanobacteria, which are Gram-negative photoautotrophs, are not clearly understood. In this study, using small carbohydrates, amino acids, and inorganic ions as permeation probes, we determined the outer membrane permeability of Synechocystis sp. strain PCC 6803 in intact cells and in proteoliposomes reconstituted with outer membrane proteins. The permeability of this cyanobacterium was >20-fold lower than that of Escherichia coli The predominant outer membrane proteins Slr1841, Slr1908, and Slr0042 were not permeable to organic nutrients and allowed only the passage of inorganic ions. Only the less abundant outer membrane protein Slr1270, a homolog of the E. coli export channel TolC, was permeable to organic solutes. The activity of Slr1270 as a channel was verified in a recombinant Slr1270-producing E. coli outer membrane. The lack of putative porins and the low outer membrane permeability appear to suit the cyanobacterial autotrophic lifestyle; the highly impermeable outer membrane would be advantageous to cellular survival by protecting the cell from toxic compounds, especially when the cellular physiology is not dependent on the uptake of organic nutrients.IMPORTANCE Because the outer membrane of Gram-negative bacteria affects the flux rates for various substances into and out of the cell, its permeability is closely associated with cellular physiology. The outer membrane properties of cyanobacteria, which are photoautotrophic Gram-negative bacteria, are not clearly understood. Here, we examined the outer membrane of Synechocystis sp. strain PCC 6803. We revealed that it is relatively permeable to inorganic ions but is markedly less permeable to organic nutrients, with >20-fold lower permeability than the outer membrane of Escherichia coli Such permeability appears to fit the cyanobacterial lifestyle, in which the diffusion pathway for inorganic solutes may suffice to sustain the autotrophic physiology, illustrating a link between outer membrane permeability and the cellular lifestyle.

Type 4 pili are dispensable for biofilm development in the cyanobacterium Synechococcus elongatus

The hair-like cell appendages denoted as type IV pili are crucial for biofilm formation in diverse eubacteria. The protein complex responsible for type IV pilus assembly is homologous with the type II protein secretion complex. In the cyanobacterium Synechococcus elongatus PCC 7942, the gene Synpcc7942_2071 encodes an ATPase homologue of type II/type IV systems. Here, we report that inactivation of Synpcc7942_2071 strongly affected the suite of proteins present in the extracellular milieu (exo-proteome) and eliminated pili observable by electron microscopy. These results support a role for this gene product in protein secretion as well as in pili formation. As we previously reported, inactivation of Synpcc7942_2071 enables biofilm formation and suppresses the planktonic growth of S. elongatus. Thus, pili are dispensable for biofilm development in this cyanobacterium, in contrast to their biofilm-promoting function in type IV pili-producing heterotrophic bacteria. Nevertheless, pili removal is not required for biofilm formation as evident by a piliated mutant of S. elongatus that develops biofilms. We show that adhesion and timing of biofilm development differ between the piliated and non-piliated strains. The study demonstrates key differences in the process of biofilm formation between cyanobacteria and well-studied type IV pili-producing heterotrophic bacteria.

Extracellular Proteins: Novel Key Components of Metal Resistance in Cyanobacteria?

Metals are essential for all living organisms and required for fundamental biochemical processes. However, when in excess, metals can turn into highly-toxic agents able to disrupt cell membranes, alter enzymatic activities, and damage DNA. Metal concentrations are therefore tightly controlled inside cells, particularly in cyanobacteria. Cyanobacteria are ecologically relevant prokaryotes that perform oxygenic photosynthesis and can be found in many different marine and freshwater ecosystems, including environments contaminated with heavy metals. As their photosynthetic machinery imposes high demands for metals, homeostasis of these micronutrients has been widely studied in cyanobacteria. So far, most studies have focused on how cells are capable of controlling their internal metal pools, with a strong bias toward the analysis of intracellular processes. Ultrastructure, modulation of physiology, dynamic changes in transcription and protein levels have been studied, but what takes place in the extracellular environment when cells are exposed to an unbalanced metal availability remains largely unknown. The interest in studying the subset of proteins present in the extracellular space has only recently begun and the identification and functional analysis of the cyanobacterial exoproteomes are just emerging. Remarkably, metal-related proteins such as the copper-chaperone CopM or the iron-binding protein FutA2 have already been identified outside the cell. With this perspective, we aim to raise the awareness that metal-resistance mechanisms are not yet fully known and hope to motivate future studies assessing the role of extracellular proteins on bacterial metal homeostasis, with a special focus on cyanobacteria.