Sustainable citric acid production from CO2 in an engineered cyanobacterium.

Citric acid is one of the most widely used organic acids in the world, with applications ranging from acidity regulation in food and beverages to metal chelation in hydrometallurgical processes. Most of its production is currently derived from fermentative processes, using plant-derived carbon feedstocks. While these are currently dominant, there is an increasing need to develop closed-loop production systems that reduce process carbon footprint. In this work, we demonstrate for the first time that an engineered marine cyanobacterium Synechococcus sp. PCC 7002 can be used as a sustainable chassis for the photosynthetic conversion of CO2 to citric acid. Decreased citric acid cycle flux, through the use of a theophylline-responsive riboswitch, was combined with improved flux through citrate synthase and enhanced citric acid excretion, resulting in a significant improvement to citric acid production. While allowing citrate production, this strategy induces a growth defect which can be overcome by glutamate supplementation or by fine-tuning aconitase levels, resulting in an increase in production relative to WT of over 100-fold. This work represents a first step toward sustainable production of a commodity organic acid from CO2.

Engineering improved ethylene production: Leveraging systems biology and adaptive laboratory evolution.

Ethylene is a small hydrocarbon gas widely used in the chemical industry. Annual worldwide production currently exceeds 150 million tons, producing considerable amounts of CO2 contributing to climate change. The need for a sustainable alternative is therefore imperative. Ethylene is natively produced by several different microorganisms, including Pseudomonas syringae pv. phaseolicola via a process catalyzed by the ethylene-forming enzyme (EFE), subsequent heterologous expression of EFE has led to ethylene production in non-native bacterial hosts including Escherichia coli and cyanobacteria. However, solubility of EFE and substrate availability remain rate-limiting steps in biological ethylene production. We employed a combination of genome-scale metabolic modelling, continuous fermentation, and protein evolution to enable the accelerated development of a high efficiency ethylene producing E. coli strain, yielding a 49-fold increase in production, the most significant improvement reported to date. Furthermore, we have clearly demonstrated that this increased yield resulted from metabolic adaptations that were uniquely linked to EFE (wild type versus mutant). Our findings provide a novel solution to deregulate metabolic bottlenecks in key pathways, which can be readily applied to address other engineering challenges.

Sub-cellular location of FtsH proteases in the cyanobacterium Synechocystis sp. PCC 6803 suggests localised PSII repair zones in the thylakoid membranes.

In cyanobacteria and chloroplasts, exposure to HL damages the photosynthetic apparatus, especially the D1 subunit of Photosystem II. To avoid chronic photoinhibition, a PSII repair cycle operates to replace damaged PSII subunits with newly synthesised versions. To determine the sub-cellular location of this process, we examined the localisation of FtsH metalloproteases, some of which are directly involved in degrading damaged D1. We generated transformants of the cyanobacterium Synechocystis sp. PCC6803 expressing GFP-tagged versions of its four FtsH proteases. The ftsH2-gfp strain was functional for PSII repair under our conditions. Confocal microscopy shows that FtsH1 is mainly in the cytoplasmic membrane, while the remaining FtsH proteins are in patches either in the thylakoid or at the interface between the thylakoid and cytoplasmic membranes. HL exposure which increases the activity of the Photosystem II repair cycle led to no detectable changes in FtsH distribution, with the FtsH2 protease involved in D1 degradation retaining its patchy distribution in the thylakoid membrane. We discuss the possibility that the FtsH2-GFP patches represent Photosystem II 'repair zones' within the thylakoid membranes, and the possible advantages of such functionally specialised membrane zones. Anti-GFP affinity pull-downs provide the first indication of the composition of the putative repair zones.

Localisation and interactions of the Vipp1 protein in cyanobacteria.

The Vipp1 protein is essential in cyanobacteria and chloroplasts for the maintenance of photosynthetic function and thylakoid membrane architecture. To investigate its mode of action we generated strains of the cyanobacteria Synechocystis sp. PCC6803 and Synechococcus sp. PCC7942 in which Vipp1 was tagged with green fluorescent protein at the C-terminus and expressed from the native chromosomal locus. There was little perturbation of function. Live-cell fluorescence imaging shows dramatic relocalisation of Vipp1 under high light. Under low light, Vipp1 is predominantly dispersed in the cytoplasm with occasional concentrations at the outer periphery of the thylakoid membranes. High light induces Vipp1 coalescence into localised puncta within minutes, with net relocation of Vipp1 to the vicinity of the cytoplasmic membrane and the thylakoid membranes. Pull-downs and mass spectrometry identify an extensive collection of proteins that are directly or indirectly associated with Vipp1 only after high-light exposure. These include not only photosynthetic and stress-related proteins but also RNA-processing, translation and protein assembly factors. This suggests that the Vipp1 puncta could be involved in protein assembly. One possibility is that Vipp1 is involved in the formation of stress-induced localised protein assembly centres, enabling enhanced protein synthesis and delivery to membranes under stress conditions.

Solar powered biohydrogen production requires specific localization of the hydrogenase.

Cyanobacteria contain a bidirectional [NiFe] hydrogenase which transiently produces hydrogen upon exposure of anoxic cells to light, potentially acting as a "valve" releasing excess electrons from the electron transport chain. However, its interaction with the photosynthetic electron transport chain remains unclear. By GFP-tagging the HoxF diaphorase subunit we show that the hydrogenase is thylakoid associated, comprising a population dispersed uniformly through the thylakoids and a subpopulation localized to discrete puncta in the distal thylakoid. Thylakoid localisation of both the HoxH and HoxY hydrogenase subunits is confirmed by immunogold electron microscopy. The diaphorase HoxE subunit is essential for recruitment to the dispersed thylakoid population, potentially anchoring the hydrogenase to the membrane, but aggregation to puncta occurs through a distinct HoxE-independent mechanism. Membrane association does not require NDH-1. Localization is dynamic on a scale of minutes, with anoxia and high light inducing a significant redistribution between these populations in favour of puncta. Since HoxE is essential for access to its electron donor, electron supply to the hydrogenase depends on a physiologically controlled localization, potentially offering a new avenue to enhance photosynthetic hydrogen production by exploiting localization/aggregation signals.

2-epi-5-epi-Valiolone synthase activity is essential for maintaining phycobilisome composition in the cyanobacterium Anabaena variabilis ATCC 29413 when grown in the presence of a carbon source.

The cyclase 2-epi-5-epi-valiolone synthase (EVS) is reported to be a key enzyme for biosynthesis of the mycosporine-like amino acid shinorine in the cyanobacterium Anabaena variabilis ATCC 29413. Subsequently, we demonstrated that an in-frame complete deletion of the EVS gene had little effect on in vivo production of shinorine. Complete segregation of the EVS gene deletion mutant proved difficult and was achieved only when the mutant was grown in the dark and in a medium supplemented with fructose. The segregated mutant showed a striking colour change from native blue-green to pale yellow-green, corresponding to substantial loss of the photosynthetic pigment phycocyanin, as evinced by combinations of absorbance and emission spectra. Transcriptional analysis of the mutant grown in the presence of fructose under dark or light conditions revealed downregulation of the cpcA gene that encodes the alpha subunit of phycocyanin, whereas the gene encoding nblA, a protease chaperone essential for phycobilisome degradation, was not expressed. We propose that the substrate of EVS (sedoheptulose 7-phosphate) or possibly lack of its EVS-downstream products, represses transcription of cpcA to exert a hitherto unknown control over photosynthesis in this cyanobacterium. The significance of this finding is enhanced by phylogenetic analyses revealing horizontal gene transfer of the EVS gene of cyanobacteria to fungi and dinoflagellates. It is also conceivable that the EVS gene has been transferred from dinoflagellates, as evident in the host genome of symbiotic corals. A role of EVS in regulating sedoheptulose 7-phosphate concentrations in the photophysiology of coral symbiosis is yet to be determined.

Control of electron transport routes through redox-regulated redistribution of respiratory complexes.

In cyanobacteria, respiratory electron transport takes place in close proximity to photosynthetic electron transport, because the complexes required for both processes are located within the thylakoid membranes. The balance of electron transport routes is crucial for cell physiology, yet the factors that control the predominance of particular pathways are poorly understood. Here we use a combination of tagging with green fluorescent protein and confocal fluorescence microscopy in live cells of the cyanobacterium Synechococcus elongatus PCC 7942 to investigate the distribution on submicron scales of two key respiratory electron donors, type-I NAD(P)H dehydrogenase (NDH-1) and succinate dehydrogenase (SDH). When cells are grown under low light, both complexes are concentrated in discrete patches in the thylakoid membranes, about 100-300 nm in diameter and containing tens to hundreds of complexes. Exposure to moderate light leads to redistribution of both NDH-1 and SDH such that they become evenly distributed within the thylakoid membranes. The effects of electron transport inhibitors indicate that redistribution of respiratory complexes is triggered by changes in the redox state of an electron carrier close to plastoquinone. Redistribution does not depend on de novo protein synthesis, and it is accompanied by a major increase in the probability that respiratory electrons are transferred to photosystem I rather than to a terminal oxidase. These results indicate that the distribution of complexes on the scale of 100-300 nm controls the partitioning of reducing power and that redistribution of electron transport complexes on these scales is a physiological mechanism to regulate the pathways of electron flow.

Loss of the SPHF homologue Slr1768 leads to a catastrophic failure in the maintenance of thylakoid membranes in Synechocystis sp. PCC 6803.

BACKGROUND: In cyanobacteria the photosystems are localised to, and maintained in, specialist membranes called the thylakoids. The mechanism driving the biogenesis of the thylakoid membranes is still an open question, with only two potential biogenesis factors, Vipp1 and Alb3 currently identified. METHODOLOGY/PRINCIPAL FINDINGS: We generated a slr1768 knockout using the pGEM T-easy vector and REDIRECT. By comparing growth and pigment content (chlorophyll a fluoresence) of the Δslr1768 mutant with the wild-type, we found that Δslr1768 has a conditional phenotype; specifically under high light conditions (130 µmol m(-2) s(-1)) thylakoid biogenesis is disrupted leading to cell death on a scale of days. The thylakoids show considerable disruption, with loss of both structure and density, while chlorophyll a density decreases with the loss of thylakoids, although photosynthetic efficiency is unaffected. Under low light (30 µmol m(-2) s(-1)) the phenotype is significantly reduced, with a growth rate similar to the wild-type and only a low frequency of cells with evident thylakoid disruption. CONCLUSIONS/SIGNIFICANCE: This is the first example of a gene that affects the maintenance of the thylakoid membranes specifically under high light, and which displays a phenotype dependent on light intensity. Our results demonstrate that Slr1768 has a leading role in acclimatisation, linking light damage with maintenance of the thylakoids.

Evidence for the intense exchange of MazG in marine cyanophages by horizontal gene transfer.

BACKGROUND: S-PM2 is a phage capable of infecting strains of unicellular cyanobacteria belonging to the genus Synechococcus. S-PM2, like other myoviruses infecting marine cyanobacteria, encodes a number of bacterial-like genes. Amongst these genes is one encoding a MazG homologue that is hypothesized to be involved in the adaption of the infected host for production of progeny phage. METHODOLOGY/PRINCIPAL FINDINGS: This study focuses on establishing the occurrence of mazG homologues in other cyanophages isolated from different oceanic locations. Degenerate PCR primers were designed using the mazG gene of S-PM2. The mazG gene was found to be widely distributed and highly conserved among Synechococcus myoviruses and podoviruses from diverse oceanic provinces. CONCLUSIONS/SIGNIFICANCE: This study provides evidence of a globally connected cyanophage gene pool, the cyanophage mazG gene having a small effective population size indicative of rapid lateral gene transfer despite being present in a substantial fraction of cyanophage. The Prochlorococcus and Synechococcus phage mazG genes do not cluster with the host mazG gene, suggesting that their primary hosts are not the source of the mazG gene.

Genetic dissection of the light-inducible carQRS promoter region of Myxococcus xanthus.

In Myxococcus xanthus photoprotective carotenoids are produced in response to illumination due to regulated expression of carotenoid biosynthesis genes at two loci. Induction of the carotenogenesis regulon is dependent on expression of the carQRS operon. The first gene product of the operon, CarQ, is a sigma factor belonging to the ECF family and is responsible for light-dependent initiation of transcription at the carQRS promoter. We defined the minimal carQRS promoter as a 145-bp fragment of DNA upstream of the carQRS transcriptional start site, which includes the promoter for a divergent gene, gufA. In order to elucidate regions with the promoter required for activity, point mutations were introduced into the carQRS promoter between positions -151 and 6. While most sequence changes abolished carQRS promoter activity, two changes enhanced promoter activity and two changes caused the mutant promoter to become constitutive and independent of CarQ. The promoter-null point mutations and 6-bp deletion mutations implied that the carQRS promoter requires a functional gufA promoter for transcriptional activity and vice versa. By mapping the extent of the promoter region, identifying sequences important for promoter activity, and highlighting potential topological effects, we provide a foundation for further analysis of the carQRS promoter.