Alteration of low-temperature susceptibility of the cyanobacterium Synechococcus sp. PCC 7002 by genetic manipulation of membrane lipid unsaturation

Synechococcus elongatus PCC 7942 as a Platform for Bioproduction of Omega-3 Fatty Acids

Alpha-linolenic acid and stearidonic acid are precursors of omega-3 polyunsaturated fatty acids, essential nutrients in the human diet. The ability of cyanobacteria to directly convert atmospheric carbon dioxide into bio-based compounds makes them promising microbial chassis to sustainably produce omega-3 fatty acids. However, their potential in this area remains unexploited, mainly due to important gaps in our knowledge of fatty acid synthesis pathways. To gain insight into the cyanobacterial fatty acid biosynthesis pathways, we analyzed two enzymes involved in the elongation cycle, FabG and FabZ, in Synechococcus elongatus PCC 7942. Overexpression of these two enzymes led to an increase in C18 fatty acids, key intermediates in omega-3 fatty acid production. Nevertheless, coexpression of these enzymes with desaturases DesA and DesB from Synechococcus sp. PCC 7002 did not improve alpha-linolenic acid production, possibly due to their limited role in fatty acid synthesis. In any case, efficient production of stearidonic acid was not achieved by cloning DesD from Synechocystis sp. PCC 6803 in combination with the aforementioned DesA and DesB, reaching maximum production at 48 h post induction. According to current knowledge, this is the first report demonstrating that S. elongatus PCC 7942 can be used as an autotrophic chassis to produce stearidonic acid.

Genetic, Genomics, and Responses to Stresses in Cyanobacteria: Biotechnological Implications

Cyanobacteria are widely-diverse, environmentally crucial photosynthetic prokaryotes of great interests for basic and applied science. Work to date has focused mostly on the three non-nitrogen fixing unicellular species Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002, which have been selected for their genetic and physiological interests summarized in this review. Extensive "omics" data sets have been generated, and genome-scale models (GSM) have been developed for the rational engineering of these cyanobacteria for biotechnological purposes. We presently discuss what should be done to improve our understanding of the genotype-phenotype relationships of these models and generate robust and predictive models of their metabolism. Furthermore, we also emphasize that because Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002 represent only a limited part of the wide biodiversity of cyanobacteria, other species distantly related to these three models, should be studied. Finally, we highlight the need to strengthen the communication between academic researchers, who know well cyanobacteria and can engineer them for biotechnological purposes, but have a limited access to large photobioreactors, and industrial partners who attempt to use natural or engineered cyanobacteria to produce interesting chemicals at reasonable costs, but may lack knowledge on cyanobacterial physiology and metabolism.

Physiological and Molecular Responses to Main Environmental Stressors of Microalgae and Bacteria in Polar Marine Environments

The Arctic and Antarctic regions constitute 14% of the total biosphere. Although they differ in their physiographic characteristics, both are strongly affected by snow and ice cover changes, extreme photoperiods and low temperatures, and are still largely unexplored compared to more accessible sites. This review focuses on microalgae and bacteria from polar marine environments and, in particular, on their physiological and molecular responses to harsh environmental conditions. The data reported in this manuscript show that exposure to cold, increase in CO₂ concentration and salinity, high/low light, and/or combination of stressors induce variations in species abundance and distribution for both polar bacteria and microalgae, as well as changes in growth rate and increase in cryoprotective compounds. The use of -omics techniques also allowed to identify specific gene losses and gains which could have contributed to polar environmental adaptation, and metabolic shifts, especially related to lipid metabolism and defence systems, such as the up-regulation of ice binding proteins, chaperones and antioxidant enzymes. However, this review also provides evidence that -omics resources for polar species are still few and several sequences still have unknown functions, highlighting the need to further explore polar environments, the biology and ecology of the inhabiting bacteria and microalgae, and their interactions.

Engineering the fatty acid synthesis pathway in Synechococcus elongatus PCC 7942 improves omega-3 fatty acid production

BACKGROUND

The microbial production of fatty acids has received great attention in the last few years as feedstock for the production of renewable energy. The main advantage of using cyanobacteria over other organisms is their ability to capture energy from sunlight and to transform CO₂ into products of interest by photosynthesis, such as fatty acids. Fatty acid synthesis is a ubiquitous and well-characterized pathway in most bacteria. However, the activity of the enzymes involved in this pathway in cyanobacteria remains poorly explored.

RESULTS

To characterize the function of some enzymes involved in the saturated fatty acid synthesis in cyanobacteria, we genetically engineered Synechococcus elongatus PCC 7942 by overexpressing or deleting genes encoding enzymes of the fatty acid synthase system and tested the lipid profile of the mutants. These modifications were in turn used to improve alpha-linolenic acid production in this cyanobacterium. The mutant resulting from fabF overexpression and fadD deletion, combined with the overexpression of desA and desB desaturase genes from Synechococcus sp. PCC 7002, produced the highest levels of this omega-3 fatty acid.

CONCLUSIONS

The fatty acid composition of S. elongatus PCC 7942 can be significantly modified by genetically engineering the expression of genes coding for the enzymes involved in the first reactions of fatty acid synthesis pathway. Variations in fatty acid composition of S. elongatus PCC 7942 mutants did not follow the pattern observed in Escherichia coli derivatives. Some of these modifications can be used to improve omega-3 fatty acid production. This work provides new insights into the saturated fatty acid synthesis pathway and new strategies that might be used to manipulate the fatty acid content of cyanobacteria.

Yeasts and moulds contaminants of food ice cubes and their survival in different drinks

AIMS

To evaluate the levels of unicellular and filamentous fungi in ice cubes produced at different levels and to determine their survival in alcoholic beverages and soft drinks.

METHODS AND RESULTS

Sixty samples of ice cubes collected from home level (HL) productions, bars and pubs (BP) and industrial manufacturing plants (MP) were investigated for the presence and cell density of yeasts and moulds. Moulds were detected in almost all samples, while yeasts developed from the majority of HL and MP samples. Representative colonies of microfungi were subjected to phenotypic and genotypic characterization. The identification was carried out by restriction fragment length polymorphism (RFLP) analysis of the region spanning the internal transcribed spacers (ITS1 and ITS2) and the 5·8S rRNA gene. The process of yeast identification was concluded by sequencing the D1/D2 region of the 26S rRNA gene. The fungal biodiversity associated with food ice was represented by nine yeast and nine mould species. Strains belonging to Candida parapsilosis and Cryptococcus curvatus, both opportunistic human pathogens, and Penicillium glabrum, an ubiquitous mould in the ice samples analysed, were selected to evaluate the effectiveness of the ice cubes to transfer pathogenic microfungi to consumers, after addition to alcoholic beverages and soft drinks. All strains retained their viability.

CONCLUSIONS

The survival test indicated that the most common mode of consumption of ice cubes, through its direct addition to drinks and beverages, did not reduce the viability of microfungi.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study evidenced the presence of microfungi in food ice and ascertained their survival in soft drinks and alcoholic beverages.

Streamlining recombination-mediated genetic engineering by validating three neutral integration sites in Synechococcus sp. PCC 7002

Background

Synechococcus sp. PCC 7002 (henceforth Synechococcus) is developing into a powerful synthetic biology chassis. In order to streamline the integration of genes into the Synechococcus chromosome, validation of neutral integration sites with optimization of the DNA transformation protocol parameters is necessary. Availability of BioBrick-compatible integration modules is desirable to further simplifying chromosomal integrations.

Results

We designed three BioBrick-compatible genetic modules, each targeting a separate neutral integration site, A2842, A0935, and A0159, with varying length of homologous region, spanning from 100 to 800 nt. The performance of the different modules for achieving DNA integration were tested. Our results demonstrate that 100 nt homologous regions are sufficient for inserting a 1 kb DNA fragment into the Synechococcus chromosome. By adapting a transformation protocol from a related cyanobacterium, we shortened the transformation procedure for Synechococcus significantly.

Conclusions

The optimized transformation protocol reported in this study provides an efficient way to perform genetic engineering in Synechococcus. We demonstrated that homologous regions of 100 nt are sufficient for inserting a 1 kb DNA fragment into the three tested neutral integration sites. Integration at A2842, A0935 and A0159 results in only a minimal fitness cost for the chassis. This study contributes to developing Synechococcus as the prominent chassis for future synthetic biology applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s13036-017-0061-8) contains supplementary material, which is available to authorized users.

Genetic tools for advancement of Synechococcus sp. PCC 7002 as a cyanobacterial chassis

Background

Successful implementation of modified cyanobacteria as hosts for industrial applications requires the development of a cyanobacterial chassis. The cyanobacterium Synechococcus sp. PCC 7002 embodies key attributes for an industrial host, including a fast growth rate and high salt, light, and temperature tolerances. This study addresses key limitations in the advancement of Synechococcus sp. PCC 7002 as an industrial chassis.

Results

Tools for genome integration were developed and characterized, including several putative neutral sites for genome integration. The minimum homology arm length for genome integration in Synechococcus sp. PCC 7002 was determined to be approximately 250 bp. Three fluorescent protein reporters (hGFP, Ypet, and mOrange) were characterized for gene expression, microscopy, and flow cytometry applications in Synechococcus sp. PCC 7002. Of these three proteins, the yellow fluorescent protein (Ypet) had the best optical properties for minimal interference with the native photosynthetic pigments and for detection using standard microscopy and flow cytometry optics. Twenty-five native promoters were characterized as tools for recombinant gene expression in Synechococcus sp. PCC 7002 based on previous RNA-seq results. This characterization included comparisons of protein and mRNA levels as well as expression under both continuous and diurnal light conditions. Promoters A2520 and A2579 were found to have strong expression in Synechococcus sp. PCC 7002 while promoters A1930, A1961, A2531, and A2813 had moderate expression. Promoters A2520 and A2813 showed more than twofold increases in gene expression under light conditions compared to dark, suggesting these promoters may be useful tools for engineering diurnal regulation.

Conclusions

The genome integration, fluorescent protein, and promoter tools developed in this study will help to advance Synechococcus sp. PCC 7002 as a cyanobacterial chassis. The long minimum homology arm length for Synechococcus sp. PCC 7002 genome integration indicates native exonuclease activity or a low efficiency of homologous recombination. Low correlation between transcript and protein levels in Synechococcus sp. PCC 7002 suggests that transcriptomic data are poor selection criteria for promoter tool development. Lastly, the conventional strategy of using promoters from photosynthetic operons as strong promoter tools is debunked, as promoters from hypothetical proteins (A2520 and A2579) were found to have much higher expression levels.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0584-6) contains supplementary material, which is available to authorized users.

Cloning and functional analysis of three diacylglycerol acyltransferase genes from peanut (Arachis hypogaea L.)

Diacylglycerol acyltransferase (DGAT) catalyzes the final and only committed acylation step in the synthesis of triacylglycerols. In this study, three novel AhDGATs genes were identified and isolated from peanut. Quantitative real-time RT-PCR analysis indicated that the AhDGAT1-2 transcript was more abundant in roots, seeds, and cotyledons, whereas the transcript abundances of AhDGAT1-1 and AhDGAT3-3 were higher in flowers than in the other tissues examined. During seed development, transcript levels of AhDGAT1-1 remained relatively low during the initial developmental stage but increased gradually during later stages, peaking at 50 days after pegging (DAP). Levels of AhDGAT1-2 transcripts were higher at 10 and 60 DAPs and much lower during other stages, whereas AhDGAT3-3 showed higher expression levels at 20 and 50 DAPs. In addition, AhDGAT transcripts were differentially expressed following exposure to abiotic stresses or abscisic acid. The activity of the three AhDGAT genes was confirmed by heterologous expression in a Saccharomyces cerevisiae TAG-deficient quadruple mutant. The recombinant yeasts restored lipid body formation and TAG biosynthesis, and preferentially incorporated unsaturated C18 fatty acids into lipids. The present study provides significant information useful in modifying the oil deposition of peanut through molecular breeding.

A desaturase gene involved in the formation of 1,14-nonadecadiene in Synechococcus sp. strain PCC 7002

The marine cyanobacterium Synechococcus sp. strain PCC 7002 synthesizes two alkenes, 1-nonadecene and 1,14-nonadecadiene. Whereas the genetic basis for the biosynthesis of the terminal double bond in both alkenes has been characterized, the origin of the internal double bond in 1,14-nonadecadiene has not. In this study, we demonstrate that a gene encoding an uncharacterized desaturase is involved in the formation of the internal double bond of 1,14-nonadecadiene. Further, at low temperatures, the desaturase gene is essential for growth, and in wild-type cells the levels of 1,14-nonadecadiene increase relative to that of cells grown at 38°C. These data suggest that 1,14-nonadecadiene plays a role in responding to cold stress.

Improved Free Fatty Acid Production in Cyanobacteria with Synechococcus sp. PCC 7002 as Host

Microbial free fatty acids (FFAs) have been proposed as a potential feedstock for renewable energy. The ability to directly convert carbon dioxide into FFAs makes cyanobacteria ideal hosts for renewable FFA production. Previous metabolic engineering efforts using the cyanobacterial hosts Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942 have demonstrated this direct conversion of carbon dioxide into FFAs; however, FFA yields in these hosts are limited by the negative impact of FFA production on the host cell physiology. This work investigates the use of Synechococcus sp. PCC 7002 as a cyanobacterial host for FFA production. In comparison to S. elongatus PCC 7942, Synechococcus sp. PCC 7002 strains produced and excreted FFAs at similar concentrations but without the detrimental effects on host physiology. The enhanced tolerance to FFA production with Synechococcus sp. PCC 7002 was found to be temperature-dependent, with physiological effects such as reduced photosynthetic yield and decreased photosynthetic pigments observed at higher temperatures. Additional genetic manipulations were targeted for increased FFA production, including thioesterases and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). Overexpression of non-native RuBisCO subunits (rbcLS) from a psbAI promoter resulted in more than a threefold increase in FFA production, with excreted FFA concentrations reaching >130 mg/L. This work illustrates the importance of host strain selection for cyanobacterial biofuel production and demonstrates that the FFA tolerance of Synechococcus sp. PCC 7002 can allow for high yields of excreted FFA.

Comparative transcriptomics of two environmentally relevant cyanobacteria reveals unexpected transcriptome diversity

Prochlorococcus is a genus of abundant and ecologically important marine cyanobacteria. Here, we present a comprehensive comparison of the structure and composition of the transcriptomes of two Prochlorococcus strains, which, despite their similarities, have adapted their gene pool to specific environmental constraints. We present genome-wide maps of transcriptional start sites (TSS) for both organisms, which are representatives of the two most diverse clades within the two major ecotypes adapted to high- and low-light conditions, respectively. Our data suggest antisense transcription for three-quarters of all genes, which is substantially more than that observed in other bacteria. We discovered hundreds of TSS within genes, most notably within 16 of the 29 prochlorosin genes, in strain MIT9313. A direct comparison revealed very little conservation in the location of TSS and the nature of non-coding transcripts between both strains. We detected extremely short 5' untranslated regions with a median length of only 27 and 29 nt for MED4 and MIT9313, respectively, and for 8% of all protein-coding genes the median distance to the start codon is only 10 nt or even shorter. These findings and the absence of an obvious Shine-Dalgarno motif suggest that leaderless translation and ribosomal protein S1-dependent translation constitute alternative mechanisms for translation initiation in Prochlorococcus. We conclude that genome-wide antisense transcription is a major component of the transcriptional output from these relatively small genomes and that a hitherto unrecognized high degree of complexity and variability of gene expression exists in their transcriptional architecture.

Engineering of photosynthetic mannitol biosynthesis from CO2 in a cyanobacterium

D-Mannitol (hereafter denoted mannitol) is used in the medical and food industry and is currently produced commercially by chemical hydrogenation of fructose or by extraction from seaweed. Here, the marine cyanobacterium Synechococcus sp. PCC 7002 was genetically modified to photosynthetically produce mannitol from CO2 as the sole carbon source. Two codon-optimized genes, mannitol-1-phosphate dehydrogenase (mtlD) from Escherichia coli and mannitol-1-phosphatase (mlp) from the protozoan chicken parasite Eimeria tenella, in combination encoding a biosynthetic pathway from fructose-6-phosphate to mannitol, were expressed in the cyanobacterium resulting in accumulation of mannitol in the cells and in the culture medium. The mannitol biosynthetic genes were expressed from a single synthetic operon inserted into the cyanobacterial chromosome by homologous recombination. The mannitol biosynthesis operon was constructed using a novel uracil-specific excision reagent (USER)-based polycistronic expression system characterized by ligase-independent, directional cloning of the protein-encoding genes such that the insertion site was regenerated after each cloning step. Genetic inactivation of glycogen biosynthesis increased the yield of mannitol presumably by redirecting the metabolic flux to mannitol under conditions where glycogen normally accumulates. A total mannitol yield equivalent to 10% of cell dry weight was obtained in cell cultures synthesizing glycogen while the yield increased to 32% of cell dry weight in cell cultures deficient in glycogen synthesis; in both cases about 75% of the mannitol was released from the cells into the culture medium by an unknown mechanism. The highest productivity was obtained in a glycogen synthase deficient culture that after 12 days showed a mannitol concentration of 1.1 g mannitol L(-1) and a production rate of 0.15 g mannitol L(-1) day(-1). This system may be useful for biosynthesis of valuable sugars and sugar derivatives from CO2 in cyanobacteria.

Identification of functional differences in metabolic networks using comparative genomics and constraint-based models

Genome-scale network reconstructions are useful tools for understanding cellular metabolism, and comparisons of such reconstructions can provide insight into metabolic differences between organisms. Recent efforts toward comparing genome-scale models have focused primarily on aligning metabolic networks at the reaction level and then looking at differences and similarities in reaction and gene content. However, these reaction comparison approaches are time-consuming and do not identify the effect network differences have on the functional states of the network. We have developed a bilevel mixed-integer programming approach, CONGA, to identify functional differences between metabolic networks by comparing network reconstructions aligned at the gene level. We first identify orthologous genes across two reconstructions and then use CONGA to identify conditions under which differences in gene content give rise to differences in metabolic capabilities. By seeking genes whose deletion in one or both models disproportionately changes flux through a selected reaction (e.g., growth or by-product secretion) in one model over another, we are able to identify structural metabolic network differences enabling unique metabolic capabilities. Using CONGA, we explore functional differences between two metabolic reconstructions of Escherichia coli and identify a set of reactions responsible for chemical production differences between the two models. We also use this approach to aid in the development of a genome-scale model of Synechococcus sp. PCC 7002. Finally, we propose potential antimicrobial targets in Mycobacterium tuberculosis and Staphylococcus aureus based on differences in their metabolic capabilities. Through these examples, we demonstrate that a gene-centric approach to comparing metabolic networks allows for a rapid comparison of metabolic models at a functional level. Using CONGA, we can identify differences in reaction and gene content which give rise to different functional predictions. Because CONGA provides a general framework, it can be applied to find functional differences across models and biological systems beyond those presented here.

Glutathione protects Lactobacillus sanfranciscensis against freeze-thawing, freeze-drying, and cold treatment

Lactobacillus sanfranciscensis DSM20451 cells containing glutathione (GSH) displayed significantly higher resistance against cold stress induced by freeze-drying, freeze-thawing, and 4 degrees C cold treatment than those without GSH. Cells containing GSH were capable of maintaining their membrane structure intact when exposed to freeze-thawing. In addition, cells containing GSH showed a higher proportion of unsaturated fatty acids in cell membranes upon long-term cold treatment. Subsequent studies revealed that the protective role of GSH against cryodamage of the cell membrane is partly due to preventing peroxidation of membrane fatty acids and protecting Na(+),K(+)-ATPase. Intracellular accumulation of GSH enhanced the survival and the biotechnological performance of L. sanfranciscensis, suggesting that the robustness of starters for sourdough fermentation can be improved by selecting GSH-accumulating strains. Moreover, the results of this study may represent a further example of mechanisms for stress responses in lactic acid bacteria.

Cyanobacterial bioreporters as sensors of nutrient availability

Due to their ubiquity in aquatic environments and their contribution to total biomass, especially in oligotrophic systems, cyanobacteria can be viewed as a proxy for primary productivity in both marine and fresh waters. In this chapter we describe the development and use of picocyanobacterial bioreporters to measure the bioavailability of nutrients that may constrain total photosynthesis in both lacustrine and marine systems. Issues pertaining to bioreporter construction, performance and field applications are discussed. Specifically, luminescent Synechococcus spp. and Synechocystis spp. bioreporters are described that allow the bioavailability of phosphorus, nitrogen and iron to be accurately measured in environmental samples.

Diversity of yeasts from puddles in the vicinity of midre lovénbreen glacier, arctic and bioprospecting for enzymes and fatty acids

A total of 132 yeast strains were characterised from 4 sediment samples collected from small puddles in the vicinity of Midre Lovénbreen glacier, Arctic. Based on the D1/D2 domain sequence similarity, the isolates could be categorised into 6 groups. The nearest phylogenetic neighbour of groups I to VI were identified as Cryptococcus gastricus, Cryptococcus terricolus, Rhodotorula muscorum, Mrakia psychrophila, Mrakia gelida and Rhodotorula glacialis, respectively. Strains representative of the six groups were psychrophilic and salt tolerant but varied in their ability to produce cold-active extracellular enzymes such as lipase, protease, pectinase, cellulase and amylase. C(18:1 (w9C)) and C(18:2 (w9,12C)) were the only two fatty acids common to all the yeasts and branched and (or) unsaturated fatty acids increased in yeasts growing at 8 degrees C compared to 22 degrees C, probably as an adaptation to low temperature. The present study establishes that psychrophilic yeasts are predominant in Arctic and could be used as work horses to produce cold-active enzymes and poly unsaturated fatty acids which have been implicated in low temperature adaptation and also for their use in biotechnology.

Influences of culture temperature on the growth, lipid content and fatty acid composition of Aurantiochytrium sp. Strain mh0186

The growth, lipid content, and fatty acid composition of Aurantiochytrium sp. strain mh0186 at different temperatures were investigated. Strain mh0186 grew well at 15-30 degrees C, but weakly at 10 degrees C. The biomass at 15-30 degrees C was significantly higher than at 10 and 35 degrees C, and the total lipid at 15-35 degrees C was significantly higher than that at 10 degrees C. The amount of DHA in the total fatty acid was highest at 10 degrees C and decreased in response to temperature increase. The content of DHA (mg/g-dry cell weight) at 15-30 degrees C were significantly higher than those at 35 degrees C and those at 15-25 degrees C were significantly higher than those at 10 and 35 degrees C. The DHA yield at 15-35 degrees C was significantly higher than those at 10 and 35 degrees C. Unsaturation of fatty acid was regulated by temperature and was enhanced in response to temperature decrease. The ratio of DHA to DPA varied at different temperatures.

Transcriptional regulation of the CO2-concentrating mechanism in a euryhaline, coastal marine cyanobacterium, Synechococcus sp. Strain PCC 7002: role of NdhR/CcmR

Cyanobacterial photosynthesis occurs in radically diverse habitats and utilizes various forms of a CO(2)-concentrating mechanism (CCM) featuring multiple inorganic carbon (C(i)) transporters. Cyanobacteria from dynamic environments can transform CCM activity depending on C(i) availability, and yet the molecular basis for this regulation is unclear, especially in coastal strains. LysR family transcription factors resembling the Calvin cycle regulator CbbR from proteobacteria have been implicated in the expression of C(i) transporter genes in freshwater cyanobacteria. Our survey of related factors revealed a group of divergent CbbR-like sequences confined to freshwater and coastal or offshore cyanobacteria. Inactivation of the single gene (termed ccmR) from this variable cluster in the euryhaline (coastal) strain Synechococcus sp. strain PCC 7002 led to constitutive expression of a high-affinity CCM. Derepression of HCO(3)(-) transporter gene transcription, including that of BicA, a recently discovered HCO(3)(-) transporter (G. D. Price et al., Proc. Natl. Acad. Sci. USA 101:18228-18233, 2004), was observed. A unique CcmR-regulated operon containing bicA plus 9 open reading frames encoding likely Na(+)/H(+) antiporters from the CPA1 and Mnh families was defined that is essential for maximal HCO(3)(-)-dependent oxygen evolution. The promoter region required for C(i)-regulated transcription of this operon was defined. We propose that CcmR (and its associated regulon) represents a specialization for species inhabiting environments subject to fluctuating C(i) concentrations.

Beyond the lipid hypothesis: mechanisms underlying phenotypic plasticity in inducible cold tolerance

The physiological adjustment of organisms in response to temperature variation is a crucial part of coping with environmental stress. An important component of the cold response is the increase in membrane lipid unsaturation, and this has been linked to an enhanced resistance to the debilitating or lethal effects of cold. Underpinning the lipid response is the upregulation of fatty acid desaturases (des), particularly those introducing double bonds at the 9-10 position of saturated fatty acids. For plants and microbes there is good genetic evidence that regulation of des genes, and the consequent changes in lipid saturation, are causally linked to generation of a cold-tolerant phenotype. In animals, however, supporting evidence is almost entirely limited to correlations of saturation with cold conditions. We describe our recent attempts to provide a direct test of this relationship by genetic manipulation of the nematode Caenorhabditis elegans. We show that this species displays a strong cold tolerant phenotype induced by prior conditioning to cold, and that this is directly linked to upregulated des activity. However, whilst genetic disruption of des activity and lipid unsaturation significantly reduced cold tolerance, animals retained a substantial component of their stress tolerant phenotype produced by cold conditioning. This indicates that mechanisms other than lipid unsaturation play an important role in cold adaptation.

Group 3 sigma factors in the marine cyanobacterium Synechococcus sp. strain PCC 7002 are required for growth at low temperature

Three genes, sigF, sigG and sigH, encoding group 3 sigma factors have been cloned and characterized in the marine cyanobacterium Synechococcus sp. strain PCC 7002. The sigF gene product was similar to sigma factors involved in general stress response and sporulation in other organisms, and the sigG and sigH gene products were similar to extracytoplasmic function (ECF) sigma factors. The sigG and sigH genes were associated with the putative regulatory genes and the sizes of transcripts for sigG and sigH genes were large enough to be cotranscribed with the associated downstream genes. The sigG downstream gene was designated sapG (sigG-associated protein), and yeast two-hybrid analysis demonstrated that SigG and SapG interact when produced in yeast cells. Null mutants of these three group 3 sigma factor genes were created by interposon mutagenesis. The growth of the sigF mutant strain was much slower than the wild-type strain at 15 degrees C, although the growth rates at 22 degrees C and 38 degrees C were identical to those of the wild-type strain. The sigG mutant could not grow continuously at 22 degrees C, and no growth occurred at 15 degrees C. Since SigG and SapG interact in yeast cells and the sigG and sapG mutants showed a similar growth phenotype, SapG is likely to be a regulatory protein for SigG involved in the same pathway in transcriptional regulation in this cyanobacterium.

Fluidization of membrane lipids enhances the tolerance of Saccharomyces cerevisiae to freezing and salt stress

Unsaturated fatty acids play an essential role in the biophysical characteristics of cell membranes and determine the proper function of membrane-attached proteins. Thus, the ability of cells to alter the degree of unsaturation in their membranes is an important factor in cellular acclimatization to environmental conditions. Many eukaryotic organisms can synthesize dienoic fatty acids, but Saccharomyces cerevisiae can introduce only a single double bond at the Delta(9) position. We expressed two sunflower (Helianthus annuus) oleate Delta(12) desaturases encoded by FAD2-1 and FAD2-3 in yeast cells of the wild-type W303-1A strain (trp1) and analyzed their effects on growth and stress tolerance. Production of the heterologous desaturases increased the content of dienoic fatty acids, especially 18:2Delta(9,12), the unsaturation index, and the fluidity of the yeast membrane. The total fatty acid content remained constant, and the level of monounsaturated fatty acids decreased. Growth at 15 degrees C was reduced in the FAD2 strains, probably due to tryptophan auxotrophy, since the trp1 (TRP1) transformants that produced the sunflower desaturases grew as well as the control strain did. Our results suggest that changes in the fluidity of the lipid bilayer affect tryptophan uptake and/or the correct targeting of tryptophan transporters. The expression of the sunflower desaturases, in either Trp(+) or Trp(-) strains, increased NaCl tolerance. Production of dienoic fatty acids increased the tolerance to freezing of wild-type cells preincubated at 30 degrees C or 15 degrees C. Thus, membrane fluidity is an essential determinant of stress resistance in S. cerevisiae, and engineering of membrane lipids has the potential to be a useful tool of increasing the tolerance to freezing in industrial strains.

Luminescent whole-cell cyanobacterial bioreporter for measuring Fe availability in diverse marine environments

A Synechococcus sp. strain PCC 7002 Fe bioreporter was constructed containing the isiAB promoter fused to the Vibrio harveyi luxAB genes. Bioreporter luminescence was characterized with respect to the free ferric ion concentration in trace metal-buffered synthetic medium. The applicability of the Fe bioreporter to assess Fe availability in the natural environment was tested by using samples collected from the Baltic Sea and from the high-nutrient, low-chlorophyll subarctic Pacific Ocean. Parallel assessment of dissolved Fe and bioreporter response confirmed that direct chemical measurements of dissolved Fe should not be considered alone when assessing Fe availability to phytoplankton.

Adaptation and acclimation of photosynthetic microorganisms to permanently cold environments

Persistently cold environments constitute one of our world's largest ecosystems, and microorganisms dominate the biomass and metabolic activity in these extreme environments. The stress of low temperatures on life is exacerbated in organisms that rely on photoautrophic production of organic carbon and energy sources. Phototrophic organisms must coordinate temperature-independent reactions of light absorption and photochemistry with temperature-dependent processes of electron transport and utilization of energy sources through growth and metabolism. Despite this conundrum, phototrophic microorganisms thrive in all cold ecosystems described and (together with chemoautrophs) provide the base of autotrophic production in low-temperature food webs. Psychrophilic (organisms with a requirement for low growth temperatures) and psychrotolerant (organisms tolerant of low growth temperatures) photoautotrophs rely on low-temperature acclimative and adaptive strategies that have been described for other low-temperature-adapted heterotrophic organisms, such as cold-active proteins and maintenance of membrane fluidity. In addition, photoautrophic organisms possess other strategies to balance the absorption of light and the transduction of light energy to stored chemical energy products (NADPH and ATP) with downstream consumption of photosynthetically derived energy products at low temperatures. Lastly, differential adaptive and acclimative mechanisms exist in phototrophic microorganisms residing in low-temperature environments that are exposed to constant low-light environments versus high-light- and high-UV-exposed phototrophic assemblages.

Expression of Delta(12) fatty acid desaturase during the induced accumulation of the antifungal diene in avocado fruits

SUMMARY The preformed (Z,Z)-1-acetoxy-2-hydroxy-4-oxo-heneicosa-12,15-diene (AFD) is the most active antifungal compound in avocado; it affects the quiescence of Colletotrichum gloeosporioides in unripe fruit. One of the genes encoding Delta(12) fatty acid desaturase (avfad12) was hypothesized to take part in the biosynthesis of AFD, and its expression pattern and enzymatic activity were determined in relation to the content of AFD. Using avfad12-3 as a probe, high levels of expression were detected in young fruits and leaves, where the level of AFD was highest. In contrast, Northern analysis of RNA from mature leaves and fruits showed no transcripts from the avfad12 gene family and lower AFD content. The transcripts from the avfad12 gene family, the enzymatic activity of Delta(12) fatty acid desaturase, and the level of AFD in unripe-resistant fruits increased transiently when the fruits were inoculated with C. gloeosporioides or exposed to ethylene (40 microL/L), low temperature (4 degrees C) or 1 mm H(2)O(2), but ripe fruits were not affected. The effect of H(2)O(2) on the transcripts from the avfad12 gene family is of specific importance, because reactive oxygen species were produced by unripe-resistant host fruit soon after inoculation of C. gloeosporioides. In addition, the fungus itself produced H(2)O(2) in culture medium at pH 5.0, which is similar to the pH of unripe-resistant fruit, but not at pH 7.0. Treatments that enhanced Delta(12) fatty acid desaturase activity increased the concentration of the AFD precursor, linoleic acid, and its incorporation into AFD; these treatments also caused a delay in decay development. The present results demonstrate temporal relationships among the transcripts from the avfad12 gene family, the synthesis of the precursor of AFD (linoleic acid), the AFD content and quiescence of C. gloeosporioides in unripe fruits.

A plant for all seasons: alterations in photosynthetic carbon metabolism during cold acclimation in Arabidopsis

Low temperatures lead to the inhibition of sucrose synthesis and photosynthesis. The biochemical and physiological adaptations of plants to low temperatures include the post-translational activation and increased expression of enzymes of the sucrose synthesis pathway, the changed expression of Calvin cycle enzymes, and changes in the leaf protein content. Recent progress has been made in understanding both the signals that trigger these processes and how the regulation of photosynthetic carbon metabolism interacts with other processes during cold acclimation.

Regulation of the desaturation of fatty acids and its role in tolerance to cold and salt stress

The expression of cold-inducible genes is regulated by a two-component system in Synechocystis and Bacillus subtilis. The cold sensors are membrane-bound histidine kinases and it seems likely that they sense and transduce changes in the fluidity of membranes. Desaturation of fatty acids in membrane lipids has been implicated in tolerance to cold and salt stress.

Iron superoxide dismutase protects against chilling damage in the cyanobacterium synechococcus species PCC7942

A strain of Synechococcus sp. PCC7942 lacking functional Fe superoxide dismutase (SOD), designated sodB-, was characterized by its growth rate, photosynthetic pigments, inhibition of photosynthetic electron transport activity, and total SOD activity at 0 degrees C, 10 degrees C, 17 degrees C, and 27 degrees C in moderate light. At 27 degrees C, the sodB- and wild-type strains had similar growth rates, chlorophyll and carotenoid contents, and cyclic photosynthetic electron transport activity. The sodB- strain was more sensitive to chilling stress at 17 degrees C than the wild type, indicating a role for FeSOD in protection against photooxidative damage during moderate chilling in light. However, both the wild-type and sodB- strains exhibited similar chilling damage at 0 degrees C and 10 degrees C, indicating that the FeSOD does not provide protection against severe chilling stress in light. Total SOD activity was lower in the sodB- strain than in the wild type at 17 degrees C and 27 degrees C. Total SOD activity decreased with decreasing temperature in both strains but more so in the wild type. Total SOD activity was equal in the two strains when assayed at 0 degrees C.

Nitrate transport and not photoinhibition limits growth of the freshwater Cyanobacterium synechococcus species PCC 6301 at low temperature

The effect of low temperature on cell growth, photosynthesis, photoinhibition, and nitrate assimilation was examined in the cyanobacterium Synechococcus sp. PCC 6301 to determine the factor that limits growth. Synechococcus sp. PCC 6301 grew exponentially between 20 degreesC and 38 degreesC, the growth rate decreased with decreasing temperature, and growth ceased at 15 degreesC. The rate of photosynthetic oxygen evolution decreased more slowly with temperature than the growth rate, and more than 20% of the activity at 38 degreesC remained at 15 degreesC. Oxygen evolution was rapidly inactivated at high light intensity (3 mE m-2 s-1) at 15 degreesC. Little or no loss of oxygen evolution was observed under the normal light intensity (250 microE m-2 s-1) for growth at 15 degreesC. The decrease in the rate of nitrate consumption by cells as a function of temperature was similar to the decrease in the growth rate. Cells could not actively take up nitrate or nitrite at 15 degreesC, although nitrate reductase and nitrite reductase were still active. These data demonstrate that growth at low temperature is not limited by a decrease in the rate of photosynthetic electron transport or by photoinhibition, but that inactivation of the nitrate/nitrite transporter limits growth at low temperature.

Structure and expression of fatty acid desaturases

Fatty acid desaturases are enzymes that introduce double bonds into fatty acyl chains. They are present in all groups of organisms, i.e., bacteria, fungi, plants and animals, and play a key role in the maintenance of the proper structure and functioning of biological membranes. The desaturases are characterized by the presence of three conserved histidine tracks which are presumed to compose the Fe-binding active centers of the enzymes. Recent findings on the structure and expression of different types of fatty acid desaturase in cyanobacteria, plants and animals are reviewed in this article. Roles of individual desaturases in temperature acclimation and principles of regulation of the desaturase genes are discussed.

Growth on urea can trigger death and peroxidation of the cyanobacterium Synechococcus sp. strain PCC 7002

Laboratory conditions have been identified that cause the rapid death of cultures of cyanobacteria producing urease. Once the death phase had initiated in the stationary growth phase, cells were rapidly bleached of all pigmentation. Null mutations in the ureC gene, encoding the alpha subunit of urease, were constructed, and these mutants were no longer sensitive to growth in the presence of urea. High levels of peroxides, including lipid peroxides, were detected in the bleaching cells. Exogenously added polyunsaturated fatty acids triggered a similar death response. Vitamin E suppressed the formation of peroxides and delayed the onset of cell bleaching. The results suggest that these cyanobacterial cells undergo a metabolic imbalance that ultimately leads to oxidative stress and lipid peroxide formation. These observations may provide insights into the mechanism of sudden cyanobacterial bloom disappearance in nature.