Bacteria can anticipate the seasons: photoperiodism in cyanobacteria

Photoperiodic Time Measurement is the ability of plants and animals to measure differences in day/night-length (photoperiod) and use that information to anticipate critical seasonal transformations such as annual temperature cycles. This timekeeping phenomenon triggers adaptive responses in higher organisms such as gonadal growth/regression, flowering, and hibernation. Unexpectedly, we discovered this capability in cyanobacteria, unicellular prokaryotes with generation times of only 5-6 h. Cyanobacteria in short winter-like days develop enhanced resistance to cold that involves desaturation of membrane lipids and differential programs of gene transcription, including stress response pathways. As in eukaryotes, this photoperiodic timekeeping requires an intact circadian clockwork and develops over multiple cycles. Therefore, photoperiodic timekeeping evolved in much simpler organisms than previously appreciated, and involved genetic responses to stresses that recur seasonally.

Transcriptional Mechanisms of Thermal Acclimation in Prochlorococcus

Low temperature limits the growth and the distribution of the key oceanic primary producer Prochlorococcus, which does not proliferate above a latitude of ca. 40°. Yet, the molecular basis of thermal acclimation in this cyanobacterium remains unexplored. We analyzed the transcriptional response of the Prochlorococcus marinus strain MIT9301 in long-term acclimations and in natural Prochlorococcus populations along a temperature range enabling its growth (17 to 30°C). MIT9301 upregulated mechanisms of the global stress response at the temperature minimum (17°C) but maintained the expression levels of genes involved in essential metabolic pathways (e.g., ATP synthesis and carbon fixation) along the whole thermal niche. Notably, the declining growth of MIT9301 from the optimum to the minimum temperature was coincident with a transcriptional suppression of the photosynthetic apparatus and a dampening of its circadian expression patterns, indicating a loss in their regulatory capacity under cold conditions. Under warm conditions, the cellular transcript inventory of MIT9301 was strongly streamlined, which may also induce regulatory imbalances due to stochasticity in gene expression. The daytime transcriptional suppression of photosynthetic genes at low temperature was also observed in metatranscriptomic reads mapping to MIT9301 across the global ocean, implying that this molecular mechanism may be associated with the restricted distribution of Prochlorococcus to temperate zones. IMPORTANCE Prochlorococcus is a major marine primary producer with a global impact on atmospheric CO₂ fixation. This cyanobacterium is widely distributed across the temperate ocean, but virtually absent at latitudes above 40° for yet unknown reasons. Temperature has been suggested as a major limiting factor, but the exact mechanisms behind Prochlorococcus thermal growth restriction remain unexplored. This study brings us closer to understanding how Prochlorococcus functions under challenging temperature conditions, by focusing on its transcriptional response after long-term acclimation from its optimum to its thermal thresholds. Our results show that the drop in Prochlorococcus growth rate under cold conditions was paralleled by a transcriptional suppression of the photosynthetic machinery during daytime and a loss in the organism's regulatory capacity to maintain circadian expression patterns. Notably, warm temperature induced a marked shrinkage of the organism's cellular transcript inventory, which may also induce regulatory imbalances in the future functioning of this cyanobacterium.

All3048, a DnaJ III homolog of Anabaena sp. PCC7120 mediates heat shock response in E. coli and its N-terminus J-domain stimulates DnaK ATPase activity

Cyanobacterial DnaJ offers thermo-tolerance and effectively prevents aggregation of denatured protein in coordination with DnaK. The hypothetical protein All3048 of Anabaena sp. PCC7120 was found to be a 24 kDa DnaJ III protein with a putative J-domain at the extreme N-terminus. This paper decodes the role of All3048 in thermo-tolerance and as a co-chaperon of DnaK. Semi-quantitative and RT-PCR results showed up-accumulation of all3048 in heat, UV-B, cadmium, arsenic and salt. BL21/pET-28a-all3048, all3048(1-95) and all3048(31-128) reduced the heat stress-induced ROS generation by 40 %, 21 % and 24 % as compared to BL21/pET-28-a. Conformational properties of All3048 and its truncated variants were assessed using bis ANS, guanidine hydrochloride and acrylamide quenching. All3048(1-95), All3048 and All3048(31-128) increased DnaK ATPase activity by 8.6, 8.2, and 2.5 fold, respectively. The thermostability investigated using DSC and DSF methods affirmed the relative stability of All3048 and All3048 (31-128), whereas All3048 (1-95) was the least stable. All3048 is a novel cyanobacterial DnaJ III that imparts heat stress tolerance in E. coli; however, only the J-domain present at N-terminus was sufficient for stimulating DnaK's ATPase activity.

DnaK and DnaJ proteins from Hsp70/40 family are involved in Rubisco biosynthesis in Synechocystis sp. PCC6803 and sustain the enzyme assembly in a heterologous system

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the first step of carbon fixation performed by photosynthetic organisms. Form I of this enzyme found in plants and cyanobacteria is composed of eight large (RbcL) and eight small (RbcS) subunits. To form a functional enzyme, Rubisco subunits need to be properly folded, with the assistance of cellular chaperone machinery, and consecutively assembled in a strictly orchestrated manner, with the help of multiple auxiliary factors. In recent years, multiple Rubisco assembly chaperones and their function in enzyme biogenesis have been extensively characterized. Little is known about the potential specialized factors involved in Rubisco subunits folding at the pre-chaperonin stage, yet this knowledge is greatly needed for the fast and efficient testing of new Rubisco variants.Synechococcus sp. PCC 6803 Rubisco shows limited solubility and a lack of assembly in the Escherichia coli expression system. In this study, we aim to identify which additional chaperones are necessary and sufficient in sustaining the heterologous assembly of native Rubisco. Our findings prove that upon the introduction of Synechocystis DnaK2 to the E. coli system, RbcL is produced in soluble form. The addition of specific DnaJ (Sll1384) enhances this effect. We explain these combined effects based on binding constancies, measured for particular partners in vitro, as well as our analysis of the putative tertiary structure of the proteins. Our results have potential implications for Rubisco engineering.

Sustainable Production of Pigments from Cyanobacteria

Pigments are intensely coloured compounds used in many industries to colour other materials. The demand for naturally synthesised pigments is increasing and their production can be incorporated into circular bioeconomy approaches. Natural pigments are produced by bacteria, cyanobacteria, microalgae, macroalgae, plants and animals. There is a huge unexplored biodiversity of prokaryotic cyanobacteria which are microscopic phototrophic microorganisms that have the ability to capture solar energy and CO2 and use it to synthesise a diverse range of sugars, lipids, amino acids and biochemicals including pigments. This makes them attractive for the sustainable production of a wide range of high-value products including industrial chemicals, pharmaceuticals, nutraceuticals and animal-feed supplements. The advantages of cyanobacteria production platforms include comparatively high growth rates, their ability to use freshwater, seawater or brackish water and the ability to cultivate them on non-arable land. The pigments derived from cyanobacteria and microalgae include chlorophylls, carotenoids and phycobiliproteins that have useful properties for advanced technical and commercial products. Development and optimisation of strain-specific pigment-based cultivation strategies support the development of economically feasible pigment biorefinery scenarios with enhanced pigment yields, quality and price. Thus, this chapter discusses the origin, properties, strain selection, production techniques and market opportunities of cyanobacterial pigments.

Growth dynamics and transcriptional responses of a Red Sea Prochlorococcus strain to varying temperatures

Prochlorococcus play a crucial role in the ocean's biogeochemical cycling, but it remains controversial how they will respond to global warming. Here we assessed the response to temperature (22-30°C) of the growth dynamics and gene expression profiles of a Red Sea Prochlorococcus strain (RSP50) in a non-axenic culture. Both the specific growth rate (0.55-0.80 day^-1 ) and cell size (0.04-0.07 μm³ ) of Prochlorococcus increased significantly with temperature. The primary production released extracellularly ranged from 20% to 34%, with humic-like fluorescent compounds increasing up to fivefold as Prochlorococcus reached its maximum abundance. At 30°C, genes involved in carbon fixation such as CsoS2 and CsoS3 and photosynthetic electron transport including PTOX were downregulated, suggesting a cellular homeostasis and energy saving mechanism response. In contrast, PTOX was found upregulated at 22°C and 24°C. Similar results were found for transaldolase, related to carbon metabolism, and citrate synthase, an important enzyme in the TCA cycle. Our data suggest that in spite of the currently warm temperatures of the Red Sea, Prochlorococcus can modulate its gene expression profiles to permit growth at temperatures lower than its optimum temperature (28°C) but is unable to cope with temperatures exceeding 30°C.

Compensatory Transcriptional Response of Fischerella thermalis to Thermal Damage of the Photosynthetic Electron Transfer Chain

Key organisms in the environment, such as oxygenic photosynthetic primary producers (photosynthetic eukaryotes and cyanobacteria), are responsible for fixing most of the carbon globally. However, they are affected by environmental conditions, such as temperature, which in turn affect their distribution. Globally, the cyanobacterium Fischerella thermalis is one of the main primary producers in terrestrial hot springs with thermal gradients up to 60 °C, but the mechanisms by which F. thermalis maintains its photosynthetic activity at these high temperatures are not known. In this study, we used molecular approaches and bioinformatics, in addition to photophysiological analyses, to determine the genetic activity associated with the energy metabolism of F. thermalis both in situ and in high-temperature (40 °C to 65 °C) cultures. Our results show that photosynthesis of F. thermalis decays with temperature, while increased transcriptional activity of genes encoding photosystem II reaction center proteins, such as PsbA (D1), could help overcome thermal damage at up to 60 °C. We observed that F. thermalis tends to lose copies of the standard G4 D1 isoform while maintaining the recently described D1INT isoform, suggesting a preference for photoresistant isoforms in response to the thermal gradient. The transcriptional activity and metabolic characteristics of F. thermalis, as measured by metatranscriptomics, further suggest that carbon metabolism occurs in parallel with photosynthesis, thereby assisting in energy acquisition under high temperatures at which other photosynthetic organisms cannot survive. This study reveals that, to cope with the harsh conditions of hot springs, F. thermalis has several compensatory adaptations, and provides emerging evidence for mixotrophic metabolism as being potentially relevant to the thermotolerance of this species. Ultimately, this work increases our knowledge about thermal adaptation strategies of cyanobacteria.

The success of the bloom-forming cyanobacteria Planktothrix: Genotypes variability supports variable responses to light and temperature stress

Cyanobacterial blooms can modify the dynamic of aquatic ecosystems and have harmful consequences for human activities. Moreover, cyanobacteria can produce a variety of cyanotoxins, including microcystins, but little is known about the role of environmental factors on the prevalence of microcystin producers in the cyanobacterial bloom dynamics. This study aimed to better understand the success of Planktothrix in various environments by unveiling the variety of strategies governing cell responses to sudden changes in light intensity and temperature. The cellular responses (photosynthesis, photoprotection, heat shock response and metabolites synthesis) of four Planktothrix strains to high-light or high-temperature were studied, focusing on how distinct ecotypes (red- or green-pigmented) and microcystin production capability affect cyanobacteria's ability to cope with such abiotic stimuli. Our results showed that high-light and high-temperature impact different cellular processes and that Planktothrix responses are heterogeneous, specific to each strain and thus, to genotype. The ability of cyanobacteria to cope with sudden increase in light intensity and temperature was not related to red- or green-pigmented ecotype or microcystin production capability. According to our results, microcystin producers do not cope better to high-light or high-temperature and microcystin content does not increase in response to such stresses.

Genome Analysis Coupled With Transcriptomics Reveals the Reduced Fitness of a Hot Spring Cyanobacterium Mastigocladus laminosus UU774 Under Exogenous Nitrogen Supplement

The present study focuses on the stress response of a filamentous, AT-rich, heterocystous cyanobacterium Mastigocladus laminosus UU774, isolated from a hot spring, Taptapani, located in the eastern part of India. The genome of UU774 contains an indispensable fragment, scaffold_38, of unknown origin that is implicated during severe nitrogen and nutrition stress. Prolonged exposure to nitrogen compounds during starvation has profound adverse effects on UU774, leading to loss of mobility, loss of ability to fight pathogens, reduced cell division, decreased nitrogen-fixing ability, reduced ability to form biofilms, reduced photosynthetic and light-sensing ability, and reduced production of secreted effectors and chromosomal toxin genes, among others. Among genes showing extreme downregulation when grown in a medium supplemented with nitrogen with the fold change > 5 are transcriptional regulator gene WalR, carbonic anhydrases, RNA Polymerase Sigma F factor, fimbrial protein, and twitching mobility protein. The reduced expression of key enzymes involved in the uptake of phosphate and enzymes protecting oxygen-sensitive nitrogenases is significant during the presence of nitrogen. UU774 is presumed to withstand heat by overexpressing peptidases that may be degrading abnormally folded proteins produced during heat. The absence of a key gene responsible for heterocyst pattern formation, patS, and an aberrant hetN without a functional motif probably lead to the formation of a chaotic heterocyst pattern in UU774. We suggest that UU774 has diverged from Fischerella sp. PCC 9339, another hot spring species isolated in the United States.

C-ferroptosis is an iron-dependent form of regulated cell death in cyanobacteria

Ferroptosis is an oxidative and iron-dependent form of regulated cell death (RCD) recently described in eukaryotic organisms like animals, plants, and parasites. Here, we report that a similar process takes place in the photosynthetic prokaryote Synechocystis sp. PCC 6803 in response to heat stress. After a heat shock, Synechocystis sp. PCC 6803 cells undergo a cell death pathway that can be suppressed by the canonical ferroptosis inhibitors, CPX, vitamin E, Fer-1, liproxstatin-1, glutathione (GSH), or ascorbic acid (AsA). Moreover, as described for eukaryotic ferroptosis, this pathway is characterized by an early depletion of the antioxidants GSH and AsA, and by lipid peroxidation. These results indicate that all of the hallmarks described for eukaryotic ferroptosis are conserved in photosynthetic prokaryotes and suggest that ferroptosis might be an ancient cell death program.

Regulation of antioxidant defense and glyoxalase systems in cyanobacteria

Oxidative stress is common consequence of abiotic stress in plants as well as cyanobacteria caused by generation of reactive oxygen species (ROS), an inevitable product of respiration and photosynthetic electron transport. ROS act as signalling molecule at low concentration however, when its production exceeds the endurance capacity of antioxidative defence system, the organisms suffer oxidative stress. A highly toxic metabolite, methylglyoxal (MG) is also produced in cyanobacteria in response to various abiotic stresses which consequently augment the ensuing oxidative damage. Taking recourse to the common lineage of eukaryotic plants and cyanobacteria, it would be worthwhile to explore the regulatory role of glyoxalase system and antioxidative defense mechanism in combating abiotic stress in cyanobacteria. This review provides comprehensive information on the complete glyoxalase system (GlyI, GlyII and GlyIII) in cyanobacteria. Furthermore, it elucidates the recent understanding regarding the production of ROS and MG, noteworthy link between intracellular MG and ROS and its detoxification via synchronization of antioxidants (enzymatic and non-enzymatic) and glyoxalase systems using glutathione (GSH) as common co-factor.

Description, Taxonomy, and Comparative Genomics of a Novel species, Thermoleptolyngbya sichuanensis sp. nov., Isolated From Hot Springs of Ganzi, Sichuan, China

Thermoleptolyngbya is a newly proposed genus of thermophilic cyanobacteria that are often abundant in thermal environments. However, a vast majority of Thermoleptolyngbya strains were not systematically identified, and genomic features of this genus are also sparse. Here, polyphasic approaches were employed to identify a thermophilic strain, PKUAC-SCTA183 (A183 hereafter), isolated from hot spring Erdaoqiao, Ganzi prefecture, China. Whole-genome sequencing of the strain revealed its allocation to Thermoleptolyngbya sp. and genetic adaptations to the hot spring environment. While the results of 16S rRNA were deemed inconclusive, the more comprehensive polyphasic approach encompassing phenetic, chemotaxic, and genomic approaches strongly suggest that a new taxon, Thermoleptolyngbya sichuanensis sp. nov., should be delineated around the A183 strain. The genome-scale phylogeny and average nucleotide/amino-acid identity confirmed the genetic divergence of the A183 strain from other strains of Thermoleptolyngbya along with traditional methods such as 16S-23S ITS and its secondary structure analyses. Comparative genomic and phylogenomic analyses revealed inconsistent genome structures between Thermoleptolyngbya A183 and O-77 strains. Further gene ontology analysis showed that the unique genes of the two strains were distributed in a wide range of functional categories. In addition, analysis of genes related to thermotolerance, signal transduction, and carbon/nitrogen/sulfur assimilation revealed the ability of this strain to adapt to inhospitable niches in hot springs, and these findings were preliminarily confirmed using experimental, cultivation-based approaches.

Genetic Responses of Metabolically Active Limnospira indica Strain PCC 8005 Exposed to γ-Radiation during Its Lifecycle

Two morphotypes of the cyanobacterial Limnospira indica (formerly Arthrospira sp.) strain PCC 8005, denoted as P2 (straight trichomes) and P6 (helical trichomes), were subjected to chronic gamma radiation from spent nuclear fuel (SNF) rods at a dose rate of ca. 80 Gy·h-1 for one mass doubling period (approximately 3 days) under continuous light with photoautotrophic metabolism fully active. Samples were taken for post-irradiation growth recovery and RNA-Seq transcriptional analysis at time intervals of 15, 40, and 71.5 h corresponding to cumulative doses of ca. 1450, 3200, and 5700 Gy, respectively. Both morphotypes, which were previously reported by us to display different antioxidant capacities and differ at the genomic level in 168 SNPs, 48 indels and 4 large insertions, recovered equally well from 1450 and 3200 Gy. However, while the P2 straight type recovered from 5700 Gy by regaining normal growth within 6 days, the P6 helical type took about 13 days to recover from this dose, indicating differences in their radiation tolerance and response. To investigate these differences, P2 and P6 cells exposed to the intermediate dose of gamma radiation (3200 Gy) were analyzed for differential gene expression by RNA-Seq analysis. Prior to batch normalization, a total of 1553 genes (887 and 666 of P2 and P6, respectively, with 352 genes in common) were selected based on a two-fold change in expression and a false discovery rate FDR smaller or equal to 0.05. About 85% of these 1553 genes encoded products of yet unknown function. Of the 229 remaining genes, 171 had a defined function while 58 genes were transcribed into non-coding RNA including 21 tRNAs (all downregulated). Batch normalization resulted in 660 differentially expressed genes with 98 having a function and 32 encoding RNA. From PCC 8005-P2 and PCC 8005-P6 expression patterns, it emerges that although the cellular routes used by the two substrains to cope with ionizing radiation do overlap to a large extent, both strains displayed a distinct preference of priorities.

Physiological and Proteomic Studies of the Cyanobacterium Anabaena sp. Acclimated to Desiccation Stress

Agricultural productivity is threatened by increasing incidence of drought and the drought tolerant cyanobacteria offer a better solution in the restoration of soil fertility and productivity. The present study describes the comparative physiological response of the cyanobacterium Anabaena sp. acclimated and un-acclimated to desiccation stress induced by polyethylene glycol (10% PEG). While, the acclimated cyanobacterial cells grew luxuriantly with optimal chlorophyll content, photosynthetic activities and nitrogen fixation, the un-acclimated cells exhibited reduced growth rate, chlorophyll content, photosynthetic activities and nitrogen fixation. Distinct differences in the accumulation of lipid peroxidation products, proline and activity of superoxide dismutase were observed under identical growth conditions in the acclimated and un-acclimated cells. Desiccation-acclimated and un-acclimated cyanobacteria showed significant alterations in the abundance of important proteins in the proteome. Two-dimensional gel electrophoresis followed by MALDI-TOF-MS/MS analysis identified twelve proteins. The acclimated cells showed the up regulation of proteins such as Rubisco, fructose-bis-phosphate aldolase, fructose 1-6 bisphosphatase, phosphoglycerate dehydrogenase and elongation factors Tu and Ts as compared to un-acclimated cells. Therefore, the ability to maintain photosynthesis, antioxidants and increased accumulation of proteins related to energy metabolism helped the acclimated cyanobacterium Anabaena sp. to grow optimally under desiccation stress conditions.

Structure, function, and substrates of Clp AAA+ protease systems in cyanobacteria, plastids, and apicoplasts: A comparative analysis

ATPases Associated with diverse cellular Activities (AAA+) are a superfamily of proteins that typically assemble into hexameric rings. These proteins contain AAA+ domains with two canonical motifs (Walker A and B) that bind and hydrolyze ATP, allowing them to perform a wide variety of different functions. For example, AAA+ proteins play a prominent role in cellular proteostasis by controlling biogenesis, folding, trafficking, and degradation of proteins present within the cell. Several central proteolytic systems (e.g., Clp, Deg, FtsH, Lon, 26S proteasome) use AAA+ domains or AAA+ proteins to unfold protein substrates (using energy from ATP hydrolysis) to make them accessible for degradation. This allows AAA+ protease systems to degrade aggregates and large proteins, as well as smaller proteins, and feed them as linearized molecules into a protease chamber. This review provides an up-to-date and a comparative overview of the essential Clp AAA+ protease systems in Cyanobacteria (e.g., Synechocystis spp), plastids of photosynthetic eukaryotes (e.g., Arabidopsis, Chlamydomonas), and apicoplasts in the nonphotosynthetic apicomplexan pathogen Plasmodium falciparum. Recent progress and breakthroughs in identifying Clp protease structures, substrates, substrate adaptors (e.g., NblA/B, ClpS, ClpF), and degrons are highlighted. We comment on the physiological importance of Clp activity, including plastid biogenesis, proteostasis, the chloroplast Protein Unfolding Response, and metabolism, across these diverse lineages. Outstanding questions as well as research opportunities and priorities to better understand the essential role of Clp systems in cellular proteostasis are discussed.

DnaK3 Is Involved in Biogenesis and/or Maintenance of Thylakoid Membrane Protein Complexes in the Cyanobacterium Synechocystis sp. PCC 6803

DnaK3, a highly conserved cyanobacterial chaperone of the Hsp70 family, binds to cyanobacterial thylakoid membranes, and an involvement of DnaK3 in the biogenesis of thylakoid membranes has been suggested. As shown here, light triggers synthesis of DnaK3 in the cyanobacterium Synechocystis sp. PCC 6803, which links DnaK3 to the biogenesis of thylakoid membranes and to photosynthetic processes. In a DnaK3 depleted strain, the photosystem content is reduced and the photosystem II activity is impaired, whereas photosystem I is regular active. An impact of DnaK3 on the activity of other thylakoid membrane complexes involved in electron transfer is indicated. In conclusion, DnaK3 is a versatile chaperone required for biogenesis and/or maintenance of thylakoid membrane-localized protein complexes involved in electron transfer reactions. As mentioned above, Hsp70 proteins are involved in photoprotection and repair of PS II in chloroplasts.

Regulatory Diversity and Functional Analysis of Two-Component Systems in Cyanobacterium Synechocystis sp. PCC 6803 by GC-MS Based Metabolomics

Two-component signal transduction systems are still poorly functionally characterized in the model cyanobacterium Synechocystis sp. PCC 6803. To address the issue, a GC-MS based comparative metabolomic analysis was conducted on a library of 44 knockout mutants for the response regulators (RRs) in Synechocystis. The metabolomic profiling analysis showed that 7 RRs mutants, namely Δslr1909, Δsll1291, Δslr6040, Δsll1330, Δslr2024, Δslr1584, and Δslr1693, were significantly different at metabolomic level, although their growth patterns are similar to the wild type under the normal autotrophic growth condition, suggesting regulatory diversity of RRs at metabolite level in Synechocystis. Additionally, a detailed metabolomic analysis coupled with RT-PCR verification led to useful clues for possible function of these 7 RRs, which were found involved in regulation of multiple aspects of cellular metabolisms in Synechocystis. Moreover, an integrative metabolomic and evolutionary analysis of all RR showed that four groups of RR genes clustered together in both metabolomic and evolutionary trees, suggesting of possible functional conservation of these RRs during the evolutionary process. Meanwhile, six groups of RRs with close evolutionary origin were found with different metabolomic profiles, suggesting possible functional changes during evolution. In contrast, more than 10 groups of RR genes with different clustering patterns in the evolutionary tree were found clustered together in metabolomics-based tree, suggesting possible functional convergences during the evolution. This study provided a metabolomic view of RR function, and the most needed functional clues for further characterization of these regulatory proteins in Synechocystis.

Proteomic De-Regulation in Cyanobacteria in Response to Abiotic Stresses

Cyanobacteria are oxygenic photoautotrophs, exhibiting a cosmopolitan distribution in almost all possible environments and are significantly responsible for half of the global net primary productivity. They are well adapted to the diverse environments including harsh conditions by evolving a range of fascinating repertoires of unique biomolecules and secondary metabolites to support their growth and survival. These phototrophs are proved as excellent models for unraveling the mysteries of basic biochemical and physiological processes taking place in higher plants. Several known species of cyanobacteria have tremendous biotechnological applications in diverse fields such as biofuels, biopolymers, secondary metabolites and much more. Due to their potential biotechnological and commercial applications in various fields, there is an imperative need to engineer robust cyanobacteria in such a way that they can tolerate and acclimatize to ever-changing environmental conditions. Adaptations to stress are mainly governed by a precise gene regulation pathways resulting in the expression of novel protein/enzymes and metabolites. Despite the demand, till date few proteins/enzymes have been identified which play a potential role in improving tolerance against abiotic stresses. Therefore, it is utmost important to study environmental stress responses related to post-genomic investigations, including proteomic changes employing advanced proteomics, synthetic and structural biology workflows. In this respect, the study of stress proteomics offers exclusive advantages to scientists working on these aspects. Advancements on these fields could be helpful in dissecting, characterization and manipulation of physiological and metabolic systems of cyanobacteria to understand the stress induced proteomic responses. Till date, it remains ambiguous how cyanobacteria perceive changes in the ambient environment that lead to the stress-induced proteins thus metabolic deregulation. This review briefly describes the current major findings in the fields of proteome research on the cyanobacteria under various abiotic stresses. These findings may improve and advance the information on the role of different class of proteins associated with the mechanism(s) of stress mitigation in cyanobacteria under harsh environmental conditions.

Role of Ca2+ as protectant under heat stress by regulation of photosynthesis and membrane saturation in Anabaena PCC 7120

The present study was aimed at understanding the effects of heat stress on selected physiological and biochemical parameters of a model cyanobacterium, Anabaena PCC 7120 in addition to amelioration strategy using exogenous Ca²⁺. A comparison of the cells exposed to heat stress (0-24 h) in the presence or absence of Ca²⁺ clearly showed reduction in colony-forming ability and increase in reactive oxygen species (ROS) leading to loss in the viability of cells of Ca²⁺-deficient cultures. There was higher level of saturation in membrane lipids of the cells supplemented with Ca²⁺ along with higher accumulation of proline. Similarly, higher quantum yield (7.8-fold) in Ca²⁺-supplemented cultures indicated role of Ca²⁺ in regulation of photosynthesis. Relative electron transport rate (rETR) decreased in both the sets with the difference in the rate of decrease (slow) in Ca²⁺-supplemented cultures. The Ca²⁺-supplemented sets also maintained high levels of open reaction centers of PS II in comparison to Ca²⁺-deprived cells. Increase in transcripts of both subunits ((rbcL and rbcS) of RubisCO from Ca²⁺-supplemented Anabaena cultures pointed out the role of Ca²⁺ in sustenance of photosynthesis of cells via CO₂ fixation, thus, playing an important role in maintaining metabolic status of the heat-stressed cyanobacterium.

Nostoc entophytum cell response to cadmium exposure: A possible role of chaperon proteins GroEl and HtpG in cadmium-induced stress

The present study is pursuing our previous research, focused on some aspects of Nostoc entophytum ISC32 cell response to the stress caused by exposure to cadmium at the cellular and molecular levels. Variations in the antioxidant system (catalase and ascorbate peroxidase activity) of N. entophytum ISC32 exposed to varying concentrations of Cd (2, and 5 mg/L) resulted in a significant increase in the activity of both catalase and peroxidase. Activity of these enzymes was, however, not significantly changed in the presence of Cd concentrations of 10 and 20 mg/L. Levels of lipid peroxidation, as measured by malondialdehyde (MDA) assay, were observed in response to exposure to Cd (20 mg/L). There was, however, a sharp drop in both antioxidant and lipid peroxidation activities of Cd treated cells after 5 days exposure, likely in consequence of cellular damage. The content of chlorophyll a and phycobiliproteins of living cells were altered under Cd-induced conditions. TEM images of cyanobacterial cells treated with Cd showed cell surface alteration and modification along with altered cellular microcompartments. Cyanobacterial cells treated with Cd at concentrations below the minimum inhibitory concentration (MIC) remained with no apparent structural changes. However, at a higher concentration of Cd (30 mg/L), a clear detachment effect was observed between the mucilage external layer and cell membrane which may be attributed to cell plasmolysis due to toxic effects of Cd. Subsequently, the thickness of the ring-shaped mucilage external layer increased likely as a result of the cell defense mechanisms against toxic concentrations of Cd. Characterization of cells treated with Cd (30 and 150 mg/L) by scanning electron microscopy (SEM) indicated cell shrinkage with varying degrees of distortion and surface wrinkling. Energy-dispersive X-ray spectrometry (EDS) analysis suggested that Cd was not present as nanoparticles within the cell, but in the form of salt or other molecular structures. The up-regulation of chaperons was confirmed for GroEL and HtpG using real-time PCR and northern blot analyses. Interestingly, the expression of GroEL was markedly increased at lower Cd concentration (5 mg/L). However, the ISC32 strain accrued higher levels of HtpG transcript in response to an elevated concentration of Cd (15 mg/L). This pattern seems to be related to the fast and early induction of GroEL, which may be necessary for induction of other factors and heat shock proteins such as HtpG in Cd-treated Nostoc cells. The result of this study paves the way for a more detailed exploration of Cd effects on the defense mechanisms of cyanobacteria. Our research also shed some light on how cyanobacterial cells have evolved to respond to the heavy metal toxicity at the cellular, molecular and ultrastructural levels.

Honoring eight senior distinguished plant biologists from India

We summarize here research contributions of eight stalwarts in photosynthesis research from India. These distinguished scientists (Shree Kumar Apte, Basanti Biswal, Udaya C. Biswal, Agepati S. Raghavendra, Attipalli Ramachandra Reddy, Prafullachandra Vishnu (Raj) Sane, Baishnab Charan Tripathy, and Dinesh C. Uprety) were honored on November 2, 2017, at the School of Life Sciences, University of Hyderabad. We include here two group photographs of this special event, which was organized by the Department of Plant Sciences, during the 8th International Conference on Photosynthesis and Hydrogen Energy Research for Sustainability-2017 ( https://prs.science/wp-content/uploads/2017/10/Photosynthesis-Research-for-Sustainability-2017.pdf , also available at: http://www.life.illinois.edu/govindjee/world-historical.html ). The main conference had honored three international scientists: William Cramer (Purdue University. West Lafayette, Indiana, USA), Govindjee (University of Illinois at Urbana-Champaign, Illinois, USA, one of the authors here); and Agepati S. Raghavendra (University of Hyderabad, India, one of those honored here as well); see papers in this Special Issue, edited by Suleyman Allakhverdiev, one of the authors here.

Desiccation and radiation stress tolerance in cyanobacteria

Cyanobacteria are among the oldest living organisms on this planet, existing since more than 3 billion years. They are ideal organisms for investigating biological processes such as photosynthesis, respiration, circadian rhythm, photoregulation of gene expression, developmental gene rearrangements, and specialized cell differentiation. They are nearly ubiquitous in distribution, have colonized a wide range of ecosystems including soil, air, dry rock, and aquatic systems, and even occupy extreme niches that are inaccessible to other organisms. Such wide ecological distribution reflects their capacity to acclimate to extreme environments. They show great adaptive abilities and have survived various adverse physiological growth conditions like desiccation, high temperatures, extreme pH, cold, osmosis, salt, light, nitrogen, and high salinity. Their ancient origin and surviving through numerous stresses during evolution indicates their remarkable capabilities to survive and prevail under different environmental and man-made stresses. It has been hypothesized that similar and overlap stress response mechanisms help them to survive different stresses. It has been stated that responses against stresses like radiation has been accidental-exhibited because of similar response against desiccation stress, which has prevailed more during evolution. These overlaps and similarities in stress responses have been instrumental in making these organisms a large class of biological entities today. Present review discuss about stress tolerance in cyanobacteria against two extreme stresses - desiccation and gamma radiation. It also discuss the commonality and underlying molecular mechanisms in these two stress responses.

Phenotypic and genotypic characterisation of an unique indigenous hypersaline unicellular cyanobacterium, Euhalothece sp.nov

A novel halotolerant species of cyanobacterium of the order Chroococcales was isolated from hypersaline estuary in Kwa-Zulu Natal, South Africa. A comprehensive polyphasic approach viz., cell morphology, pigment composition and complete genome sequence analysis was conducted to elucidate the taxonomic position of the isolated strain. The blue-green oval to rod-shaped cells were 14-18 μm in size, and contained a high amount of phycocyanin pigments. The strain was moderate thermotolerant/alkalitolerant halophile with the optimum conditions for growth at 35 °C, pH 8.5 and 120 g/l of NaCl. Based on 16S rRNA gene sequence phylogeny, the strain was related to members of the 'Euhalothece' subcluster (99%). The whole genome sequence was determined, and the annotated genes showed a 90% sequence similarity to the gas-vacuolate, spindle-shaped Dactylococcopsis salina PCC 8305. The size of the genome was determined to be 5,113,178 bp and contained 4332 protein-coding genes and 69 RNA genes with a G + C content of 46.7%. Genes encoding osmoregulation, oxidative stress, heat shock, persister cells, and UV-absorbing secondary metabolites, among others, were identified. Based on the phylogenetic analysis of the 16S rRNA gene sequences, physiological data, pigment compositions and genomic data, the strain is considered to represent a novel species of Euhalothece.

Niche separation of Baltic Sea cyanobacteria during bloom events by species interactions and autecological preferences

Cyanobacterial blooms regularly occur in the Baltic Sea during the summer months, with filamentous, heterocystous Nodularia spumigena and Dolichospermum sp. and the coccoid picocyanobacterium Synechococcus spp. as important species. Under calm conditions, N. spumigena accumulate at the surface, whereas Dolichospermum sp. and Synechococcus sp. remain at the subsurface, in the upper water layer. This vertical separation allows co-occurring species to compete for the same resources. The factors that determine the vertical distribution within blooms, however, are mostly unknown. The present study examined the growth rates of these three cyanobacterial species in a two-factorial experiment, with temperature (16 and 24 °C) and radiation (38 and 150 μmol photons m^-2 s^-1) conditions mimicking those at the water surface and directly below. To determine whether interactions between the three species influenced their growth rates (and therewith also their vertical distribution), paired and multi-species cultures were established. In the single-species cultures, the autecological preferences of the species matched the assumed natural occurrence in bloom events: N. spumigena grew best under high and Dolichospermum sp. and Synechococcus sp. under low light conditions (maximum growth rates at the preferred conditions: μ = 0.48 ± 0.017, 0.88 ± 0.092, and 0.67 ± 0.012, respectively). High temperatures were tolerated by N. spumigena, but inhibited the growth of Dolichospermum sp. and Synechococcus sp. These results suggested that the factors resulting in the vertical separation of species within a bloom are species-specific: N. spumigena responded predominantly to irradiance and only slightly to temperature, Dolichospermum sp. was intensely affected by temperature and less by irradiance, and Synechococcus sp. responded more strongly to irradiance than to temperature. The interactions in paired and multi-species cultures revealed beneficial and detrimental effects, depending on species composition and abiotic conditions. Under the environmental conditions in which the three species occur, however, similar interactions resulted in no or only slight inhibition. Our observations demonstrate how autecological preferences together with the avoidance of negative interactions determine the vertical distribution of cyanobacteria during bloom events in the Baltic Sea.

Regulation of heat-shock genes in bacteria: from signal sensing to gene expression output

The heat-shock response is a mechanism of cellular protection against sudden adverse environmental growth conditions and results in the prompt production of various heat-shock proteins. In bacteria, specific sensory biomolecules sense temperature fluctuations and transduce intercellular signals that coordinate gene expression outputs. Sensory biomolecules, also known as thermosensors, include nucleic acids (DNA or RNA) and proteins. Once a stress signal is perceived, it is transduced to invoke specific molecular mechanisms controlling transcription of genes coding for heat-shock proteins. Transcriptional regulation of heat-shock genes can be under either positive or negative control mediated by dedicated regulatory proteins. Positive regulation exploits specific alternative sigma factors to redirect the RNA polymerase enzyme to a subset of selected promoters, while negative regulation is mediated by transcriptional repressors. Interestingly, while various bacteria adopt either exclusively positive or negative mechanisms, in some microorganisms these two opposite strategies coexist, establishing complex networks regulating heat-shock genes. Here, we comprehensively summarize molecular mechanisms that microorganisms have adopted to finely control transcription of heat-shock genes.

Quantitative proteomics analysis by iTRAQ revealed underlying changes in thermotolerance of Arthrospira platensis

Growth temperature is a critical factor that affects cultivation of Arthrospira platensis which is a type of cyanobacterium widely known as Spirulina that has significant commercial value. To investigate the molecular mechanism underlying the thermotolerance of Spirulina, differential protein expression profiling was carried out using iTRAQ-based quantitative proteomic analysis. This study only analyzed changes in thylakoids. Among the 2085 proteins quantified, 43 differentially expressed proteins were selected based on the fold change cutoff scores of ≥2 or ≤0.5 for up-regulation or down-regulation, respectively. An analysis of these 43 proteins found that 23% of them are photosynthetic system proteins which include photosynthetic enzymes and pigment proteins. The dynamic change of these proteins indicates that photosynthetic system functions were profoundly affected under heat stress and the light-dependent reactions were probably the most sensitive to temperature changes. Meanwhile, to cope with the low energy production due to impaired photosynthesis there was a considerable down-shift in protein synthesis which is a very energy demanding process. The impaired photosynthesis led to low energy generation that was compensated by a down-shift in translation (the most energy-demanding process) and an up-shift of glycolysis. The reduction of many ribosome proteins may lead to a loss in translation efficiency; therefore, Spirulina may adopted a different mechanism to increase translational elongation under heat stress to compensate for this loss, such as elevate L7/L12 proteins. Changes were also found in the classical heat shock proteins, the ROS scavenging system, DNA-binding proteins, and some membrane proteins. In conclusion, this research demonstrate that heat stress induces profound changes in cellular physiology and shed light on the mechanism of the heat stress response and thermotolerance of Arthrospira platensis.

BIOLOGICAL SIGNIFICANCE

Arthrospira platensis, widely known as Spirulina, is a type of cyanobacteria which is cultivated at large scale for it great commercial value. It has become a consensus that continually increasing temperature due to global warming is bringing serious threat to agriculture, including the Spirulina cultivation. High temperature not only limits biomass accumulation by Spirulina, but also changes the composition of nutrition. Therefore there is a greater need than ever before to understand how Spirulina tolerates and cope with high temperature. In this study, we for the first time applied the iTRAQ-based quantitative proteomic technology to investigate the thermotolerance of Spirulina. Our results showed that many biological processes were altered by heat stress. Most significantly, we found that heat stress harmed the photosynthesis ability and caused low energy production, and to deal with this situation, energy demanding processes like protein synthesis were down-shifted and the alternative energy metabolism process glycolysis was up-shifted. Our results also show other important proteins, like the classical heat shock proteins and some antioxidant proteins, are also increased. Thus our study sheds light to our understanding of the mechanism underlying the thermotolerance of Spirulina.

Conserved two-component Hik34-Rre1 module directly activates heat-stress inducible transcription of major chaperone and other genes in Synechococcus elongatus PCC 7942

Bacteria and other organisms, including cyanobacteria, employ two-component signal transducing modules comprising histidine kinases and response regulators to acclimate to changing environments. While the number and composition of these modules differ among cyanobacteria, two response regulators that contain DNA binding domains, RpaB and Rre1, are conserved in all sequenced cyanobacterial genomes and are essential for viability. Although RpaB negatively or positively regulates high light and other stress-responsive gene expression, little is known about the function of Rre1. Here, they investigated the direct regulatory targets of Rre1 in the cyanobacterium Synechococcus elongatus PCC 7942. Chromatin immunoprecipitation and high-density tiling array analysis were used to map Rre1 binding sites. The sites included promoter regions for chaperone genes such as dnaK2, groESL-1, groEL-2, hspA and htpG, as well as the group 2 sigma factor gene rpoD2. In vivo and in vitro analyses revealed that Rre1 phosphorylation level, DNA binding activity and adjacent gene transcription increased in response to heat stress. These responses were much diminished in a knock-out mutant of Hik34, a previously identified heat shock regulator. Based on our results, we propose Hik34-Rre1 is the heat shock-responsive signaling module that positively regulates major chaperone and other genes in cyanobacteria.

Metaproteomics reveals metabolic transitions between healthy and diseased stony coral Mussismilia braziliensis

Infectious diseases such as white plague syndrome (WPS) and black band disease (BBD) have caused massive coral loss worldwide. We performed a metaproteomic study on the Abrolhos coral Mussismilia braziliensis to define the types of proteins expressed in healthy corals compared to WPS- and BBD-affected corals. A total of 6363 MS/MS spectra were identified as 361 different proteins. Healthy corals had a set of proteins that may be considered markers of holobiont homoeostasis, including tubulin, histone, Rab family, ribosomal, peridinin-chlorophyll a-binding protein, F0F1-type ATP synthase, alpha-iG protein, calmodulin and ADP-ribosylation factor. Cnidaria proteins found in healthy M. braziliensis were associated with Cnidaria-Symbiodinium endosymbiosis and included chaperones (hsp70, hsp90 and calreticulin), structural and membrane modelling proteins (actin) and proteins with functions related to intracellular vesicular traffic (Rab7 and ADP-ribosylation factor 1) and signal transduction (14-3-3 protein and calmodulin). WPS resulted in a clear shift in the predominance of proteins, from those related to aerobic nitrogen-fixing bacteria (i.e. Rhizobiales, Sphingomonadales and Actinomycetales) in healthy corals to those produced by facultative/anaerobic sulphate-reducing bacteria (i.e. Enterobacteriales, Alteromonadales, Clostridiales and Bacteroidetes) in WPS corals. BBD corals developed a diverse community dominated by cyanobacteria and sulphur cycle bacteria. Hsp60, hsp90 and adenosylhomocysteinase proteins were produced mainly by cyanobacteria in BBD corals, which is consistent with elevated oxidative stress in hydrogen sulphide- and cyanotoxin-rich environments. This study demonstrates the usefulness of metaproteomics for gaining better comprehension of coral metabolic status in health and disease, especially in reef systems such as the Abrolhos that are suffering from the increase in global and local threatening events.

NADPH-Thioredoxin Reductase C Mediates the Response to Oxidative Stress and Thermotolerance in the Cyanobacterium Anabaena sp. PCC7120

NADPH-thioredoxin reductase C (NTRC) is a bimodular enzyme composed of an NADPH-thioredoxin reductase and a thiioredoxin domain extension in the same protein. In plants, NTRC has been described to be involved in the protection of the chloroplast against oxidative stress damage through reduction of the 2-Cys peroxiredoxin (2-Cys Prx) as well as through other functions related to redox enzyme regulation. In cyanobacteria, the Anabaena NTRC has been characterized in vitro, however, nothing was known about its in vivo function. In order to study that, we have generated the first knockout mutant strain (ΔntrC), apart from the previously described in Arabidopsis. Detailed characterization of this strain reveals a differential sensitivity to oxidative stress treatments with respect to the wild-type Anabaena strain, including a higher level of ROS (reactive oxygen species) in normal growth conditions. In the mutant strain, different oxidative stress treatments such as hydrogen peroxide, methyl-viologen or high light irradiance provoke an increase in the expression of genes related to ROS detoxification, including AnNTRC and peroxiredoxin genes, with a concomitant increase in the amount of AnNTRC and 2-Cys Prx. Moreover, the role of AnNTRC in the antioxidant response is confirmed by the observation of a pronounced overoxidation of the 2-Cys Prx and a time-delay recovery of the reduced form of this protein upon oxidative stress treatments. Our results suggest the participation of this enzyme in the peroxide detoxification in Anabaena. In addition, we describe the role of Anabaena NTRC in thermotolerance, by the appearance of high molecular mass AnNTRC complexes, showing that the mutant strain is more sensitive to high temperature treatments.

Role of calcium in the mitigation of heat stress in the cyanobacterium Anabaena PCC 7120

The effects of exogenously added CaCl₂ (0.25mM) on photopigments, photosynthetic O₂-evolution, antioxidative enzyme activity, membrane damage, expression of two heat shock genes (groEL and groES) and apoptotic features in Anabaena 7120 under heat stress (45°C) for up to 24h were investigated. Heat stress lowered the level of photopigments; however, Ca²⁺--supplemented cultures showed a low level reduction in Chl a but induced accumulation of carotenoids and phycocyanin under heat stress. Photosynthetic O₂-evolving capacity was maintained at a higher level in cells from Ca²⁺-supplemented medium. Among the antioxidative enzymes, superoxide dismutase activity was unaffected by the presence or absence of Ca²⁺ in contrast to increases in catalase, ascorbate peroxidase and glutathione reductase activities in cells grown in Ca²⁺-supplemented medium. Lower levels of lipid peroxidation were recorded in Anabaena cells grown in Ca²⁺-supplemented medium in comparison to cells from Ca²⁺--deprived medium. Target cells grown in Ca²⁺-deprived medium developed apoptotic features in the early stages of heat shock, while Ca²⁺ application seemed to interfere with apoptosis because only a few cells showed such features after 24 h of heat exposure, indicating a role for Ca²⁺ in maintaining cell viability under heat stress. There was also continuous up regulation of two important heat shock genes (groEL and groES) in Ca²⁺-supplemented cultures, exposed to heat shock, again indicating a role for Ca²⁺ in stress management.

Complete genome sequence of the bioleaching bacterium Leptospirillum sp. group II strain CF-1

We describe the complete genome sequence of Leptospirillum sp. group II strain CF-1, an acidophilic bioleaching bacterium isolated from an acid mine drainage (AMD). This work provides data to gain insights about adaptive response of Leptospirillum spp. to the extreme conditions of bioleaching environments.

In silico analysis and experimental validation of lipoprotein and novel Tat signal peptides processing in Anabaena sp. PCC7120

Signal peptide (SP) plays a pivotal role in protein translocation. Lipoprotein- and twin arginine translocase (Tat) dependent signal peptides were studied in All3087, a homolog of competence protein of Synechocystis PCC6803 and in two putative alkaline phosphatases (ALPs, Alr2234 and Alr4976), respectively. In silico analysis of All3087 is shown to possess the characteristics feature of competence proteins such as helix-hairpin-helix, N and C-terminal HKD endonuclease domain, calcium binding domain and N-terminal lipoprotein signal peptide. The SP recognition-cleavage site in All3087 was predicted (AIA-AC) using SignalP while further in-depth analysis using Pred-Lipo and WebLogo analysis for consensus sequence showed it as IAA-C. Activities of putative ALPs were confirmed by heterologous overexpression, activity assessment and zymogram analysis. ALP activity in Anabaena remains cell bound in log-phase, but during late log/stationary phase, an enhanced ALP activity was detected in extracellular milieu. The enhancement of ALP activity during stationary phase was not only due to inorganic phosphate limitation but also contributed by the presence of novel bipartite Tat-SP. The Tat signal transported the folded active ALPs to the membrane, followed by anchoring into the membrane and successive cleavage enabling transportation of the ALPs to the extracellular milieu, because of bipartite architecture and processing of transit Tat-SP.

GroEL of the nitrogen-fixing cyanobacterium Anabaena sp. strain L-31 exhibits GroES and ATP-independent refolding activity

The nitrogen-fixing cyanobacterium, Anabaena L-31 has two Hsp60 proteins, 59 kDa GroEL coded by the second gene of groESL operon and 61 kDa Cpn60 coded by cpn60 gene. Anabaena GroEL formed stable higher oligomer (>12-mer) in the presence of K(+) and prevented thermal aggregation of malate dehydrogenase (MDH). Using three protein substrates (MDH, All1541 and green fluorescent protein), it was found that the refolding activity of Anabaena GroEL was lower than that of Escherichia coli GroEL, but independent of both GroES and ATP. This correlated with in vivo data. GroEL exhibited ATPase activity which was enhanced in the presence of GroES and absence of a denatured protein, contrary to that observed for bacterial GroEL. However, a significant role for ATP could not be ascertained during in vitro folding assays. The monomeric Cpn60 exhibited much lower refolding activity than GroEL, unaffected by GroES and ATP. In vitro studies revealed inhibition of the refolding activity of Anabaena GroEL by Cpn60, which could be due to their different oligomeric status. The role of GroES and ATP may have been added during the course of evolution from the ancient cyanobacteria to modern day bacteria enhancing the refolding ability and ensuring wider scope of substrates for GroEL.

Understanding the Mechanism of Thermotolerance Distinct From Heat Shock Response Through Proteomic Analysis of Industrial Strains of Saccharomyces cerevisiae

Saccharomyces cerevisiae has been intensively studied in responses to different environmental stresses such as heat shock through global omic analysis. However, the S. cerevisiae industrial strains with superior thermotolerance have not been explored in any proteomic studies for elucidating the tolerance mechanism. Recently a new diploid strain was obtained through evolutionary engineering of a parental industrial strain, and it exhibited even higher resistance to prolonged thermal stress. Herein, we performed iTRAQ-based quantitative proteomic analysis on both the parental and evolved industrial strains to further understand the mechanism of thermotolerant adaptation. Out of ∼ 2600 quantifiable proteins from biological quadruplicates, 193 and 204 proteins were differentially regulated in the parental and evolved strains respectively during heat-stressed growth. The proteomic response of the industrial strains cultivated under prolonged thermal stress turned out to be substantially different from that of the laboratory strain exposed to sudden heat shock. Further analysis of transcription factors underlying the proteomic perturbation also indicated the distinct regulatory mechanism of thermotolerance. Finally, a cochaperone Mdj1 and a metabolic enzyme Adh1 were selected to investigate their roles in mediating heat-stressed growth and ethanol production of yeasts. Our proteomic characterization of the industrial strain led to comprehensive understanding of the molecular basis of thermotolerance, which would facilitate future improvement in the industrially important trait of S. cerevisiae by rational engineering.

Translational control of small heat shock genes in mesophilic and thermophilic cyanobacteria by RNA thermometers

Cyanobacteria constitute a heterogeneous phylum of oxygen-producing, photosynthetic prokaryotes. They are susceptible to various stress conditions like heat, salt, or light stress, all inducing the cyanobacterial heat shock response (HSR). Cyanobacterial small heat shock proteins (sHsps) are known to preserve thylakoid membrane integrity under stress conditions, thereby protecting the photosynthesis machinery. In Synechocystis sp PCC 6803, synthesis of the sHsp Hsp17 is regulated by an RNA thermometer (RNAT) in the 5′-untranslated region (5′-UTR) of the hsp17 mRNA. RNATs are direct temperature sensors that control expression of many bacterial heat shock and virulence genes. They hinder translation at low temperatures by base pairing, thus blocking ribosome access to the mRNA.

 

To explore the temperature range in which RNATs act, we studied various RNAT candidates upstream of sHsp genes from mesophilic and thermophilic cyanobacteria. The mesophilic cyanobacteria Anabaena variabilis and Nostoc sp chromosomally encode two sHsps each. Reporter gene studies suggested RNAT-mediated post-transcriptional regulation of shsp expression in both organisms. Detailed structural analysis of the two A. variabilis candidates revealed two novel RNAT types. The first, avashort, regulates translation primarily by masking of the AUG translational start codon. The second, featuring an extended initial hairpin, thus named avalong, presumably makes use of complex tertiary interaction. The 5′-UTR of the small heat shock gene hspA in the thermophile Thermosynechococcus elongatus is predicted to adopt an extended secondary structure. Structure probing revealed that the ribosome binding site was blocked at temperatures below 55 °C. The results of this study demonstrate that cyanobacteria commonly use RNATs to control expression of their small heat shock genes.