Cold-induced gene expression and ω3 fatty acid unsaturation is controlled by red light in Synechocystis

50 years since the concept of homeoviscous adaptation

This mini review focuses on the phenomenon of homeoviscous adaptation (HVA). The concept, which dominated for decades, had a significant impact on membrane and lipid research. It includes the functional characterization of biological membranes and their domains, the role of lipids and fatty acids in cell metabolic control, and the characterization of fatty acid desaturases and their roles in membrane properties modulation. This hypothesis led to the discovery of a feed-back manner of desaturase expression and membrane-associated temperature sensors in bacteria.

Red-light-dependent chlorophyll synthesis kindles photosynthetic recovery of chlorotic dormant cyanobacteria using a dark-operative enzyme

Chlorosis dormancy resulting from nitrogen starvation and its resuscitation upon available nitrogen contributes greatly to the fitness of cyanobacterial population under nitrogen-fluctuating environments. The reinstallation of the photosynthetic machinery is a key process for resuscitation from a chlorotic dormant state; however, the underlying regulatory mechanism is still elusive. Here, we reported that red light is essential for re-greening chlorotic Synechocystis sp. PCC 6803 (a non-diazotrophic cyanobacterium) after nitrogen supplement under weak light conditions. The expression of dark-operative protochlorophyllide reductase (DPOR) governed by the transcriptional factor RpaB was strikingly induced by red light in chlorotic cells, and its deficient mutant lost the capability of resuscitation from a dormant state, indicating DPOR catalyzing chlorophyll synthesis is a key step in the photosynthetic recovery of dormant cyanobacteria. Although light-dependent protochlorophyllide reductase is widely considered as a master switch in photomorphogenesis, this study unravels the primitive DPOR as a spark to activate the photosynthetic recovery of chlorotic dormant cyanobacteria. These findings provide new insight into the biological significance of DPOR in cyanobacteria and even some plants thriving in extreme environments.

Transmembrane and PAS domains of the histidine kinase Hik33 as regulators of cold and light responses in the cyanobacterium Synechocystis sp. PCC 6803

The PAS (Per-ARNT-Sim) domain is a sensory protein regulatory module found in archaea, prokaryotes, and eukaryotes. Histidine and serine/threonine protein kinases, chemo- and photoreceptors, circadian rhythm regulators, ion channels, phosphodiesterases, and other cellular response regulators are among these proteins. Hik33 is a multifunctional sensory histidine kinase that is implicated in cyanobacterial responses to cold, salt, hyperosmotic, and oxidative stressors. The functional roles of individual Hik33 domains in signal transduction were investigated in this study. Synechocystis Hik33 deletion variants were developed, in which either both or a portion of the transmembrane domains and/or the PAS domain were deleted. Cold stress was applied to the mutant strains either under illumination or in the dark. The findings show that the transmembrane domains govern temperature responses, whereas PAS domain may be involved in regulation of downstream gene expression in light-dependent manner.

Multiple Light-Dark Signals Regulate Expression of the DEAD-Box RNA Helicase CrhR in Synechocystis PCC 6803

Since oxygenic photosynthesis evolved in the common ancestor of cyanobacteria during the Archean, a range of sensing and response strategies evolved to allow efficient acclimation to the fluctuating light conditions experienced in the diverse environments they inhabit. However, how these regulatory mechanisms are assimilated at the molecular level to coordinate individual gene expression is still being elucidated. Here, we demonstrate that integration of a series of three distinct light signals generate an unexpectedly complex network regulating expression of the sole DEAD-box RNA helicase, CrhR, encoded in Synechocystis sp. PCC 6803. The mechanisms function at the transcriptional, translational and post-translation levels, fine-tuning CrhR abundance to permit rapid acclimation to fluctuating light and temperature regimes. CrhR abundance is enhanced 15-fold by low temperature stress. We initially confirmed that the primary mechanism controlling crhR transcript accumulation at 20 °C requires a light quantity-driven reduction of the redox poise in the vicinity of the plastoquinone pool. Once transcribed, a specific light quality cue, a red light signal, was required for crhR translation, far-red reversal of which indicates a phytochrome-mediated mechanism. Examination of CrhR repression at 30 °C revealed that a redox- and light quality-independent light signal was required to initiate CrhR degradation. The crucial role of light was further revealed by the observation that dark conditions superseded the light signals required to initiate each of these regulatory processes. The findings reveal an unexpected complexity of light-dark sensing and signaling that regulate expression of an individual gene in cyanobacteria, an integrated mechanism of environmental perception not previously reported.

Universal Molecular Triggers of Stress Responses in Cyanobacterium Synechocystis

Systemic analysis of stress-induced transcription in the cyanobacterium Synechocystis sp. strain PCC 6803 identifies a number of genes as being induced in response to most abiotic stressors (heat, osmotic, saline, acid stress, strong light, and ultraviolet radiation). Genes for heat-shock proteins (HSPs) are activated by all these stresses and form a group that universally responds to all environmental changes. The functions of universal triggers of stress responses in cyanobacteria can be performed by reactive oxygen species (ROS), in particular H₂O₂, as well as changes in the redox potential of the components of the photosynthetic electron transport chain. The double mutant of Synechocystis sp. PCC 6803 (katG/tpx, or sll1987/sll0755), which is defective in antioxidant enzymes catalase (KatG) and thioredoxin peroxidase (Tpx), cannot grow in the presence of exogenous hydrogen peroxide (H₂O₂); and it is extremely sensitive to low concentrations of H₂O₂, especially under conditions of cold stress. Experiments on this mutant demonstrate that H₂O₂ is involved in regulation of gene expression that responds to a decrease in ambient temperature, and affects both the perception and the signal transduction of cold stress. In addition, they suggest that formation of ROS largely depends on the physical state of the membranes such as fluidity or viscosity. In cyanobacteria, an increase in membrane turnover leads to a decrease in the formation of ROS and an increase in resistance to cold stress. Therefore: (1) H₂O₂ is the universal trigger of stress responses in cyanobacterial cells; (2) ROS formation (in particular, H₂O₂) depends on the physical properties of both cytoplasmic and thylakoid membranes; (3) The destructive effect of H₂O₂ is reduced by increasing of fluidity of biological membranes.

Photosystem I oligomerization affects lipid composition in Synechocystis sp. PCC 6803

In cyanobacteria, increasing growth temperature decreases lipid unsaturation and the ratio of monomer/trimer photosystem I (PSI) complexes. In the present study we applied Fourier-transform infrared (FTIR) spectroscopy and lipidomic analysis to study the effects of PSI monomer/oligomer ratio on the physical properties and lipid composition of thylakoids. To enhance the presence of monomeric PSI, a Synechocystis sp. PCC6803/ΔpsaL mutant strain (PsaL) was used which, unlike both trimeric and monomeric PSI-containing wild type (WT) cells, contain only the monomeric form. The protein-to-lipid ratio remained unchanged in the mutant but, due to an increase in the lipid disorder in its thylakoids, the gel to liquid-crystalline phase transition temperature (Tm) is lower than in the WT. In thylakoid membranes of the mutant, digalactosyldiacylglycerol (DGDG), the most abundant bilayer-forming lipid is accumulated, whereas those in the WT contain more monogalactosyldiacylglycerol (MGDG), the only non-bilayer-forming lipid in cyanobacteria. In PsaL cells, the unsaturation level of sulphoquinovosyldiacylglycerol (SQDG), a regulatory anionic lipid, has increased. It seems that merely a change in the oligomerization level of a membrane protein complex (PSI), and thus the altered protein-lipid interface, can affect the lipid composition and, in addition, the whole dynamics of the membrane. Singular value decomposition (SVD) analysis has shown that in PsaL thylakoidal protein-lipid interactions are less stable than in the WT, and proteins start losing their native secondary structure at much milder lipid packing perturbations. Conclusions drawn from this system should be generally applicable for protein-lipid interactions in biological membranes.

Novel Histidine Kinase Gene HisK2301 from Rhodosporidium kratochvilovae Contributes to Cold Adaption by Promoting Biosynthesis of Polyunsaturated Fatty Acids and Glycerol

Hybrid histidine kinase (HHKs) are widespread in fungi, but their roles in the regulation of fungal adaptation to environmental stresses remain largely unclear. To elucidate this, we cloned HisK2301 from Rhodosporidium kratochvilovae strain YM25235, characterized HisK2301 as a novel HHK, and further investigated the role of HisK2301 by overexpressing it in YM25235. Our results revealed that HisK2301 can promote adaptation of YM25235 to cold, osmotic, and salt stresses. During cold stress, HisK2301 significantly enhanced the biosynthesis of polyunsaturated fatty acids (PUFA) and intracellular glycerol. HisK2301 also augmented the expression levels of Δ¹²/Δ¹⁵ fatty acid desaturase ( RKD12) and glycerol-3-phosphate dehydrogenase1 ( GPD1), which are responsible for PUFA and glycerol biosynthesis, respectively. To conclude, our findings give the first insight into the defense and mechanisms of HisK2301 in fungi against cold stress and suggest the potential use of the novel cold-adapted HHK HisK2301 in industrial processes, such as large-scale production of PUFA.

Membrane physical state and stress regulation in Synechocystis: fluidizing alcohols repress fatty acid desaturation

Cyanobacteria are prokaryotic photosynthetic organisms widely used in biotechnology, photosynthesis and abiotic stress research. There are several cyanobacterial strains modified to produce biofuels, but the influence of alcohols on cyanobacterial cell physiology is poorly understood. Here, we conducted a systematic study of the effects of nine primary aliphatic alcohols and an aromatic benzyl alcohol on both membrane physical state and the expression of genes for fatty acid desaturases (FADs) in a model cyanobacterium Synechocystis sp. strain PCC 6803. Hexan-1-ol was found to have the most membrane fluidizing action among all alcohols studied, with its efficiency correlating with both duration of treatment and alcohol concentration. A prolonged exposure to alcohol results in a continuous loss of unsaturated fatty acids (FAs) followed by cell death, an undesired challenge that should be considered in cyanobacterial biotechnology. We suggest that membrane fluidization is the key component in alcohol stress causing inactivation of FADs and resulting in a lethal depletion of unsaturated FAs. Due to the most pronounced effects of alcohol- and heat-induced membrane fluidization on desB encoding a terminal ω3-FAD, we propose to call desB a 'viscosity gene' in analogy to heat-induced 'fluidity gene' hspA.

Hydrogen Peroxide Participates in Perception and Transduction of Cold Stress Signal in Synechocystis

The double mutant ΔkatG/tpx of cyanobacterium Synechocystis sp. strain PCC 6803, defective in the anti-oxidative enzymes catalase (KatG) and thioredoxin peroxidase (Tpx), is unable to grow in the presence of exogenous H2O2. The ΔkatG/tpx mutant is shown to be extremely sensitive to very low concentrations of H2O2, especially when intensified with cold stress. Analysis of gene expression in both wild-type and ΔkatG/tpx mutant cells treated by combined cold/oxidative stress revealed that H2O2 participates in regulation of expression of cold-responsive genes, affecting either signal perception or transduction. The central role of a transmembrane stress-sensing histidine kinase Hik33 in the cold/oxidative signal transduction pathway is discussed.

Membrane fluidity controls redox-regulated cold stress responses in cyanobacteria

Membrane fluidity is the important regulator of cellular responses to changing ambient temperature. Bacteria perceive cold by the transmembrane histidine kinases that sense changes in thickness of the cytoplasmic membrane due to its rigidification. In the cyanobacterium Synechocystis, about a half of cold-responsive genes is controlled by the light-dependent transmembrane histidine kinase Hik33, which also partially controls the responses to osmotic, salt, and oxidative stress. This implies the existence of some universal, but yet unknown signal that triggers adaptive gene expression in response to various stressors. Here we selectively probed the components of photosynthetic machinery and functionally characterized the thermodynamics of cyanobacterial photosynthetic membranes with genetically altered fluidity. We show that the rate of oxidation of the quinone pool (PQ), which interacts with both photosynthetic and respiratory electron transport chains, depends on membrane fluidity. Inhibitor-induced stimulation of redox changes in PQ triggers cold-induced gene expression. Thus, the fluidity-dependent changes in the redox state of PQ may universally trigger cellular responses to stressors that affect membrane properties.

New insights in cyanobacterial cold stress responses: Genes, sensors, and molecular triggers

BACKGROUND

Cold stress strongly induces the expression of ~100 genes in cyanobacteria. Some of these genes are necessary to protect cellular functions by adjustment of membranes, as well as transcriptional and translational machineries. About a half of cold-induced genes are not functionally characterized. A part of cold-induced genes is under control of a two-component regulatory system, consisting of histidine kinase Hik33 and response regulator Rre26. The mechanism(s) that control another part of cold-inducible genes are still unknown.

SCOPE OF REVIEW

The aim of this review is to summarise the latest findings in cyanobacterial cold-stress responses including transcriptomics, cold sensing, and molecular triggers.

MAJOR CONCLUSIONS

A feedback loop between the membrane fluidity and transcription of genes for fatty acid desaturases operates via the transmembrane red-light-activated cold sensor Hik33, which perceives cold-induced membrane rigidification as a change in its thickness. The cold-induced kinase activity of Hik33 is facilitated by interaction with a small protein, Ssl3451 - the third contributor to a canonical two-component regulatory system, which may explain the ability of some cyanobacterial histidine kinases to interact with different response regulators under different stress conditions. Other regulatory systems that control cold-stress responses operate via Ser/Thr protein kinase, SpkE, and via temperature-dependent changes in DNA supercoiling. Transcriptomic analysis shows that universal triggers of stress responses are reactive oxygen species and changes in redox status of plastoquinone pool.

GENERAL SIGNIFICANCE

Deeper understanding of molecular mechanisms of temperature sensing and regulation of cold-stress responses in photosynthetic cells provide a background for generation of cold-resistant crops.

Temperature effects on bacterial phytochrome

Bacteriophytochromes (BphPs) are light-sensing regulatory proteins encoded in photosynthetic and non-photosynthetic bacteria. This protein class incorporate bilin as their chromophore, with majority of them bearing a light- regulated His kinase or His kinase related module in the C-terminal. We studied the His kinase actives in the temperature range of 5°C to 40°C on two BphPs, Agp1 from Agrobacterium tumefaciens and Cph1 from cyanobacterium Synechocystis PCC 6803. As reported, the phosphorylation activities of the far red (FR) irradiated form of the holoprotein is stronger than that of the red (R) irradiated form in both phytochromes. We observed for the apoprotein and FR irradiated holoprotein of Agp1 an increase in the phosphorylation activities from 5°C to 25°C and a decrease from 25°C to 40°C. At 5°C the activities of the apoprotein were significantly lower than those of the FR irradiated holoprotein, which was opposite at 40°C. A similar temperature pattern was observed for Cph1, but the maximum of the apoprotein was at 20°C while the maximum of the FR irradiated holoprotein was at 10°C. At 40°C, prolonged R irradiation leads to an irreversible bleaching of Cph1, an effect which depends on the C-terminal His kinase module. A more prominent and reversible temperature effect on spectral properties of Agp1, mediated by the His kinase, has been reported before. His kinases in phytochromes could therefore share similar temperature characteristics. We also found that phytochrome B mutants of Arabidopsis have reduced hypocotyl growth at 37°C in darkness, suggesting that this phytochrome senses the temperature or mediates signal transduction of temperature effects.