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Membrane Fluidity and the Perception of Environmental Signals In Progress in Lipid Research 42 (2003) 527–543 www.elsevier.com/locate/plipres Review Membrane fluidity and the perception of environmental signals in cyanobacteria and plants Koji Mikami, Norio Murata* Department of Regulation Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan Accepted 22 April 2003 Abstract Photosynthetic organisms, namely, plants and cyanobacteria, are directly exposed to changes in their environment and their survival depends on their ability to acclimate to such changes. Several lines of evi- dence suggest that temperature stress, such as unusually low or high temperatures, and osmotic stress might be perceived by plants and cyanobacteria via changes in the fluidity of their cell membranes. The availability of techniques for gene-targeted mutagenesis and gene transfer, as well as for the analysis of genomes and transcripts, has allowed us to examine and evaluate this hypothesis and its implications. In this review, we summarize recent studies of the regulation of gene expression by changes in the extent of unsaturation of fatty acids and membrane fluidity, and we present a discussion of the induction of gene expression by environmental stress and of sensors of environmental conditions and relationships between their activity and the fluidity of membranes in cyanobacteria and plants. # 2003 Elsevier Ltd. All rights reserved. Contents 1. Introduction ........................................................................................................................................................... 528 2. Modulation of membrane fluidity .......................................................................................................................... 530 2.1. Effects of temperature and hyperosmotic stress ............................................................................................ 530 2.2. Effects of the unsaturation of fatty acids in membrane lipids....................................................................... 532 2.3. Homeostatic regulation of membrane fluidity by fatty acid desaturation..................................................... 532 * Corresponding author. Tel.: +81-564-55-7600; fax: +81-564-54-4866. E-mail address: [email protected] (N. Murata). 0163-7827/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0163-7827(03)00036-5 528 K. Mikami, N. Murata / Progress in LipidResearch 42 (2003) 527–543 3. Perception of low temperature ............................................................................................................................... 533 3.1. Cold-induced gene expression in cyanobacteria ............................................................................................ 533 3.2. Sensors of low temperature ........................................................................................................................... 534 3.3. Cold-inducible gene expression and transduction of cold signals in plants................................................... 535 4. Perception of high temperature.............................................................................................................................. 537 4.1. High temperature-induced gene expression ................................................................................................... 537 4.2. Sensors of high temperature.......................................................................................................................... 537 5. Perception of hyperosmotic stress .......................................................................................................................... 538 5.1. Hyperosmotic stress-induced gene expression ............................................................................................... 538 5.2. Sensors of hyperosmotic stress ...................................................................................................................... 539 6. A multi-stress sensor in Synechocystis.................................................................................................................... 539 7. Conclusion and future perspectives........................................................................................................................ 540 Acknowledgements...................................................................................................................................................... 540 References ................................................................................................................................................................... 541 Nomenclature PG phosphatidylglycerol DPH 1, 6-diphenyl-1, 3, 5-hexatriene Hik histidine kinase FTIR Fourier transform infrared 1. Introduction The plasma membranes and thylakoid membranes of cyanobacteria resemble the inner envel- ope and thylakoid membranes of the chloroplasts of higher plants in terms of both ultrastructure and lipid composition [1]. These membranes in cyanobacterial cells and chloroplasts are char- acterized by their unique lipid composition: the major components are glycolipids, such as monogalactosyl diacylglycerol, digalactosyl diacylglycerol and sulfoquinovosyl diacylglycerol [2]. Phosphatidylglycerol (PG) is the only phospholipid in these membranes [2] and accounts for approximately 10–20% of the total membrane lipids [3–8]. In plants, there is a very close relationship between the level of unsaturation of the fatty acids in PG and sensitivity to cold [9–11]. Genetic manipulation of levels of unsaturated fatty acids led to the eventual modification of the cold sensitivity of tobacco plants [12]. In cyanobacteria, sensi- tivity to cold is also closely correlated with the level of unsaturation of membrane lipids [13,14]. K. Mikami, N. Murata / Progress in LipidResearch 42 (2003) 527–543 529 Thus, sensitivity to cold stress in photosynthetic organisms seems to be determined by the level of unsaturation of fatty acids in their unique membrane lipids. Poikilothermic organisms, such as cyanobacteria and plants, modulate the composition of their membrane lipids in response to changes in environmental conditions [15], and modulation of levels of unsaturation of membrane lipids has been widely observed in a variety of species, such as cyanobacteria, Tetrahymena thermophila, Acanthamoeba castellanii and carp [16–20]. Modulation of the level of unsaturation of membrane lipids is regulated mainly by the low temperature- dependent desaturation of fatty acids. We have focussed our attention on a strain of cyano- bacteria, Synechocystis sp. PCC 6803 (hereafter, Synechocystis), that introduces double bonds at the Á6, Á9, Á12and Á15 (o3) positions in C-18 fatty acids at the sn-1 position in reactions cat- alysed by fatty acid desaturases that are encoded by the desD, desC, desA and desB genes, respectively [21,22]. It was postulated from the results of early experiments that the rigidification of membrane lipids might be the primary signal of cold stress [16–20] and that the activity of desaturases might be regulated by the fluidity of membrane lipids [23–25]. Indeed, a downward or an upward shift in temperature, as well as osmotic stress, induces variations in membrane fluidity that are due to the physical properties of the membrane lipids [26–29] and are related to the physical phase transition of membrane lipids from a liquid-crystalline phase to a gel phase [30,31]. To prove the above-mentioned hypothesis, an attempt was made to determine whether mem- brane fluidity could act directly to regulate the specific activity of desaturases [25,32]. Horva´ th et al. [25] suggested that desaturases might have a negative temperature coefficient, with higher activities at lower temperatures. However, a biochemical analysis of the dependence on tem- perature of the activity of cyanobacterial DesA, the Á12desaturase, which had been over- expressed in Escherichia coli, demonstrated that DesA has a positive temperature coefficient, as do most other enzymes [32]. Another mechanism proposed was that cold stress might enhance the expression of genes for desaturases. Los et al. [22] demonstrated that cold stress accelerated the transcription of the desA, desB and desD genes but not that of the desC gene. Such enhanced transcription of genes for desaturases has been observed in many poikilothermic organisms (see [18]). Transcription of desA, desB and desD genes in Synechocystis seems to be regulated via the rigidification of membrane lipids because catalytic hydrogenation, which decreases levels of unsaturated fatty acids in plasma membranes, induced a 10-fold increase in the expression of the desA gene with kinetics similar to those of the induction of this gene by cold [33]. These results imply that maintenance of the fluidity of membranes at physiological levels can be achieved under stress conditions in order to sustain the functional activity of membrane proteins and the membranes themselves [20]. The details of the involvement of membrane fluidity in the regulation of gene expression remain to be characterized. In Saccharomyces cerevisiae, addition of linoleic acid to the culture medium increases the temperature that is critical for the induction of heat-inducible genes [34]. Thus, the heat inducibility of such gene expression seems to depend on the level of unsaturated fatty acids and, thus, on the fluidity of membrane lipids. However, in Synechocystis, membrane fluidity might not be involved in the induction of gene expression by high temperature (see
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