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Lipid droplets are ubiquitous cellular compartments that store neutral lipids and specific proteins localize on their surface. These proteins work as a scaffold in maintaining the lipid droplet structure or as regulators of lipogenesis or lipolysis. Previously, the most abundant lipid droplet protein, namely stramenopile-type lipid droplet protein (StLDP), was identified in the marine diatom Phaeodactylum tricornutum; however, its function remains unclear because StLDP does not reveal homology with known lipid droplet proteins and lacks a predictable domain. In this study, P. tricornutum was transformed to express a homologous StLDP gene under an fcpA promoter in order to determine its function. StLDP expression was strongly enhanced in the mutant (H8), especially in nitrogen-sufficient conditions; however, it was attenuated in nitrogen-deficient conditions. Despite the strong expression, no significant difference was observed in the lipid composition between the wild type (WT) and H8 under nitrogen-sufficient conditions. After cultivation in nitrogen-free medium for 6 days, neutral lipid content significantly increased in H8 than in WT. After 2 days of cultivation in nitrogen-free medium, 97.0% of single cells in WT formed one or two lipid droplets, whereas in H8, this proportion decreased to 78.8%, and the proportion of cells forming three or four lipid droplets increased. Thus, the function of StLDP was speculated to sequester triacylglycerol on the initial lipid droplet formation.

The perception of environmental stress and the subsequent transduction of stress signals are primary events in the acclimation of all organisms to changes in their environment. Many of the molecular sensors and transducers of environmental stress cannot be identified by traditional and conventional methods. Based on genomic information, a systematic approach has been applied to the solution of this problem in cyanobacteria, involving mutagenesis of potential sensors and signal transducers in combination with DNA microarray analyses for the genome-wide expression of genes. Almost all of the histidine kinases (Hiks) and response regulators (Rres) have been successfully inactivated by targeted mutagenesis in the unicellular cyanobacterium Synechocystis sp. PCC 6803. Screening of mutant libraries by genome-wide DNA microarray analysis under various stress and non-stress conditions has allowed identification of the Hiks and Rres that perceive and transduce signals of environmental stress. In this chapter, we summarize recent progress in the identification of regulatory two-component systems. In addition, we discuss the potential roles of Spks, DNA supercoiling, sigma factors and transcription factors in the regulation of the responses of cyanobacterial cells to various types of stress.

Cyanobacteria exhibit considerable tolerance to salt stress and are useful for studies of acclimation and tolerance to such stress. In a previous study we isolated a desA−/desD− mutant strain of Synechocystis sp. PCC 6803 in which the desA and desD genes for the Δl2 and Δ6 desaturases, respectively, had been inactivated by targeted mutagenesis (1). The desA−/desD− cells contain monounsaturated but not polyunsaturated fatty acids, whereas wild-type cells contain polyunsaturated fatty acids such as di-, tri-, and tetraunsaturated fatty acids (2,3). In the present study, we investigated the contribution of the unsaturation of fatty acids in membrane lipids to tolerance to salt stress by comparing desA − /desD − cells to wild-type cells of Synechocystis sp. PCC 6803. We demonstrated that the unsaturation of fatty acids is associated with the ability of the photosynthetic machinery to tolerate salt stress.