Osmotic stress causes water molecules to efflux from cells through the cytoplasmic membrane. This study reveals that targeted mutation of the aqpZ gene, encoding an aquaporin water channel protein, in the cyanobacterium Synechocystis sp. PCC 6803 prevents the osmotic shrinkage of cells, suggesting that it is the water channel rather than the lipid bilayer that is primarily responsible for water transition through the membrane of this organism. The observations suggest that the aquaporin-mediated shrinkage of the Synechocystis cells plays an important role in changes of gene expression in response to hyperosmotic stress.

A fragment of the gene des3-1 encoding ω3 fatty acid desaturase was cloned from a cDNA library of the unicellular green galophilic alga Dunaliella salina. The comparative phylogenetic analysis of ω3-desaturase amino acid sequences from diverse organisms placed the desaturase of D. salina between cyanobacteria and higher plants in the evolutionary range of desaturases. The expression of des3-1 was studied in D. salina cells exposed to low temperatures, high irradiance, and high CO 2 concentrations. Lowering the external temperature from 32 to 22°C produced a transient increase in the level of specific mRNA. Considerable accumulation of mRNA for ω3-desaturase was also observed when CO 2 concentration in gas–air mixture was raised from 2 to 10%. An irradiation increase from 70 to 500 μmol/(m 2 s) did not affect the level of specific mRNA. The latter evidence presumes that in Dunaliella cells, this desaturase is probably located in the endoplasmic reticulum, rather than in the chloroplast.

Cells of the cyanobacterium Synechocystis sp. PCC 6803 are equipped with a mechanosensitive ion channel MscL that is located in their plasma membrane. However, the exact function of the channel in this freshwater cyanobacterium is unknown. This study shows that cells of Synechocystis are capable of releasing Ca2+ in response to depolarization of the plasma membrane by the K+ ionophore valinomycin in the presence of K+ or by tetraphenylphosphonium (TPP+). A fluorescent dye, diS-C-3-(5), sensitive to membrane potential and the metallochromic Ca2+ indicator arsenazo III were used to follow the plasma membrane depolarization and the Ca2+ release, respectively. The Ca2+ release from wild-type cells was temperature-dependent and it was strongly inhibited by the Ca2+ channel blocker verapamil and by the mechanosensitive channel blocker amiloride. In MscL-deficient cells, Ca2+ release was about 50 %of that from the wild-type cells. The mutant cells had lost temperature sensitivity of Ca2+ release completely. However, verapamil and amiloride inhibited Ca2+ release from these cells in same manner as in the wild-type cells. This suggests the existence of additional Ca2+ transporters in Synechocystis, probably of a mechanosensitive nature. Evidence for the putative presence of intracellular Ca2+ stores in the cells was obtained by following the increase in fluorescence intensity of the Ca2+ indicator chlortetracycline. These results suggest that the MscL of Synechocystis might operate as a verapamil/amiloride-sensitive outward Ca2+ channel that is involved in the plasma-membrane depolarization-induced Ca2+ release from the cells under temperature stress conditions.