[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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 CO2 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 CO2 concentration in gas–air mixture was raised from 2 to 10%. An irradiation increase from 70 to 500 μmol/(m2 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.
[Show abstract][Hide abstract] ABSTRACT: 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 Ca(2+) 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 Ca(2+) indicator arsenazo III were used to follow the plasma membrane depolarization and the Ca(2+) release, respectively. The Ca(2+) release from wild-type cells was temperature-dependent and it was strongly inhibited by the Ca(2+) channel blocker verapamil and by the mechanosensitive channel blocker amiloride. In MscL-deficient cells, Ca(2+) release was about 50 % of that from the wild-type cells. The mutant cells had lost temperature sensitivity of Ca(2+) release completely. However, verapamil and amiloride inhibited Ca(2+) release from these cells in same manner as in the wild-type cells. This suggests the existence of additional Ca(2+) transporters in Synechocystis, probably of a mechanosensitive nature. Evidence for the putative presence of intracellular Ca(2+) stores in the cells was obtained by following the increase in fluorescence intensity of the Ca(2+) indicator chlortetracycline. These results suggest that the MscL of Synechocystis might operate as a verapamil/amiloride-sensitive outward Ca(2+) channel that is involved in the plasma-membrane depolarization-induced Ca(2+) release from the cells under temperature stress conditions.
[Show abstract][Hide abstract] ABSTRACT: Mn is an essential component of the oxygen-evolving machinery of photosynthesis and is an essential cofactor of several important enzymes, such as Mn-superoxide dismutase and Mn-catalase. The availability of Mn in the environment varies, and little is known about the mechanisms for maintaining cytoplasmic Mn(2+) ion homeostasis. Using a DNA microarray, we screened knockout libraries of His kinases and response regulators of Synechocystis sp PCC 6803 to identify possible participants in this process. We identified a His kinase, ManS, which might sense the extracellular concentration of Mn(2+) ions, and a response regulator, ManR, which might regulate the expression of the mntCAB operon for the ABC-type transporter of Mn(2+) ions. Furthermore, analysis with the DNA microarray and by reverse transcription PCR suggested that ManS produces a signal that activates ManR, which represses the expression of the mntCAB operon. At low concentrations of Mn(2+) ions, ManS does not generate a signal, with resulting inactivation of ManR and subsequent expression of the mntCAB operon.
[Show abstract][Hide abstract] ABSTRACT: A single-copy gene resembling the gene for the delta9 acyl-lipid desaturase (desC) was cloned from the thermophilic cyanobacterium Synechococcus vulcanus. Expression of desC in Escherichia coli confirmed that it encodes the delta9 desaturase. The nucleotide sequence of the desC was characterized by high G+C content that is typical of the sequences of thermophilic bacteria. The deduced amino acid sequence exhibited low Cys content and high Arg/Lys ratio that are the attributes of thermostable enzymes. A low level of the desC mRNA was detected in the cells grown at 55 degrees C, the optimum growth temperature for S. vulcanus. About a 10-fold increase was observed in the levels of the transcript and the protein during the shift in temperature from 55 to 45 degrees C. At 35 degrees C the amount of the desC mRNA and of the enzyme accumulated in the cells, was 3 to 4 times smaller than at 45 degrees C. At both temperatures, however, lipids were desaturated at similar rates. These results suggest that in S. vulcanus the conversion of stearic acid into oleic acid may be controlled not only by the de novo synthesis of the delta9 desaturase but, possibly, by the activation of the pre-existing enzyme.
Full-text · Article · Aug 2000 · Journal of Molecular Microbiology and Biotechnology