External spermine prevents UVA-induced damage of <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 via increased catalase activity and decreased H<Subscript>2</Subscript>O<Subscript>2</Subscript> and malonaldehyde levels

Common polyamines, putrescine (Put), spermidine (Spd), and spermine (Spm), are cationic compounds known as beneficial factors for many cellular processes including cell division, proliferation, differentiation, and stress response in all living organisms. Effects of exogenous Spm on the protective responses of Synechocystis sp. PCC 6803 exposed to UVA were investigated. The presence of 0.5 mM Spm in the culture medium significantly reduced cell growth after 60 min under white light condition but protected the cells after growing for 60 min under UVA. The stress-tolerant response of Synechocystis cells represented by the ratio of putrescine/spermidine (Put/Spd) showed about a 6-fold increase after 60 min UVA in the presence of Spm. In addition, those levels of chlorophyll a, carotenoids, and photosynthetic oxygen evolution were increased by Spm supplementation in UVA-treated cells. Exogenous Spm induced the activity of catalase but not superoxide dismutase in cells under UVA treatment. On the other hand, Spm treatment enabled cells to apparently decrease the intracellular free radical H₂O₂ and malonaldehyde (MDA) levels. Overall results suggested that Spm supplementation could protect Synechocystis sp. PCC 6803 cells via the increase of Put/Spd ratio and the reduction of both H₂O₂ and MDA levels in conjunction with the induction of catalase activity. Interestingly, UVA-treated cells as compared to non-treated cells with exogenous Spm showed a decrease of Spm with an increase of Put and no change in Spd. This suggested the back conversion of Spm to Spd and finally to Put as cellular mechanism in response to UVA.     

Na+-stimulated nitrate uptake with increased activity under osmotic upshift in Synechocystis sp. strain PCC 6803

In the non-diazotrophic cyanobacterium Synechocystis sp. strain PCC 6803, an osmolality of 30 and 40 mosmol/kg sorbitol and NaCl resulted in 3.5- and 4.5-fold increase of nitrate uptake, respectively. The NaCl-stimulated uptake was abolished by treatment with chloramphenicol. At 25 mosmol/kg or higher, NaCl induced higher nitrate uptake than sorbitol suggesting an ionic effect of Na⁺. The nitrate uptake in Synechocystis showed K _s and V _max values of 46 μM and 1.37 μmol/min/mg Chl, respectively. Mutants disrupted in nitrate and nitrite reductase exhibited a decreased nitrate uptake. Ammonium, chlorate, and dl-glyceraldehyde caused a reduction of nitrate uptake. Dark treatment caused a drastic reduction of uptake by 70% suggesting an energy-dependent system. Nitrate transport was sensitive to various metabolic inhibitors including those dissipating proton gradients and membrane potential. The results suggest that nitrate uptake in Synechocystis is stimulated by Na⁺ ions and requires energy provided by the functioning electron transport chain.