EFFECT OF SORBITOL STRESS ON POLYAMINE CONTENTS IN Synechocystis sp. PCC 6803 MUTANTS OF sll1077 AND sll0228 GENES

Abstract

Effect of sorbitol-induced osmotic stress on growth rate, chlorophyll a and carotenoid pigment contents, oxygen evolution, polyamine contents, gene expression and agmatinase activity was investigated in cyanobacterium Synechocystis sp. PCC 6803 wild type and two mutants lacking putative agmatinase gene (∆sll1077) and putative arginase gene (∆sll0288), respectively. Although the pairwise alignment of sll1077 and sll10228 amino acid sequences showed 35.3% of similarity and 20.7% of identity, their phylogenetic tree demonstrated that they were in the same clade of agmatinase when aligned to known amino acid sequences of other species. All three strains of wild type, ∆sll1077 and ∆sll0228 were grown in BG11 media containing various sorbitol concentrations of 250, 500 and 700 mM, respectively. The growth rate, chlorophyll a and carotenoid contents of wild type and mutant strains were not different under 250 mM and 500 mM sorbitol conditions while 700 mM sorbitol condition significantly inhibited them compared to those under control condition. Interestingly, ∆sll1077 cells slightly growed under 700 mM sorbitol condition and could maintain the chlorophyll a and carotenoid contents along 14 days of treatment. The O2 evolution rates of those three strains were not significantly different under 0, 250 and 500 mM sorbitol conditions whereas 700 mM sorbitol condition obviously decreased their O2 evolution rate. Total polyamine content of wild type cells was apparently induced by 700 mM sorbitol stress whereas ∆sll1077 showed partial decreases of total polyamine contents under all stressed conditions. It was strikingly found that ∆sll0228 cells completely diminished their total polyamines. Moreover, norspermidine, uncommon polyamine, was also found under sorbitol stress. The agmatinase activity of wild type was increased by sorbitol stress whereas ∆sll1077 gave lower enzyme activity. The trace level of ∆sll0228 agmatinase activity was clearly observed. On the other hand, the transcript levels of sll1077 and sll0228 under 700 mM sorbitol condition were up-regulated in both wild type and ∆sll1077. In addition, the transcript level of putative N-carbamoylputrescine amidohydrolase (sll0601) in ∆sll1077 strain under 700 mM sorbitol condition was up-regulated whereas down-regulation of this sll0601 transcript was observed in ∆sll0228 strain. The 700 mM sorbitol condition also induced transcript levels of sll0873 encoding carboxynorspermidine decarboxylase in all strains. Altogether, our findings are indicated that agmatinase has an important role for Synechocystis stress response and adaptation although it is not the key enzyme of polyamine-biosynthetic pathway. The lack of putative agmatinase genes, sll1077 and sll0228, did not harm to cell growth but indirectly enhanced other adaptive mechanisms, such as norspermidine production and gene expression of putative N-carbamoylputrescine amidohydrolase.