NaCl acts as a direct modulator in the salt adaptive response: Salt-dependent activation of glucosylglycerol synthesis in vivo and in vitro

The formation of glucosylglycerol (GG) was studied in dependence on the NaCl concentration in vivo after moderate salt shocks of cells and in vitro after enzyme activation in cell extracts of Synechocystis sp. PCC 6803. After salt shock treatments between 342 and 684 mmol/L NaCl GG was accumulated with about the same rate without a lag-phase. In vitro enzyme assays showed that the GG synthesizing enzyme system was activated immediately after a shock of 342 mmol/L NaCl. The amount of these enzymes was further enhanced about 3 fold within the first 3 hours of the adaptation process. Only the latter increase of enzyme activity could be inhibited by chloramphenicol treatment indicating an immediate activation of preformed enzymes combined with a weak activation of gene expression of the GG synthesizing enzymes. In vitro a 15 fold increase of the activity of the GG synthesizing enzymes were achieved by adding NaCl in the concentration range of 171 to 342 mmol/L to protein extracts. In contrast to the remarkable high salt tolerance of the GG-phosphate synthase (GGP-S) in vitro the GG-phosphate phosphatase was fairly inhibited in the presense of higher NaCl concentrations. Their salt tolerance in vitro did not change regardless whether the enzymes were extracted from control or NaCl-adapted cells. Ic is assumed that the reversible modulation of the GGP-S activity by NaCl may function as a control mode in other living cells, too.