NMR-STUDIES ON NA+ TRANSPORT IN SYNECHOCOCCUS PCC-6311

The freshwater cyanobacterium Synechococcus PCC 6311 is able to adapt to grow after sudden exposure to salt (NaCl) stress. We have investigated the mechanism of Na+ transport in these cells during adaptation to high salinity. Na+ influx under dark aerobic conditions occurred independently of ΔpH or Δψ across the cytoplasmic membrane, ATPase activity, and respiratory electron transport. These findings are consistent with the existence of Na+/monovalent anion cotransport or simultaneous Na+/H+ + anion/OH- exchange. Na+ influx was dependent on Cl-, Br-, NO3 -, or NO2 -. No Na+ uptake occurred after addition of NaI, NaHCO3, or Na2SO4. Na+ extrusion was absolutely dependent on ΔpH and on an ATPase activity and/or on respiratory electron transport. This indicates that Na+ extrusion via Na+ H+ exchange is driven by primary H+ pumps in the cytoplasmic membrane. Cells grown for 4 days in 0.5 m NaCl medium, "salt-grown cells," differ from control cells by a lower vmax of Na+ influx and by lower steady-state ratios of [Na+]in [Na+]out. These results indicate that cells grown in high-salt medium increase their capacity to extrude Na+. During salt adaptation Na+ extrusion driven by respiratory electron transport increased from about 15 to 50%. © 1992.