Expression of nhaAv gene encoding Na+/H+ antiporter from Vibrio alginolyticus in a freshwater cyanobacterium Synechococcus sp PCC 7942 confers lithium tolerance, but not sodium tolerance

Salt tolerance of a freshwater Synechococcus sp. PCC 7942 transformed with a shuttle vector, that contains the 1.4 kb fragment encoding a Vibrio alginolyticus Na+/H+ antiporter, was investigated. Northern blot and reverse transcript-PCR analysis showed the expression of the Vibrio nhaAv gene in Synechococcus. Increased activity of the Na+/H+ antiporter in the transformant was observed by using acridine orange fluorescence. In growth medium containing low concentrations of NaCl, the growth rate of the transformant was similar to that of the control cells, but it became lower under high concentrations of NaCl, indicating the sodium sensitivity of the transformant. Electron transport activities and the levels of ATP and Chi were lower in the transformant at high salinity. The intracellular Na+ level was 2.4 times higher in the transformant. Acridine orange fluorescence analysis of thylakoid membranes in Synechococcus cells revealed the functional expression of the Vibrio Na+/H+ antiporter in thylakoid membranes. E. coli cells transformed with Vibrio nhaAv exhibited sodium tolerance. The expression of the nhaAv gene conferred tolerance to LiCl although not to NaCl in Synechococcus cells. The homeostatic mechanisms regulating intracellular ion concentrations were discussed.