OSMOTICALLY INDUCED RESPONSE IN REPRESENTATIVES OF HALOPHILIC PROKARYOTES - THE BACTERIUM HALOMONAS-ELONGATA AND THE ARCHAEON HALOFERAX-VOLCANII

Haloferax volcanii and Halomonas elongata have been selected as repres entatives of halophilic Archaea and Bacteria, respectively, to analyze the responses to various osmolaritres at the protein synthesis level. We have identified a set of high-salt-related proteins (39, 24, 20, a nd 15.5 kDa in H. elongata; 70, 68, 48, and -16 kDa in H. volcanii) wh ose synthesis rates increased with increasing salinities. A different set of proteins (60, 42, 15, and 6 kDa for H. elongata; 63, 44, 34, 18 , 17, and 6 kDa for H. volcanii), some unique for low salinities, was induced under low-salt conditions. For both organisms, and especially for the haloarchaeon, adaptation to low-salt conditions involved a str onger and more specific response than adaptation to high-salt conditio ns, indicating that unique mechanisms may have evolved for low-salinit y adaptation. In the case of H. volcanii, proteins with a typical tran sient response to osmotic shock, induced by both hypo-and hyperosmotic conditions, probably corresponding to described heat shock proteins a nd shelving the characteristics of general stress proteins, have also been identified. Cell recovery after a shift to low Salinities was imm ediate in both organisms. In contrast, adaptation to higher salinities in both cases involved a lag period during which growth and general p rotein synthesis were halted, although the high-salt-related proteins were induced rapidly. In H. volcanii, this lag period corresponded exa ctly to the time needed for cells to accumulate adequate intracellular potassium concentrations, white extrusion of potassium after the down -shift was immediate. Thus, reaching osmotic balance must be the main limiting factor for recovery of tell functions after the variation in salinity.