Disaccharides as a new class of nonaccumulated osmoprotectants for Sinorhizobium meliloti

Sucrose and ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidine carboxylic ac id) are very unusual osmoprotectants for Sinorhizobium meliloti because the se compounds, unlike other bacterial osmoprotectants, do not accumulate as cytosolic osmolytes in salt-stressed S. meliloti cells. Here, we show that, in fact, sucrose and ectoine belong to a new family of nonaccumulated sino rhizobial osmoprotectants which also comprises the following six disacchari des: trehalose, maltose, cellobiose, gentiobiose, turanose, and palatinose. Also, several of these disaccharides were very effective exogenous osmopro tectants for strains of Rhizobium leguminosarum biovars phaseoli and trifol ii. Sucrose and trehalose are synthesized as endogenous osmolytes in variou s bacteria, but the other five disaccharides had never been implicated befo re in osmoregulation in any organism. All of the disaccharides that acted a s powerful osmoprotectants in S. meliloti and R. leguminosarum also acted a s very effective competitors of [C-14]sucrose uptake in salt-stressed cultu res of these bacteria. Conversely, disaccharides that were not osmoprotecti ve for S. meliloti and R. leguminosarum did not inhibit sucrose uptake in t hese bacteria. Hence, disaccharide osmoprotectants apparently shared the sa me uptake routes in these bacteria. Natural-abundance C-13 nuclear magnetic resonance spectroscopy and quantification of cytosolic solutes demonstrate d that the novel disaccharide osmoprotectants were not accumulated to osmot ically significant levels in salt-stressed S. meliloti cells; rather, these compounds, like sucrose and ectoine, were catabolized during early exponen tial growth, and contributed indirectly to enhance the cytosolic levels of two endogenously synthesized osmolytes, glutamate and the dipeptide N-acety lglutaminylglutamine amide. The ecological implication of the use of these disaccharides as osmoprotectants is discussed.