TRANSIENT ACCUMULATION OF GLYCINE BETAINE AND DYNAMICS OF ENDOGENOUS OSMOLYTES IN SALT-STRESSED CULTURES OF SINORHIZOBIUM-MELILOTI

The fate of exogenously supplied glycine betaine and the dynamics of e ndogenous osmolytes were investigated throughout the growth cycle of s alt-stressed cultures of strains of Sinorhizobium meliloti which diffe r in their ability to use glycine betaine as a growth substrate, but n ot as an osmoprotectant. We present C-13 nuclear magnetic resonance sp ectral and radiotracer evidence which demonstrates that glycine betain e is only transiently accumulated as a cytoplasmic osmolyte in young c ultures of wild-type strains 102F34 and RCR2011. Specifically, these s trains accumulate glycine betaine as a preferred osmolyte which virtua lly prevents the accumulation of endogenous osmolytes during the lag a nd early exponential phases of growth. Then, betaine levels in stresse d cells decrease abruptly during the second half of the exponential ph ase. At this stage, the levels of glutamate and the dipeptide N-acetyl glutaminylglutamine amide increase sharply so that the two endogenous solutes supplant glycine betaine in the ageing culture, in which it be comes a minor osmolyte because it is progressively catabolized. Ultima tely, glycine betaine disappears when stressed cells reach the station ary phase. At this stage, wild-type strains of S. meliloti also accumu late the disaccharide trehalose as a third major endogenous osmolyte. By contrast, glycine betaine is always the dominant osmolyte and stron gly suppresses the buildup of endogenous osmolytes at all stages of th e growth cycle of a mutant strain, S. meliloti GMI766, which does not catabolize this exogenous osmoprotectant under any growth conditions.