OSMOADAPTATION IN RHIZOBIA - ECTOINE-INDUCED SALT TOLERANCE

After having shown that ectoine (a tetrahydropyrimidine) displays osmo protective properties towards Escherichia coil (M. Jebbar, R. Talibart , K. Gloux, T. Bernard, and C. Bianco, J. Bacteriol. 174:5027-5035, 19 92), we have investigated the involvement of this molecule in the osmo tic adaptation of Rhizobium meliloti. Ectoine appeared almost as effec tive as glycine betaine in improving the growth of R. meliloti under a dverse osmotic conditions (0.5 M NaCl). Moreover, improvement of growt h of rhizobial strains insensitive to glycine betaine was also observe d. Ectoine transport proved inducible, periplasmic protein dependent, and, as shown by competition experiments, distinct from the transport of glycine betaine. Medium osmolarity had little effect on the uptake characteristics, since the rate of influx increased from 12 to only 20 nmol min(-1) mg of protein(-1) when NaCl concentrations were raised f rom 0 to 0.3 or 0.5 M, with a constant of transport of 80 mu M. Natura l-abundance C-13-nuclear magnetic resonance and radiolabelling assays showed that ectoine, unlike glycine betaine, is not intracellularly ac cumulated and, as a consequence, does not repress the synthesis of end ogenous compatible solutes (glutamate, N-acetylglutaminylglutamine ami de, and trehalose). Furthermore, the strong rise in glutamate content in cells osmotically stressed in the presence of ectoine suggests that , instead of being involved in osmotic balance restoration, ectoine sh ould play a key role in triggering the synthesis of endogenous osmolyt es. Hence, we believe that there are at least two distinct classes of osmoprotectants: those such as glycine betaine or glutamate, which act as genuine osmolytes, and those such as ectoine, which act as chemica l mediators.