Salt tolerance of a Sinorhizobium meliloti strain isolated from dry lands: growth capacity and protein profile changes

Sinorhizobium meliloti strain A5, isolated from root nodules of Medicago truncatula growing spontaneously in Tunisian dry lands, showed better growth performance under different sodium chloride concentrations than the standard reference strain RCR2011. Strain A5 has the ability to grow even at 684 mM NaCl. Two-dimensional (2D) electrophoretic analysis of the salt effect on proteome profiles revealed the greatest change on the 4th day of exposure. The halotolerant strain A5 showed more upregulated protein spots than the standard RCR2011 strain (23% of the analyzed protein spots vs only 10% in strain RCR2011) and fewer downregulated protein spots (28% vs 69%). Proteins over-expressed in salty medium as determined by Matrix-assisted laser desorption ionization time-of-flight mass spectrometry for both strains were identified as an outer-membrane protein, an l-amino-acid binding periplasmic transporter protein, a 50S ribosomal protein (L25), a heat shock protein, and a putative oxido-reductase protein. Down-regulated proteins were identified as a putative iron-binding periplasmic transporter protein, a putative lipoprotein precursor, a putative spermidine/putrescine-binding periplasmic transporter protein, and a putative amino-acid binding ABC transporter protein. The profile of strain A5 showed higher over-expression of an oxidoreductase and a heat-shock protein, and specifically an induction of a putative oligopeptide ABC transporter ATP-binding protein.