Omeostasi e signalling del calcio in batteri azotofissatori (Rhizobium) simbionti di leguminose.

The involvement of a Ca2+-binding protein in the regulation of intracellular free Ca2+ concentration has been recently demonstrated in the cyanobacterium Anabaena during heterocyst differentiation. Within a project aimed at unraveling the participation of Ca2+ ions in rhizobia during the early stages of their symbiotic interactions with legumes, we checked in the nitrogen-fixing bacterium Mesorhizobium loti the possible occurrence of Ca2+ buffering proteins as a component of the bacterial Ca2+ homeostat. As no sequence homologous to that encoding the Ca2+-binding protein of the cyanobacterium Anabaena was found in the Mesorhizobium loti genoma DataBase, a biochemical approach was carried out. Evidence was provided for a 20 kDa soluble protein which was found to share with eukaryotic calsequestrin and calreticulin several biochemical features: i) purification by an ammonium sulphate precipitation procedure followed by anion-exchange chromatography; ii) an acidic isoelectric point (pI 4.2); iii) Stains All blue staining on SDS-PAGE; iiii) Ca2+ dependent mobility shift.