A STOMATIN-LIKE PROTEIN ENCODED BY THE SIP GENE OF RHIZOBIUM-ETLI IS REQUIRED FOR NODULATION COMPETITIVENESS ON THE COMMON BEAN

Rhizobium etli strain TAL182 is a competitive strain for effective nod ulation of beans. From this strain, a novel gene was isolated, sip, wh ich is 669 bp in size and required for nodulation competition on the c ommon bean. The sip knockout mutant of TAL182 is defective in nodulati on competition, shows reduced growth in the presence of 200 mM NaCl, K Cl or LiCl and is complemented by the cloned sip gene. The deduced ami no acid sequence of sip shows 66-72% similarity to stomatin proteins o f Homo sapiens, Mus musculus and Caenorhabditis elegans. Expression of sip in Escherichia coil from a T7 promoter shows a 26 kDa protein whi ch cross-reacts with human-stomatin-specific polyclonal antibody. Like the human stomatin protein, the sip-deduced protein, Sip, is very hyd rophilic except for a single hydrophobic membrane-spanning domain. Amo ng various bean-nodulating rhizobia, sip is present in R. etli, Rhizob ium leguminosarum by. phaseoli and Rhizobium tropici type A strains bu t is absent in R. tropici type B strains. It is also absent in Bradyrh izobium and several other Rhizobium spp.