Protein and lipopolysaccharide profiles of a salt-sensitive Rhizobium sp. and its exopolysaccharide-deficient mutant

A fast-growing, salt-sensitive rhizobium (Rhizobium sp. ST1) with a narrow host range of infectivity was isolated from the root nodules of locally grown pigeonpea (Cajanus cajan). One of the Tn5 mutants of Rhizobium sp. ST1 was exopolysaccharide (EPS) deficient (exo−) and showed a 50% growth inhibition (GI 50) at 350 mM NaCl, compared to the GI50 value of the wild type strain at 250 mM NaCl. Whole cell protein profiles of the wild type in the presence of NaCl showed an overall increase in the levels of several proteins (22, 38, 68, >97 kDa), whereas in its exo− mutant, certain low molecular weight outer membrane proteins (38 and 22 kDa) decreased. Other outer membrane proteins (22, 38, 40, 42,62 and 68 kDa) also markedly decreased in both the wild type and the exo− mutant in the presence of salt. Similarly, both the wild type and the exo− mutant showed decreased levels of both the lipopolysaccharide (LPS) components (LPS I and LPS II) in the presence of NaCl. These observations suggest the possible involvement of the outer membrane components, along with other factors, during growth under salt stress, in both salt-sensitive and relatively salt-tolerant strains of rhizobia.