Isolation of salt-sensitive mutants of Sinorhizobium meliloti strain Rm1021

Abstract
The determinants necessary for adaptation to high NaCl concentrations and competition for nodule occupancy inSinorhizobium melilotiwere investigated genetically. Mutations infabGas well assmc02909(transmembrane transglycosylase), trigger factor (tig) andsmc00717(probablyftsE) gave rise to strains that were unable to tolerate high salt and were uncompetitive for nodule occupancy relative to the wild-type. MoreoverexoF1,exoAandpgmdeterminants were determined to be necessary for strain Rm1021 to survive high NaCl and/or MgCl2concentrations. The introduction of anexpR+allele was capable of suppressing the Mg2+sensitivity associated with theexoF1, but not theexoA, mutation in a manner independent of exopolysaccharide II (EPS II)-associated mucoidy. The results also show that the EPS II-associated mucoid phenotype was affected by either Mg2+or K+, but not by Li+, Ca2+, or high osmolarity.

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