Regulation of pssA and pssB gene expression in Rhizobium leguminosarum bv. trifolii in response to environmental factors.

Exopolysaccharide (EPS) produced by Rhizobium leguminosarum plays an important role in indeterminate-type nodulation. The pssA gene, encoding the first glycosyl transferase and pssB, encoding inositol monophosphatase, are involved in EPS synthesis and regulation in R. leguminosarum bv. trifolii. Using pssA-lacZ and pssB-lacZ plasmid-borne transcriptional fusions, a stimulating effect of phosphate on the transcription of both genes was observed. The increase in the transcription of pssA and pssB genes was correlated with a significant enhancement in EPS production in rhizobia grown in the high phosphate concentrations. In the presence of ammonium, transcription of the pss-lacZ fusions was repressed, especially the pssB-lacZ fusion, but EPS production still remained enhanced. Root exudate positively influenced the level of pss-lacZ transcription. These results indicate a complex regulation of pss gene transcription by several environmental factors. In addition, the expression of pssA and pssB genes in the symbiotic state was examined using pss-gusA plasmid-borne transcriptional fusions. It was found that the pssA-gusA fusion was not expressed in the nodules. The pssB-gusA fusion was expressed mainly in the infection zone and interzone of the nodule and the pattern of expression was dependent on the plant phosphate availability.