Environmental modulation of thepssTNOPgene expression inRhizobium leguminosarumbv.trifolii

Abstract

Exopolysaccharide production by Rhizobium leguminosarum bv. trifolii is required for successful establishment of nitrogen-fixing symbiosis with clover (Trifolium pratense L.). Using plasmid-borne transcriptional fusions of promoters of pss genes with promoterless lacZ the effect of root exudate, phosphate, and ammonia on expression of pssT, pssN, pssO, and pssP genes in wild-type strain RtTA1 background was determined. A stimulating effect of these environmental factors on pssO and pssP gene expression was observed. The putative pssO gene promoter was determined to be a strong promoter within which the divergent nod-box element was identified. The pssO promoter was slightly inducible in a flavonoid-dependent manner in wild-type R. leguminosarum bv. trifolii strains RtTA1 and ANU843 and very weakly active in a mutant of strain ANU843 that lacks the regulatory nodD gene. The expression of pssO and pssP genes in planta was investigated using plasmid-borne pssO–gusA and pssP–gusA fusions under different phosphate availability to clover. The level of pssO–gusA fusion expression was shown to be dependent on phosphate concentration in the plant growth medium.Key words: Rhizobium leguminosarum, exopolysaccharide, nod box, transcriptional fusion.