The psi operon of Rhizobium leguminosarum biovar phaseoli: identification of two genes whose products are located at the bacterial cell surface

We have delineated three short open reading frames, psiA, ORF-P and psiB within the psi operon of Rhizobium leguminosarum biovar phaseoli. psiA, in a multi-copy plasmid, causes inhibition of exopolysaccharide synthesis in R. leguminosarum. In addition, the suppression of exopolysaccharide synthesis due to the multi-copy psiA caused R. leguminosarum strains to stain with the dye calcofluor, a response that does not occur with wild-type strains of this species. Insertions of a defective phoA gene (lacking its promoter, ribosomal binding site and leader sequence) into psiA and psiB were isolated and the precise locations of the insertions were established. PsiA-PhoA and PsiB-PhoA protein fusions were found to express alkaline phosphatase activity indicating that PsiA and PsiB span the inner membrane or are translocated across it.