LOW-MOLECULAR-WEIGHT SUCCINOGLYCAN IS PREDOMINANTLY PRODUCED BY RHIZOBIUM-MELILOTI STRAINS CARRYING A MUTATED EXOP PROTEIN CHARACTERIZED BYA PERIPLASMIC N-TERMINAL DOMAIN AND A MISSING C-TERMINAL DOMAIN

The membrane topology of the Rhizobium meliloti 2011 ExoP protein invo lved in polymerization and export of succinoglycan was analysed by tra nslational fusions of lacZ and phoA reporter genes to the exoP gene. B ased on this analysis, the ExoP protein could be divided into an N-ter minal domain mainly located in the periplasmic space and a C-terminal domain located in the cytoplasm. Whereas the C-terminal domain of ExoP is characterized by a potential nucleotide-binding motif, the N-termi nal ExoP domain contains the sequence motif 'PX(2)PX(4)SPKX(11)GXMXG', which is also present in proteins involved in the determination of O- antigen chain length. R. meliloti strains carrying mutated exoP genes , exclusively encoding the N-terminal ExoP domain, produced a reduced amount of succinoglycan. This reduction could be suppressed by a mutat ion in the regulatory gene exoR. The ratio of low-molecular-weight to high-molecular-weight succinoglycan was significantly increased in the exoP mutant strain. In the exoP*/exoR mutant strain only low-molecul ar-weight succinoglycan could be detected. Based on sequence homologie s and similar hydropathic profiles, the N-terminal domain of ExoP was proposed to be a member of a protein family thought to be involved in polysaccharide chain-length determination.