A symbiotic mutant of Sinorhizobium meliloti reveals a novel genetic pathway involving succinoglycan biosynthetic functions.

A large-scale screen for symbiotic mutants was carried out using the model root nodulating bacterium Sinorhizobium meliloti. Several mutations in the previously uncharacterized gene msbA2 were isolated. msbA2 encodes a member of the ATP-binding cassette exporter family. This protein family is known to export a wide variety of compounds from bacterial cells. S. meliloti MsbA2 is required for the invasion of nodule tissue, with msbA2 mutant cells stimulating nodule primordium morphogenesis, but failing to invade plant tissue beyond the epidermal cell layer. msbA2 mutants do not exhibit any of the free-living traits often found to correlate with symbiotic defects, suggesting that MsbA2 may take part in a specifically symbiotic function. In strains that overproduce the symbiotic signalling polysaccharide succinoglycan, loss of MsbA2 function is extremely deleterious. This synthetic lethal phenotype can be suppressed by disrupting the succinoglycan biosynthetic genes exoY or exoA. It can also be suppressed by disrupting putative glycosyltransferase-encoding genes found upstream of msbA2. Finally, the symbiotic phenotype of a msbA2 null mutant is suppressed by secondary mutations in these upstream transferase genes, indicating that the msbA2 mutant phenotype may be caused by an inhibitory accumulation of a novel polysaccharide that is synthesized from succinoglycan precursors.