Production of complex extracellular polysaccharides (EPSs) by the nitrogen-fixing soil bacterium Sinorhizobium meliloti is required for efficient invasion of root nodules on the host plant alfalfa. Any one of three S. meliloti polysaccharides, succinoglycan, EPS II, or K antigen, can mediate infection thread initiation and extension (root nodule invasion) on alfalfa. Of these three polysaccharides, the only symbiotically active polysaccharide produced by S. meliloti wild-type strain Rm1021 is succinoglycan. The expR101 mutation is required to turn on production of symbiotically active forms of EPS II in strain Rm1021. In this study, we have determined the nature of the expR101 mutation in S. meliloti. The expR101 mutation, a spontaneous dominant mutation, results from precise, reading frame-restoring excision of an insertion sequence from the coding region of expR, a gene whose predicted protein product is highly homologous to the Rhizobium leguminosarum bv. viciae RhiR protein and a number of other homologs of Vibrio fischeri LuxR that function as receptors for N-acylhomoserine lactones (AHLs) in quorum-sensing regulation of gene expression. S. meliloti ExpR activates transcription of genes involved in EPS II production in a density-dependent fashion, and it does so at much lower cell densities than many quorum-sensing systems. High-pressure liquid chromatographic fractionation of S. meliloti culture filtrate extracts revealed at least three peaks with AHL activity, one of which activated ExpR-dependent expression of the expE operon.The soil bacterium Sinorhizobium meliloti fixes atmospheric dinitrogen to ammonia in symbiotic association with the host plant Medicago sativa (alfalfa). A successful symbiosis is the result of a complex series of interactions between the host and the symbiont (10,18,48,55,78). A broad range of plant compounds can function to influence the production of Nod factors by S. meliloti. Nod factors stimulate root hair curling and root nodule formation. S. meliloti cells colonize curled root hairs and invade developing root nodules via extended invaginations of the root hair cell membrane called infection threads. The S. meliloti cells are individually surrounded by host cell membrane and released into the host nodule cells, where they differentiate into bacteroids, the nitrogen-fixing form of the bacteria.Extracellular polysaccharides (EPSs) produced by S. meliloti are crucial for establishing a successful nitrogen-fixing symbiosis with alfalfa. S. meliloti mutants that are unable to produce symbiotically active polysaccharides are defective in nodule invasion and primarily induce the formation of symbiotically ineffective root nodules that are devoid of bacteria and bacteroids (34,45,62). Any one of three S. meliloti polysaccharides, succinoglycan, EPS II, or K antigen, can mediate alfalfa root nodule invasion. By using green fluorescent protein-expressing S. meliloti strains, we have recently demonstrated that each of these three polysaccharides functions to mediate infection thread ini...