Competitive and Cooperative Effects in Quorum-Sensing-Regulated Galactoglucan Biosynthesis in Sinorhizobium meliloti

The symbiotic nitrogen-fixing bacteriumSinorhizobium meliloti possesses the Sin quorum-sensing system based onN -acyl homoserine lactones (AHLs) as signal molecules. The Sin system consists of SinI, the AHL synthase, and SinR, the LuxR-type regulator. This system regulates the expression of a multitude ofS. meliloti genes through ExpR, another LuxR-type regulator. Analysis of the activity of thesinI promoter showed that the expression ofsinI is dependent onsinR and enhanced by a combination ofexpR and Sin AHLs. The characterization of the ExpR binding site upstream ofsinI and the identification of binding sites upstream of the galactoglucan biosynthesis geneswgaA (expA1 ) andwgeA (expE1 ) allowed the definition of a consensus sequence for these binding sites. Based on this consensus, two additional ExpR binding sites in the promoter regions ofexoI andexsH , two genes related to the production of succinoglycan, were found. The specific binding of ExpR to thewgaA andwgeA promoters was enhanced in the presence of oxo-C14 -HL. Positive regulation of the galactoglucan biosynthesis genes by ExpR was shown to be dependent on WggR (ExpG) and influenced by MucR, both of which are previously characterized regulators of these genes. Based on these results, a reworked model of the Sin-ExpR quorum-sensing regulation scheme of galactoglucan production inS. meliloti is suggested.