The Sin quorum sensing system of Sinorhizobium meliloti depends upon at least three genes, sinR, sinI and expR, and N-acyl homoserine lactones (AHLs) as signals to regulate multiple processes in its free-living state in the rhizosphere and in the development towards symbiosis with its plant host. In this study, we have characterized novel mechanisms of transcription control through which the system regulates itself. At low AHL levels a positive feedback loop activates expression of sinI (AHL synthase), resulting in amplification of AHL levels. At high AHL levels, expression of sinI is reduced by a negative feedback loop. These feedback mechanisms are mediated by the LuxR-type regulators ExpR and SinR. Expression of sinR and expR is regulated by ExpR in the presence of AHLs. A novel ExpR binding site in the promoter of sinR is responsible for the reduction of expression of this gene. In addition, expression of sinR, upon which sinI expression is dependent, is induced by phoB during growth under phosphate-limiting conditions. This indicates that this response ensures quorum sensing in phosphate-restricted growth.