The Chaperone GroESL Enhances the Accumulation of Soluble, Active TraR Protein, a Quorum-Sensing Transcription Factor fromAgrobacterium tumefaciens

TraR ofAgrobacterium tumefaciens is a LuxR-type quorum-sensing transcription factor that regulates genes required for replication and conjugation of the tumor-inducing (Ti) plasmid. TraR requires its cognate autoinducerN -3-oxooctanoyl-homoserine lactone (OOHL) for resistance of proteolysis in wild-type bacteria and for correct protein folding and solubility when overexpressed inE. coli . In this study, we ask whether GroESL might also play a role in TraR folding, as this molecular chaperone assists many proteins in attaining their native tertiary structure. Expression ofE. coli GroESL in a strain expressing TraR increases the solubility of TraR and increases transcriptional activity of a TraR-dependent promoter. Both solubility and activity still require OOHL. We also studied the folding of TraR in the closely related bacteriumSinorhizobium meliloti . A mutation in onegroEL gene slightly decreased the expression of a TraR-dependent promoter, strongly decreased the accumulation of TraR in Western immunoblot assays, and also strongly influenced the fate of pulse-labeled TraR.