Bacteria sense the presence of each other through the production and detection of pheromones. Agrobacterium tumefaciens , a plant pathogen, produces Agrobacterium autoinducer (AAI), which is a ligand for the transcription factor TraR. Zhu and Winans expressed native TraR and a TraR maltose binding protein fusion protein in E. coli and found that, in the absence of AAI, TraR cannot fold properly and is rapidly degraded by cytosolic proteases. Only TraR synthesized in the presence of AAI is stable and forms DNA-binding dimers that are active. Apo-TraR cannot form dimers, appears not to fold in a stable tertiary structure, and cannot be induced to fold by the addition of AAI after synthesis. Thus, TraR appears to be a receptor whose activity is controlled by its ligand, because only TraR synthesized in the presence of AAI folds properly and is stable. J. Zhu, S. C. Winans, The quorum-sensing transcriptional regulator TraR requires its cognate signaling ligand for protein folding, protease resistance, and dimerization. Proc. Natl. Acad. Sci. U.S.A. 98 , 1507-1512 (2001). [Abstract] [Full Text]