Analysis of membrane protein interaction: ToxR can dimerize the amino terminus of phage lambda repressor

The ToxR protein of Vibrio cholerae is an integral membrane protein that co-ordinately regulates virulence determinant expression. ToxR directiy activates the cholera toxin operon, but maximal activation is achieved in the presence of ToxS, an integral membrane protein thought to interact with ToxR periplasmic sequences. Studies that substitute alkaline phosphatase sequences for the periplasmic domain of ToxR have led to a model for ToxR activation based on dimerization and ToxS interaction. We constructed λ-ToxR chimeric proteins using the DNA-binding domain of the phage λ repressor, which cannot effectively dimerize by itself, to assess the ability of ToxR to form dimers in Escherichia coli The results suggest that ToxR sequences can propagate dimerization, and that ToxS can influence the ability to dimerize.     

Identification of a regulatory protein required for pressure-responsive gene expression in the deep-sea bacterium Photobacterium species strain SS9

Here, we report the characterization of a gene necessary for hydrostatic pressure regulation of gene expression in the deep-sea bacterium Photobacterium species strain SS9. The deduced amino acid sequence of the gene product shares extensive similarity to ToxR, a transmembrane DNA-binding protein first discovered as a virulence determinant in the pathogenic bacterium Vibrio cholerae . Changes in hydrostatic pressure induce changes in both the abundance and the activity of the SS9 ToxR protein (or the activity of a ToxR-regulated protein). As with other high-pressure-inducible phenomena observed in higher organisms, anaesthetics antagonize high-pressure signalling mediated by ToxR. It is suggested that SS9 ToxR has evolved the ability to respond to pressure-mediated alterations in membrane structure. V. cholerae and SS9 also share similarity in a ToxR-regulated protein, indicating that part of the ToxR regulon is conserved in diverse members of the family Vibrionaceae. The SS9 ToxR system represents a useful model for studies of signal transduction and environmental adaptation in the largest portion of the biosphere, the deep sea.