The molecular biology of chemotactic signal transduction in Rhodobacter sphaeroides
This study has succeeded in identifying, cloning and sequencing three genes previously unknown in R. sphaeroides, che W, che R and che Y2. These genes are homologous to genes recently discovered in Rhizobium meliloti and also show a lesser homology to genes in other organisms including the well characterised enteric chemotaxis genes. A comparative analysis of the deduced proteins has been made to determine structural and functional similarities between the R. sphaeroides genes and their homologues in R. meliloti and in E. coli. Considerable conservation of functionally important amino acid residues was revealed. In vivo complementation was done using the R. sphaeroides Che W protein to complement Che W- strain of E. coli. The R. sphaeroides protein complemented the E. coli strain empirically proving similarity of function of the protein between these widely divergent genera. Homologous recombination using transposon-interrupted genes and internal gene fragments was attempted and met with limited success because R. sphaeroides proved permissive to the suicide vectors used. The most significant result of this study has been not in the similarities revealed between the chemotaxis systems of R. sphaeroides and the enterics, but in the differences discovered. It is now known that R. sphaeroides possesses two Che Y proteins, Che Y and Che Y2, and that whereas the R. sphaeroides Che Y2 protein is closely related to the E. coli Che Y protein, the R. sphaeroides Che Y protein shows considerable evolutionary divergence from it's enteric homologue. The significance of a second copy of Che Y protein suggests that these two response regulators act independently at the motor/switch complex and that they represent the final elements of two functionally distinct chemotactic sensory transduction pathways. This dual pathway system is not present in the enteric bacteria (a member of the γ-group of proteobacteria) but, we propose, may be present in a large group of environmentally important bacteria, the α-group of proteobacteria. Caulobacter, Agrobacterium, Rhizobium and Azospirillum species (α-group proteobacteria) all show behavioural similarities, and there are genetic clues to suggest that these organisms possess a dual chemotactic sensory transduction similar to the one we have found in R. sphaeroides. Apart from being fundamentally important, the dual sensory pathway hypothesis explains the wild-type behaviour of R. sphaeroides, as well as the difficulty in obtaining behavioural mutant phenotypes using methods developed during the investigation of E. coli.