CHEMOSENSORY AND PHOTOSENSORY PERCEPTION IN PURPLE PHOTOSYNTHETIC BACTERIA UTILIZE COMMON SIGNAL-TRANSDUCTION COMPONENTS

The chemotaxis gene cluster from the photosynthetic bacterium Rhonospi rillum centenum contains five open reading frames (ORFs) that have sig nificant sequence homology to chemotaxis genes from other bacteria. To elucidate the functions of each ORF, we have made various mutations i n the gene cluster and analyzed their phenotypic defects, Deletion of the entire che operon (Delta che), as well as nonpolar disruptions of cheAY, cheW, and cheR, resulted in a smooth-swimming phenotype, wherea s disruption of cheB resulted in a locked tumbly phenotype. Each of th ese mutants was defective in chemotactic response. Interestingly, disr uption of cheY resulted in a slight increase in the frequency of tumbl ing/reversal with no obvious defects in chemotactic response. In contr ast to observations with Escherichia coli and several other bacteria, we found that all of the che mutant cells were capable of differentiat ing into hyperflagellated swarmer cells when plated on a solid agar su rface. When viewed microscopically, the smooth-swimming che mutants ex hibited active surface motility but were unable to respond to a step-d own in light intensity, Both positive and negative phototactic respons es were abolished in all che mutants, including the cheY mutant, These results indicate that eubacterial photosensory perception is mediated by light-generated signals that are transmitted through the chemotaxi s signal transduction cascade.