Novel Photosensory Two-Component System (PixA-NixB-NixC) Involved in the Regulation of Positive and Negative Phototaxis of Cyanobacterium Synechocystis sp PCC 6803

Two wild-type substrains of a motile cyanobacterium Synechocystis sp. PCC 6803 show positive phototaxis toward a light source (PCC-P) and negative phototaxis away from light (PCC-N). In this study, we found that a novel two-component system of photoresponse is involved in the phototactic regulation. Inactivation of slr1212 (pixA), which encodes a photoreceptor histidine kinase, reverted the positive phototaxis of PCC-P to negative phototaxis, and inactivation of the downstream slr1213 (nixB) and slr1214 (nixC), which encode AraC-like transcription factor-type and PatA-type response regulators, respectively, reverted the negative phototaxis of PCC-N to positive phototaxis. Opposite effects of pixA and nixBC disruption implies an unexpected signal transduction pathway in the switching of positive and negative phototaxis. The blue/green-type cyanobacteriochrome GAF domain of PixA was expressed in Synechocystis and phycocyanobilin-producing Escherichia coli. The holoprotein covalently bound a chromophore phycoviolobilin and showed reversible photoconversion between the violet- (Pv, lambda(peak) = 396 nm) and green-absorbing (Pg, lambda(peak) = 533 nm) forms, although the protein from E. coli partially bound a precursor phycocyanobilin. These results were discussed with regard to an idea that PixA serves as a violet light receptor for switching of positive and negative phototaxis by transcriptional and functional regulation.