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Plant Cell Physiol. 2011 Dec;52(12):2214-24. doi: 10.1093/pcp/pcr155. Epub 2011 Nov 7.

Novel photosensory two-component system (PixA-NixB-NixC) involved in the regulation of positive and negative phototaxis of cyanobacterium Synechocystis sp. PCC 6803.

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Department of Life Sciences (Biology), Graduate School of Art and Sciences, University of Tokyo, Komaba 3-8-1, Meguro, Tokyo, 153-8902 Japan.


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, λ(peak) = 396 nm) and green-absorbing (Pg, λ(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.

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