Characterization of the Cph1 holo-phytochrome from Synchocystis sp PCC 6803

The cph1 gene from the unicellular cyanobacterium Synechoycstis sp. PCC 680 3 encodes a protein with the characteristics of plant phytochromes and hist idine kinases of two-component phospho-relay systems. Spectral and biochemi cal properties, of Cph1 have been intensely studied in vitro using protein from recombinant systems, but virtually nothing is known about the situatio n in the natural host. In the present study, His(6)-tagged Cph1 was isolated from Synechocystis ce lls. The cph1-his gene was expressed either under the control of the natura l cph1 promoter or over-expressed using the strong promoter of the psbA2 ge ne. Upon purification with nickel affinity chromatography, the presence of Cph1 in extracts was confirmed by immunoblotting and Zn2+-induced fluoresce nce. The Cph1 extracts exhibited a red/far-red photoactivity characteristic of phytochromes. Difference spectra were identical with those of the phyco cyanobilin adduct of recombinant Cph1, implying that phycocyanobilin is the chromophore of Cph1 in Synechocystis.