Abstract
A unicellular cyanobacterium Synechocystis sp. PCC 6803 shows positive and negative phototaxis toward or away from the light source, respectively, via pili. Previously, we reported that the phytochrome-like photoreceptor, PixJ1 (formerly PisJ1), is involved in positive phototaxis of Synechocystis. In the present study, to examine the spectroscopic properties, histidine-tagged PixJ1 was expressed and isolated from Synechocystis cells. SDS-PAGE of the purified His-PixJ1 showed Zn2+-enhanced fluorescence, indicative of covalent binding of linear tetrapyrrole. Native His-PixJ1 showed maximum fluorescence at 640 nm and maximum excitation at 580 nm. Notably, the excitation peak of PixJ1 was significantly shorter in wavelength than that of the cyanobacterial phytochrome Cph1. It is speculated that PixJ1 binds phycoerythrobilin. However, Synechocystis genome does not apparently harbor homologs for the enzymes responsible for phycoerythrobilin biosynthesis. We will discuss a possible chromophore attached to PixJ1 and a role of PixJ1 in phototaxis in Synechocystis.
