Abstract
Phytochromes are well-known photoreceptors mediating photomorphogenetic responses in plants. Plant phytochromes and bacteriophytochromes covalently bind a linear tetrapyrrole as a chromophore, and show red/far-red reversible photoconversion. PixJ1 is a phytochrome-like photoreceptor necessary for positive phototaxis in a unicellular cyanobacterium Synechocystis sp. PCC 6803. His-tagged PixJ1 protein purified from the Synechocystis cells covalently bound an unknown linear tetrapyrrole and showed a novel blue (λAmax= 435 nm)/green (λAmax= 535 nm) reversible photoconversion. Assembly with red light-absorbing chromophore phycocyanobilin (PCB) well simulated the blue/green photoconversion, indicating that a PCB or PCB-like chromophore binds to PixJ1. In this study, site-directed mutations were introduced at the potential chromophore-binding site(s) to investigate the manner of the blue-shifted photoconversion. Mutation at the either conserved Cys or His residue bound a tetrapyrrole covalently, however, were deficient in reversible photoconversion. We will discuss the possible involvements of PCB to the blue/green photoconversion of PixJ1.
