Abstract
Phytochromes are well-known photoreceptors mediating photomorphogenetic responses in plants. Plant phytochromes and bacteriophytochromes covalently bind a linear tetrapyrrole as a chromophore at a conserved GAF domain, 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. Co-expression of GAF domain of PixJ1 with red light absorbing open tetrapyrrole phycocyanobilin (PCB) well simulated the blue/green photoconversion, indicating that a PCB or PCB-like chromophore binds to PixJ1. It is suggested that a chromophore may be attached to the protein moiety in a twisted conformation that interrupts the π-electron conjugation.
The clustering analysis together with known phytochromes and phytochrome-related proteins classified PixJ1 into the different group from red/far-red absorbing phytochromes.
