Abstract
Phytochromes are well-known photoreceptors mediating photomorphogenesis in plants. Recently, distinct homologs of plant phytochromes have been found in several bacteria. These proteins covalently bind a linear tetrapyrrole as chromophore, and show red/fared reversible photoconversion. pixJ1 was found as one of the genes necessary for positive phototaxis in a unicellular cyanobacterium Synechocystis sp. PCC 6803. PixJ1 has a similar amino acid sequence to chromophore-binding regions of phytochromes. His-PixJ1 protein purified from Synechocystis cells covalently bound an unknown linear tetrapyrrole and showed the novel blue (λAmax=435 nm)/green (λAmax=535 nm) reversible photoconversion. In this study, the assemblies of the chromophore-binding domain of PixJ1 with several linear tetrapyrroles and their absorption spectral properties were studied. Assembly with phycocyanobilin (PCB) well simulated the blue/green photoconversion, although PCB-bound cyanobacterial phytochrome, Cph1, is known to show a red/far-red reversible photoconversion. We will discuss the possible involvements of PCB to the blue/green photoconversion of PixJ1.
