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. We have already reported that PixJ1 protein covalently bound a linear tetrapyrrole phycocyanobilin (PCB) and showed a novel blue (λAmax= 435 nm)/green (λAmax= 535 nm) reversible photoconversion. In this study, the interconvertible photoreactions of PixJ1 were investigated by light-induced UV-Vis difference spectroscopy at low temperature. The photoreactions of a PCB-bound cyanobacterial phytochrome, Cph1, were also studied. For the photoconversion of Cph1 Pr to Pfr, an intermediate lumi-R was observed at 133 K, while the photoconversion of PixJ1 from Pb to Pg showed a lumi-R-like intermediate below 77 K. Above 250 K, Pg form was obtained. We will discuss the possible photoreactions of PixJ1.
