Phytochrome-like PixJ1 holoprotein from Synechocystis shows novel reversible photoconversion between blue- and green-absorbing Forms

Abstract

The gene, pixJ1, is predicted to encode a phytochrome-like photoreceptor for the positive phototaxis in the unicellular cyanobacterium Synechocystis sp. PCC 6803. Previously, we reported that PixJ1 protein, which was overexpressed as a fusion with His-tag and isolated from Synechocystis cells, covalently bound a linear tetrapyrrole like phytochromes. In the present study, PixJ1 showed novel photoreversible conversion between a blue light-absorbing form (P_b, λ_Amax = 435 nm) and a green light-absorbing form (P_g , λ_Amax = 535 nm). Red light irradiation, which is effective for the photochemical changes of known phytochromes, gave no obvious change in the spectrum. Phycocyanobilin, the chromophore of cyanobacterial phytochrome Cph1 (λ_Amax = 656 nm for P_r and λ_Amax = 706 nm for P_fr), is unlikely to attach to PixJ1. To characterize the chromophore species, we searched possible genes responsible for the chromophore biogenesis and disrupted them. The chromophore attached to PixJ1 will be will discussed.