A Second Photochromic Bacteriophytochrome from Synechocystis sp. PCC 6803: Spectral Analysis and Down-Regulation by Light

Abstract

It now appears that photosynthetic prokaryotes and lower eukaryotes possess higher plant phytochrome-like proteins. In this work, a second phytochrome-like gene was isolated, in addition to the recently identified Cph1 phytochrome, from the Synechocystis sp. PCC 6803, and its gene product was characterized photochemically. The open reading frame sll0821 (designated cph2 in this work) has structural characteristics similar to those of the plant phytochromes and the Synechocystis Cph1 with high amino acid sequence homology in the N-terminal chromophore binding domain. The predicted Cph2 protein consists of 1276 amino acids with a calculated molecular mass of 145 kDa. Interestingly, the Cph2 protein has two putative chromophore binding domains, one around Cys-129 and the other around Cys-1022. The Cph2 was overexpressed in E. coli as an Intein/CBD (chitin binding domain) fusion and in vitro reconstituted with phycocyanobilin (PCB) or phytochromobilin (PΦB). Both the Cph2−PCB and Cph2−PΦB adducts showed the typical photochromic reversibility with the difference spectral maxima at 643/690 and 655/701 nm, respectively. The Cys-129 was confirmed to be the chromophore binding residue by in vitro mutagenesis and Zn²⁺ fluorescence. The microenvironment of the chromophore in Cph2 seems to be similar to that in plant phytochromes. The cph2 gene expression was dark-induced and down-regulated to a basal level by light, like the cph1 gene. These observations suggest that Synechocystis species have multiple photosensory proteins, probably with distinct roles, as in higher plants.