It now appears that photosynthetic prokaryotes and lower eukaryotes possess higher plantphytochrome-like proteins. In this work, a second phytochrome-like gene was isolated, in addition to therecently identified Cph1 phytochrome, from the
Synechocystis sp. PCC 6803, and its gene product wascharacterized photochemically. The open reading frame
sll0821 (designated
cph2 in this work) has structuralcharacteristics sim
ilar to those of the plant phytochromes and the
Synechocystis Cph1 with high aminoacid sequence ho
mology in the N-terminal chro
mophore binding domain. The predicted Cph2 proteinconsists of 1276 amino acids with a calculated
molecular mass of 145 kDa. Interestingly, the Cph2 proteinhas two putative chro
mophore 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 vitroreconstituted with phycocyanob
ilin (PCB) or phytochro
mob
ilin (P
B). Both the Cph2-PCB and Cph2-P
B adducts showed the typical photochromic reversib
ility with the difference spectral maxima at 643/690 and 655/701 nm, respectively. The Cys-129 was confirmed to be the chro
mophore binding residueby in vitro mutagenesis and Zn
2+ fluorescence. The microenvironment of the chro
mophore in Cph2 seemsto be sim
ilar 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
Synechocystisspecies have multiple photosensory proteins, probably with distinct roles, as in higher plants.