Structural and functional analysis of the cyanobacterial phytochrome Cph2 from Synechocystis sp.

Cph2 from Synechocystis sp. PCC 6803 is a non-canonical phytochrome which contains multiple GAF domains (GAF1 binds PCB) instead of the common PAS and PHY domains. It provides two GGDEF and one EAL domain for signaling that are supposed to be involved in c-di-GMP turnover.

We solved the structure of the two N-terminal GAF domains in the P_r form at 3.2 Å resolution. The overall structure resembles the GAF and PHY domain of known phytochrome structures including the α-helical linker between GAF1 and 2 and the tongue-like protrusion that covers the cofactor binding pocket.

Spectroscopic analysis of this photosensory module shows that the P_fr form is thermally instable. This and the large P_fr to P_r ratio at the photoequilibrium suggest that the P_fr state is the physiologically active conformation [1].

We generated a panel of mutants for analysing the signal transfer from the PCB binding site. P_fr formation and stability is altered in mutants of the tongue region and of amino acids surrounding the PCB D-ring, suggesting an involvement of the tongue region in signaling.

cph2 deletion mutants exhibit phototaxis towards blue light [2]. This blue light reception is caused by the GAF3 domain which resembles cyanobacteriochromes. We produced the C-terminal GAF3/GGDEF2 domains providing BV and PCB in vivo. The GAF3 domain undergoes photoconversion between a green (P_g) and blue light (P_b) absorbing form. This fragment of Cph2 is capable to produce c- di-GMP from GTP with P_g as signaling state.