Unusual ring D fixation by three crucial residues promotes phycoviolobilin formation in the DXCF-type cyanobacteriochrome without the second Cys

Cyanobacteriochromes are linear tetrapyrrole-binding photoreceptors produced by cyanobacteria. Their chromophore-binding GAF domains categorized into many lineages. Among them, dual Cys-type cyanobacteriochrome possessing not only a highly conserved ‘first Cys' but also ‘second found from multiple The first Cys stably attaches to C31 of the A-ring, while second mostly shows reversible ligation C10 chromophore. Notably, position in primary sequence is diversified, and most abundant have within DXCF motif, which called domains. It has been long known that involved isomerization activity (reduction C4=C5 double bond) initially incorporated phycocyanobilin phycoviolobilin. However, comprehensive site-directed mutagenesis on domains, AM1_6305g1 AM1_1499g1, revealed dispensable for activity, three residues participate fixing C- D-rings. Fixation chromophore both sides C5 bridge necessary, even though one side fixation site far this bridge, with other at fixed Cys.