Bacterial phytochromes (Bphs) are ancestors of the well characterized plant photoreceptors. Whereas plant phyto-chromes perform their photoisomerization exclusively via a covalently bound bilin chromophore, Bphs are variable in their chromophore selection. This is demonstrated in the cyanobacterium CalothrixPCC7601 that expresses two Bphs, CphA and CphB. CphA binds phycocyanobilin (PCB) covalently, whereas CphB, lacking the covalently binding cysteine of the plant phytochromes, carries biliver-din IXa (BV) as the chromophore
