CpeR is an activator required for expression of the phycoerythrin operon (cpeBA) in the cyanobacterium Fremyella diplosiphon and is encoded in the phycoerythrin linker-polypeptide operon (cpeCDESTR) IntroductionIn cyanobacteria much of the light that drives photosynthesis is absorbed by the phycobiliproteins. The chromophores that confer on these proteins the ability to absorb light are the bilins (open chain tetrapyrroles), which are covalently attached by thioether linkages at specific cysteinyl residues. The biliproteins, together with colourless linker polypeptides, self-assemble into intensely absorbing particles called phycobilisomes (for a review, see Sidler, 1994). The mass of a phycobilisome is approximately 10 7 Da and within each phycobilisome are bound 400-700 bilin moieties. Bilins thus make up about 1-2% of the total phycobilisome mass. Phycobilisomes have evolved not only to have a high specific absorbance but also to efficiently funnel excitons into the thylakoid membrane to which the phycobilisomes are superficially attached (Glazer, 1989). In the cyanobacterial thylakoid membrane, photosynthesis proceeds by a mechanism similar to that of higher plants, namely two photosystems containing chlorophyll a, which cooperate to oxidize water.Three major types of bilin attached to phycobiliproteins have been identified. These bilins, when isolated as short bilipeptides, show light absorption at different parts of the visible spectrum. Phycocyanobilin absorbs maximally at the longest wavelengths (orange light), next phycoerythrobilin (green light) and finally phycourobilin (blue-green light). Specific non-covalent interactions occurring between a bilin and the protein to which it is covalently bound can significantly change the absorption properties of the bilin. For example, the two phycobiliproteins, phycocyanin (PC) and allophycocyanin (APC), contain only one type of bilin (phycocyanobilin), but they differ in wavelength of maximum absorption by as much as 50 nm. Phycobiliproteins containing phycoerythrobilin are known as phycoerythrins (PEs). Some phycoerythrins contain both phycoerythrobilin and phycourobilin. In almost all cases, each phycobiliprotein has two distinct polypeptide subunits (a and b) which aggregate either into trimeric (ab) 3 or hexameric (ab) 6 structures. Amino acid sequence alignments and phylogenetic analyses suggest that the phycobiliproteins have evolved from a single common ancestral gene (Apt et al., 1995). Molecular Microbiology (2002) 44(6),