Phycobilisomes, the major light-harvesting complexes of cyanobacteria are 
multimolecular structures made up of chromophoric proteins called 
phycobiliproteins and non chromophoric linker polypeptides. We report here the 
isolation and nucleotide sequence of the genes, cpeA and cpeB, which in 
Calothrix PCC 7601 encode the alpha and beta subunits of phycoerythrin, one of 
the major phycobiliproteins. In Calothrix PCC 7601, modulation of the 
polypeptide composition of the phycobilisomes occurs in response to changes of 
the light wavelength, a phenomenon known as complementary chromatic adaptation. 
Under green illumination, cells synthesize phycoerythrin and its two 
specifically associated linker polypeptides (LR35 and LR36), while under red 
illumination none of these proteins are detected. Using specific probes, a 
single transcript (1450 nucleotide long) corresponding to the cpe genes was 
detected but only in green-light-grown cells, establishing the occurrence of 
transcriptional regulation for the expression of this operon in response to 
light wavelength changes. The size of this transcript excludes the possibility 
that the phycoerythrin-associated LR35 and LR36 could be cotranscribed with the 
cpeA and cpeB genes.