Genes encoding both subunits of phycoerythrocyanin, a light-harvesting biliprotein from the cyanobacterium Mastigocladus laminosus.

Phycocyanin (PC) and phycoerythrocyanin (PEC) are light-harvesting components of the phycobilisome (PbS) from the cyanobacterium Mastigocladus laminosus. These two biliproteins are closely related, and show a particularly high degree of sequence homology in the C-terminal part of their beta-subunits. A 198-bp gene fragment encoding this region of PC from M. laminosus was therefore used as a heterologous hybridization probe to identify the genes coding for PEC from the same organism. A 1.7-kb HindIII fragment was cloned and its sequence determined. Three open reading frames (ORFs) were found on this fragment. The gene coding for the beta-subunit of PEC (pecB) was followed downstream by the alpha-subunit encoding gene (pecA). This gene arrangement had also been found in the PC-encoding (cpc) gene pair from M. laminosus, and is conserved in cpc genes from other organisms. This finding is compatible with a model of evolution of the cpc and pec gene pairs as the product of gene duplication of an ancestral beta- and alpha-subunit-encoding pair. A third ORF starts downstream from pecA. It codes for the 34.5-kDa linker protein, which forms complexes with PEC with a 1:6 stoichiometry in the PbS. Biliprotein- and linker protein-encoding genes are frequently clustered, and this provides mechanisms for the production of the different stoichiometric amounts of these gene products required in the PbS and for coregulation by environmental factors.