A multigene family in Calothrix sp. PCC 7601 encodes phycocyanin, the major component of the cyanobacterial light-harvesting antenna.

In cyanobacteria, light is harvested by phycobilisomes which are essentially made up of chromophoric proteins called phycobiliproteins. We have characterized two gene clusters (cpcB1, cpcA1 and cpcB3, cpcA3) each encoding the two subunits of phycocyanin (βPC and αPC, respectively), one of the major phycobiliproteins in Calothrix 7601. Downstream from the gene encoding the PCα subunit in cluster 1, an open reading frame was found, cpcE1. These genes are organized in two transcriptional units, namely: cpcB3 A3 and cpcB1 A1 E1. All these genes are transcribed whatever the chromatic light received during cell growth. Consequently, although only one type of “constitutive” PC has been biochemically characterized, we have demonstrated that there are two cpc operons “constitutively” transcribed in this strain. With the previously described red light “inducible” cpcB2 A2 operon, there are three copies of the PC encoding genes in Calothrix 7601. The significance of this newly described multigene family in cyanobacteria is discussed. We have also mapped the 5′ and 3′ termini of the major transcript from the cpc1 operon. Analysis of the 5′ untranslated region of this transcript has revealed alternative secondary structures which are proposed to play a role in the regulation of the expression of this operon.