Cyanobacterial light-harvesting complex subunits encoded in two red light-induced transcripts.

The major light-harvesting complex in cyanobacteria and red algae, the phycobilisome, is composed of chromophoric and nonchromophoric polypeptides. Two linked genes encoding major chromophoric components, the polypeptide subunits of phycocyanin, were isolated from the cyanobacterium Fremyella diplosiphon. Transcripts from this phycocyanin subunit gene cluster were present as major species in the cyanobacterium grown in red light, but not in cultures maintained in green light. The genes for the subunits of the red light-induced phycocyanin were transcribed together (beta-phycocyanin followed by alpha-phycocyanin) on two messenger RNA species; one contained 1600 bases while the other had 3800 bases. The latter, which encompassed the smaller transcript, contained additional sequences extending from the 3' end of the coding region of the alpha-phycocyanin gene. It may encode other light-induced components of the phycobilisome. Since phycocyanin, which effectively absorbs red light, becomes a dominant constituent of the phycobilisome in red light, these different levels may reflect an important adaptive mechanism of these organisms to their environment.