A small multigene family encodes the rod-core linker polypeptides of Anabaena sp. PCC7120 phycobilisomes

Abstract

The cpc operon of Anabaena sp. PCC7120 is shown to encode ten genes: 5'-cpcB-cpcA-cpcC-cpcD-cpcE-cpcF- cpcG1-cpcG2-cpcG3-cpcG4-3'. The 3' portion of this operon includes four tandemly repeated genes encoding phycocyanin (PC)-associated, rod-core linker polypeptides of the phycobilisomes (PBS). The products of these four genes are most similar at their N termini, and overall are 50-61% identical and 68-76% similar to one another. The four CpcG proteins of Anabaena sp. PCC7120 are 41-47% identical and 62-65% similar to the single CpcG rod-core linker protein in Synechococcus sp. PCC7002. The N-terminal domains of the polypeptides are also more distantly related to the conserved domains of other types of rod-linker polypeptides associated with PC, phycoerythrin, and allophycocyanin (AP). Three of these rod-core linker proteins (CpcG1, CpcG2, and CpcG4) were demonstrated to occur in isolated PBS by N-terminal amino acid sequence analyses. These results indicate that previously proposed models for the PBS of Anabaena sp. are incorrect. It is suggested that the PBS of Anabaena sp. have eight peripheral rods, each of which interacts with the AP of the core via a specific rod-core linker (CpcG) polypeptide.