Molecular characterization of the terminal energy acceptor of cyanobacterial phycobilisomes (Calothrix sp. PCC 7601/apcE sequence/LcM polypeptide/phycobiliprotein/secondary structure)

Cyanobacteria harvest light energy through multimolecular structures, the phycobilisomes, regularly ar- rayed at the surface of the photosynthetic membranes. Phyco- bilisomes consist of a central core from which rods radiate. A large polypeptide (LCM, 75-120 kDa) is postulated to act both as terminal energy acceptor and as a linker polypeptide that stabilizes the phycobilisome architecture. We report here the characterization of the gene (apcE) that encodes this LCM polypeptide in Calothrix sp. PCC 7601. It is located upstream from the genes encoding the major components of the phyco- bilisome core (alHophycocyanin) and is part of the same operon. The deduced amino acid sequence shows that the N-terminal region of LCM shares homology with the other phycobiliprotein subunits and thus constitutes the chromoprotein domain. The other part of the molecule is made up of four repeated domains that are highly homologous to the N-terminal regions of the phycocy,nin rod linker polypeptides. The predicted secondary structure of the different domains of the LCM is discussed in relation to the different roles and properties of this large molecule.