STRUCTURE OF THE GENES ENCODING THE ROD-CORE LINKER POLYPEPTIDES OF MASTIGOCLADUS-LAMINOSUS PHYCOBILISOMES AND FUNCTIONAL-ASPECTS OF THE PHYCOBILIPROTEIN LINKER-POLYPEPTIDE INTERACTIONS

The 3' portion of the cpc operon in Mastigocladus laminosus encloses the genes 5'-cpcF-cpcG1-cpcG2-cpcG3 3'. The three cpcG genes encode different phycocyanin-associated rod-core linker polypeptides of the phycobilisomes with predicted 279, 247 and 254 amino acids in length. The gene products CpcG show a high similarity at their N-terminal domains (190 amino acids) and an overall identity of 47-53% to one another. Each of the three CpcG polypeptides is highly related to one of the four CpcG gene products of Anabaena sp. PCC 7120 (66-81% identity). It is suggested that these pairs of rod-core linker polypeptides mediate the same specific type of phycocyanin --> allophycocyanin interaction in the similar phycobilisomes of M. laminosus and Anabaena sp. PCC 7120. The similarity of the CpcG1, CpcG2 and CpcG3 polypeptides to the single CpcG rod-core linker polypeptide of Synechococcus sp. PCC 7002 (36-41% identity) is lower. The rod-core linker polypeptides are more distantly related to the rod linker polypeptides associated with phycocyanin or phycoerythrin. However, six conserved domains were identified within the N-terminal 190 amino acids of these linker proteins, which bear similar amino acid sequences, including highly conserved basic amino acids. A similar amino acid sequence but with conserved acidic amino acids can be found in the beta-subunits of phycocyanin, phycoerythrin and phycoerythrocyanin, which is protruding into the central cavity of the phycobiliprotein hexamers. It is suggested that these domains are sites of phycobiliprotein-hexamer/rod and rod-core linker interactions.