Structure of the genes encoding the rod-core linker polypeptides of Mastigocladus laminusus phycobilisomes and functional aspects of the phycobiliprotein/linker-polypeptide interactions

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Abstract
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.