The blue-colored linker polypeptide L-55 is a fusion protein of phycobiliproteins in the cyanobacterium Synechocystis sp strain BO 8402

The cyanobacterium Synechocystis sp. strain BO 8402, isolated from Lake Constance, lacks phycobilisomes but instead forms inclusion bodies containing remnants of phycobiliproteins. The inclusion bodies are surrounded by a proteinaceous capsule and contain alpha-phycocyanin and beta-phycocyanin, the rod linker polypeptide (LRPC)-P-35 and a novel blue-colored protein L-55 with an apparent molecular mass of 55 kDa. An antibody raised against beta-phycocyanin showed a strong cross-reaction with L- 55. Mass spectrometry analysis of proteolytic peptides from L- 55 revealed mass identity to proteolytic peptides derived from (LRPC)-P-35 and beta-phycocyanin. However, analysis of the genome of strain BO 8402 revealed only one cpcBACE operon, encoding the apoproteins of beta-phycocyanin and alpha- phycocyanin, (LRPC)-P-35 and a subunit of the phycocyanin alpha subunit phycocyanobilin lyase, respectively. The gene structure, sequence and transcription of these genes were identical to that of a revertant strain, Synechocystis sp. strain BO 9201, which formed phycobilisomes and did not express L-55. Based on these observations, we concluded that L-55 did not derive from a particular gene or from a special form of mRNA-processing. We propose that L-55 is formed by post- translational fusion of (LRPC)-P-35 and beta-phycocyanin. Cross-linking may stabilize the formation of the large paracrystalline phycocyanin aggregates unique to Synechocystis sp. strain BO 8402. [KEYWORDS: cross-link; inclusion body; phycobiliproteins;post- translational fusion; Synechocystis sp strain; BO 8402 Mastigocladus-laminosus phycobilisomes; subunit phycocyanobilinlyase; light-harvesting complexes; crystal-structure analysis;c-phycocyanin; rod substructures; core substructure; bilin attachment; gamma-subunit; 2.2 angstrom]