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 Con stance, lacks phycobilisomes but instead forms inclusion bodies containing remnants of phycobiliproteins. The inclusion bodies are surrounded by a pro teinaceous capsule and contain alpha-phycocyanin and beta-phycocyanin, the rod linker polypeptide (LRPC)-P-35 and a novel blue-colored protein L-55 wi th an apparent molecular mass of 55 kDa. An antibody raised against beta-ph ycocyanin showed a strong cross-reaction with L-55. Mass spectrometry analy sis 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, respectiv ely. The gene structure, sequence and transcription of these genes were ide ntical to that of a revertant strain, Synechocystis sp. strain BO 9201, whi ch formed phycobilisomes and did not express L-55. Based on these observati ons, 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-tr anslational fusion of (LRPC)-P-35 and beta-phycocyanin. Cross-linking may s tabilize the formation of the large paracrystalline phycocyanin aggregates unique to Synechocystis sp. strain BO 8402.