Phycoerythrins of the oxyphotobacterium Prochlorococcus marinus are associated to the thylakoid membrane and are encoded by a single large gene cluster

An intrinsic divinyl-chlorophyll a/b antenna and a particular form of phyco biliprotein, phycoerythrin (PE) III, coexist in the marine oxyphotobacteriu m Prochlorococcus marinus CCMP 1375. The genomic region including the cpeB/ A operon of P. marinus was analysed. It encompasses 10 153 nucleotides that encode three structural phycobiliproteins and at least three (possibly fiv e) different polypeptides analogous to cyanobacterial or red algal proteins involved either in the linkage of subunits or the synthesis and attachment of chromophoric groups. This gene cluster is part of the chromosome and is located within a distance of less than 110 kb from a previously characteri zed region containing the genes aspA-psbA-aroC. Whereas the Prochlorococcus phycobiliproteins are characterized by distinct deletions and amino acid r eplacements with regard to analogous proteins from other organisms, the gen e arrangement resembles the organization of phycobiliprotein genes in some other cyanobacteria, in particular marine Synechococcus strains. The expres sion of two of the Prochlorococcus polypeptides as recombinant proteins in Escherichia coli allowed the production of individual homologous antisera t o the Prochlorococcus alpha and beta PE subunits. Experiments using these s era show that the Prochlorococcus PEs are specifically associated to the th ylakoid membrane and that the protein level does not significantly vary as a function of light irradiance or growth phase.