DELETION OF THE PB-LOOP IN THE L-CM SUBUNIT DOES NOT AFFECT PHYCOBILISOME ASSEMBLY OR ENERGY-TRANSFER FUNCTIONS IN THE CYANOBACTERIUM SYNECHOCYSTIS SP. PCC6714

In cyanobacteria, light energy is mainly harvested for photosynthesis by the phycobilisome (PBS): a large pigment-protein complex. This comp lex is composed of heterodimeric phycobiliproteins that :Ire assembled with the aid of linker polypeptides in order to optimize light-energy absorbance and transfer to photosystem II. The cure membrane linker s ubunit (L-CM) is a fascinating multifunctional polypeptide that partic ipates in the PBS structure. function and anchoring to the photosynthe tic membrane. Sequence analysis has defined several domains within the L-CM polypeptide. The C-terminal portion contains two to four repeate d domains that are similar to the conserved domains of linker polypept ides and are believed to play the same role. The N-terminal portion is similar to phycobiliproteins (PB-domain) and carries, like phycobilip roteins. a covalently linked phycobilin chromophore. This domain is in terrupted by a so-called PB-loop insertion. The PB-domain of the I,,, is thus regarded as one of the core subunits, with its PB-loop protrud ing towards the photosynthetic membrane. The PB-loop was thought to be involved in the attachment of the PBS to the photosynthetic membrane. Wt generated an apcE gene (encoding L-CM), in which we deleted the se quence encoding 54 amino acids of the PB-loop domain. The modified gen e was expressed in a Synechocystis PCC6714 strain in which the apcE ge ne had been inactivated. The truncated polypeptide was functionally eq uivalent to the wild-type L-CM; PBSs were assembled and functioned as in the wild-type. The PB-loop of the L-CM seems thus dispensable for t he PBS biogenesis and function.