Deletion of the PB-loop in the L sub(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 complex is composed of heterodimeric phycobiliproteins that are assembled with the aid of linker polypeptides in order to optimize light-energy absorbance and transfer to photosystem II. The core membrane linker subunit (L sub(CM)) is a fascinating multifunctional polypeptide that participates in the PBS structure, function and anchoring to the photosynthetic membrane. Sequence analysis has defined several domains within the L sub(CM) polypeptide. The C-terminal portion contains two to four repeated domains that are similar to the conserved domains of linker polypeptides and are believed to play the same role. The N-terminal portion is similar to phycobiliproteins (PB-domain) and carries, like phycobiliproteins, a covalently linked phycobilin chromophore. This domain is interrupted by a so-called PB-loop insertion. The PB-domain of the L sub(CM) is thus regarded as one of the core subunits, with its PB-loop protruding towards the photosynthetic membrane. The PB-loop was thought to be involved in the attachment of the PBS to the photosynthetic membrane. We generated an apcE gene (encoding L sub(CM)), in which we deleted the sequence encoding 54 amino acids of the PB-loop domain. The modified gene was expressed in a Synechocystis PCC6714 strain in which the apcE gene had been inactivated. The truncated polypeptide was functionally equivalent to the wild-type L sub(CM); PBSs were assembled and functioned as in the wild-type. The PB-loop of the L sub(CM) seems thus dispensable for the PBS biogenesis and function.