Deletion of the PB‐loop in the LCM subunit does not affect phycobilisome assembly or energy transfer functions in the cyanobacterium Synechocystis sp. PCC6714

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Abstract
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 (LCM) 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 LCM 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 LCM 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 LCM), 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 LCM; PBSs were assembled and functioned as in the wild‐type. The PB‐loop of the LCM seems thus dispensable for the PBS biogenesis and function.

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