Assembly of CpcL-phycobilisomes.

CpcL-phycobilisomes (CpcL-PBSs) are a reduced type of phycobilisome (PBS) found in several cyanobacteria. They lack the traditional PBS terminal energy emitters, but still show the characteristic red-shifted fluorescence at ~670 nm. We established a method of assembling in vitro a rod-membrane linker protein, CpcL, with phycocyanin, generating complexes with the red-shifted spectral features of CpcL-PBSs. The red-shift arises from the interaction of a conserved key glutamine, Q57 of CpcL in Synechocystis sp. PCC 6803, with a single phycocyanobilin chromophore of trimeric phycocyanin at one of the three ß82-sites. This chromophore is the terminal energy acceptor of CpcL-PBSs and donor to the photosystem(s). This mechanism also operates in PBSs from Acaryochloris marina MBIC11017. We then generated multichromic complexes harvesting light over nearly the complete visible range via the replacement of phycocyanobilin chromophores at sites α84 and ß153 of phycocyanins by phycoerythrobilin and/or phycourobilin. The results demonstrate the rational design of biliprotein-based light-harvesting elements by engineering CpcL and phycocyanins, which broadens the light-harvesting range and accordingly improves the light-harvesting capacity and may be potentially applied in solar energy harvesting.

Dichromic Allophycocyanin Trimer Covering a Broad Spectral Range (550-660†nm).

Phycobilisomes, the light-harvesting complexes of cyanobacteria and red algae, are a resource for photosynthetic, photonic and fluorescence labeling elements. They cover an exceptionally broad spectral range, but the complex superstructure and assembly have been an obstacle. By replacing in Synechocystis sp. PCC 6803 the biliverdin reductases, we studied the role of chromophores in the assembly of the phycobilisome core. Introduction of the green-absorbing phycoerythrobilin instead of the red-absorbing phycocyanobilin inhibited aggregation. A novel, trimeric allophycocyanin (Dic-APC) was obtained. In the small (110 kDa) unit, the two chromophores, phycoerythrobilin and phytochromobilin, cover a wide spectral range (550 to 660 nm). Due to efficient energy transfer, it provides an efficient artificial light-harvesting element. Dic-APC was generated in vitro by using the contained core-linker, LC , for template-assisted purification and assembly. Labeling the linker provides a method for targeting Dic-APC.