Structure and energy transfer of the phycobilisome in a linker protein replacement mutant of cyanobacterium Synechococcus 7942

Abstract

The role of the linker proteins in the biogenesis and energy transfer of the phycobilisome rod was monitored by making insertional inactivation in the cpcI gene coding for the core-proximal 33 kilodalton (kDa) protein in the cyanobacterium Synechococcus 7942. The insertion leaves the cpcH gene coding for the core-distal 30 kDa protein intact and functional. Analysis of the phycobilisome protein composition of the cpcI mutant shows that the 30 kDa protein is present in normal amounts in the rod, indicating that the 30 kDa linker protein can replace the 33 kDa protein in the biogenesis and structural integrity of the rod. The absorption and fluorescence characteristics of the mutated phycobilisome is almost indistinguishable from that of the wild-type of the same rod length. The fluorescence kinetics from the cpcI mutant show that the dominating decay component has a lifetime from phycocyanin of 69 ps as compared to 72 ps found for the wild-type phycobilisome with the same rod length. The results show that replacing the 33 kDa for the 30 kDa linker in the rod does not alter the energy harvesting or the energy transfer characteristics of the rod in contrast to what has been concluded from data obtained from in vitro experiments. We conclude that the linker polypeptides have only a minor influence on the energy transfer characteristics of the rod but are mainly involved in determining the length of the rod in response to changing environmental light conditions.