Version française

Tuesday 01 December 2009

BOULAY Clémence

Title : Characterization of the Orange Carotenoid Protein associated photo-protective mechanism in cyanobacteria.

Jury :

  • Président : Michel Dron
  • Rapporteurs : Roberto Bassi, Frédéric Partensky
  • Examinateurs : Margaret Ahmad, Giovanni Finazzi
  • Directeur de thèse : Diana Kirilovsky

Summary :

Cyanobacteria have developed a photoprotective mechanism, named qEcya, to protect themselves from high light intensities. It diminishes the formation of dangerous oxygen species by increasing energy dissipation into heat and thus, decreases the energy arriving at the reaction centers from the phycobilisomes (PBS, the cyanobacterial external antennae). The mechanism is induced by the absorption of blue-green light by the carotenoid of the soluble photoactive Orange Carotenoid Protein, OCP. The process is accompanied by a reversible decrease (quenching) of fluorescence. In this work, we first demonstrated that, in the cyanobacterium Synechocystis PCC 6803 under iron starvation conditions, the large blue-green light-induced fluorescence quenching is related to OCP and its associated qEcya mechanism, and not to the iron-starvation-induced IsiA protein. Then, I showed that the OCP and the qEcya mechanism are widespread among phycobilisome-containing cyanobacteria. While in the OCP containing cyanobacteria, energy dissipation and fluorescence quenching are increased under stress conditions like iron starvation, another strategy based on a quick degradation of their PBS was developed in the few cyanobacteria without OCP. Preliminary results about OCP-PBS interactions are also described. It was not possible to isolate OCP-containing-PBS. However after incubation of PBS in the presence of the OCP, I was able to isolated OCP-PBS complexes in a sucrose gadient, even when only the core of the PBS was put in the presence of the OCP. These results strongly suggest that OCP and the core of the PBS are interacting. However I did not succeed to induce the qEcya mechanism in these purified OCP-PBS complexes in vitro. The most important result of this PhD is the discovery and the first characterization of a new component essential for the qEcya mechanism : a 13kDa protein strongly bound to the membrane encoded by the slr1964 gene in Synechocystis PCC 6803. This protein appeared to be conserved in almost all the OCP cyanobacteria. We have named this protein the Fluorescence Recovery Protein, FRP, because it is involved in the recovery of the phycobilisome fluorescence quenching induced by strong blue-green light. The characterization of the slr1964 gene showed that it is conserved downstream to the gene of OCP in cyanobacteria, and that it can be transcribed independantly or co-transcribed with the gene of OCP. Our results also strongly suggested that the FRP and the OCP interact. PDF

Mots-clefs : photo-protection