PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

The role of the cytochrome C₅₅₀ in the cyanobacterium Thermosynechoccocus elongatus : A study of redox mutants. Diana Kirilovsky^*,1, Alain Boussac¹, José Maria Ortega³, Mercedes Roncel³, Adjélé Wilson¹, Hervé Bottin¹, Jorge Zurita³, A. Williams Rutherford¹, ¹ Service de Bioenergetique,URA2096 CNRS, DBJC/DSV, Gif sur Yvette, France³ Instituto de Bioquimica Vegetal y Fotosintesis, Sevilla, Spain

ABSTRACT- Cytochrome c550 (cyt c550), encoded by the psbV gene, is a water soluble c-type cytochrome containing a monoheme with a bihistidine coordination. This cytochrome is one of the extrinsic photosystem II (PS II) subunits involved in oxygen evolution in cyanobacteria cells. While deletion of the gene encoding cyt c550 results in impaired growth and diminished oxygen evolution, the role of the heme is unknown. Two populations of cyt c550, one soluble and one membrane-bound were detected in Termosynechoccocus elongatus cells. The soluble cyt c550 presented a low mid-potential (about −250 mV at pH 7) with large pH dependence. The mid-potential of the membrane-bound cyt c550 is largely higher (about −80/−100 mV). To study the possible role of the heme of the cyt c500 on oxygen evolution we constructed two mutants of T elongatus in which the residue His 92, the sixth ligand of the heme, was replaced by a Met or a Cys in order to modify the redox properties of the heme. In addition, in these two mutants, the psbV2 gene, encoding a cyt c550-like protein, was disrupted by and antibiotic cassette to allow their selection. The behavior of the H92M and H92C mutants has been compared to that of a psbV2-disruptant mutant (psbV2), and to that of a psbV1-disruptant mutant (psbV1) that presented different characteristics than those reported for the psbV Synechocystis 6803 mutant. The PS II complex of the T elongatus mutant was more stable than that of the Synechocystis mutant. Compared to psbV Synechocystis cells, the loss of oxygen evolving activity was less important (35-40% versus 50-60%), the retardation of oxygen release was smaller in thylakoids and inexistent in cells, the up-shift of the maximum of the thermoluminescence Q band was null in cells and very small in thylakoids and finally no fast dark deactivation was observed. The H92M and H92C mutations rend the cyt c550 unstable and easily detachable from the PS II. However, mutant cells and PS II activity seem to be only slightly modified by the mutation.

KEY WORDS: cyanobacteria, T. elongatus, photosystem II, cytochrome c550