A genetic suppressor approach of the c-type cytochrome maturation system involved in the assembly of the cytochrome b6f complex in Chlamydomonas

Alizée Malnoë1,2, Jacqueline Girard-Bascou1,2, Jean Alric1,2, Fabrice Rappaport 1,2, Richard Kuras1,2, Francis-André Wollman1,2 and Catherine de Vitry1,2

1CNRS, UMR 7141, Laboratoire de Physiologie Membranaire et Moléculaire du Chloroplaste, Institut de Biologie Physico-Chimique, 13 Rue Pierre et Marie Curie, 75005 Paris, France; 2UPMC Univ Paris 06.

Our studies of Chlamydomonas mutants revealed that the covalent binding of c type hemes localized on the electronegative side of the thylakoid membranes (case of heme ci' covalent binding to cytochrome b6) requires a specific maturation pathway named system IV or CCB (cofactor assembly, complex C (b6f), subunit B (PetB)) implicating at least four chloroplast membranes proteins [Kuras, Saint-Marcoux, Wollman, de Vitry (2007) A specific c-type cytochrome maturation system is required for oxygenic photosynthesis. Proc Natl Acad Sci USA 104, 9906-9910]. Nuclear ccb mutants preventing heme ci' binding and chloroplast mutants lacking the cysteine residue involved in covalent binding of heme ci' accumulate very little assembled b6f complex and are non phototrophic. We started a genetic and phenotypic characterization for the phototrophic suppressors of the ccb mutants with the aim to reach an in depth understanding of the biogenesis and function of heme ci'. In our search for extragenic suppressors of ccb mutants, we obtained a particularly interesting revertant that is phototrophic, accumulates in vivo wild-type levels of b6f complex that may be devoid of covalent heme ciÕ and is very photosensitive only in presence of oxygen. This revertant should offer a unique possibility to test the role of heme ci'. The suppressor mutation of this revertant is recessive. The suppressor spectrum is being characterized. We attempt to clone the suppressor gene by complementation with a selection on the restoration of photoresistance. Other ccb mutants are analysed to establish whether the four CCB factors represent the entire CCB pathway or to identify additional CCB partners.

e-mail address of presenting author: alizee.malnoe@ibpc.fr