A new subunit of cytochrome b(6)f complex undergoes reversible phosphorylation upon state transition

A 15.2-kDa polypeptide, encoded by the nuclear gene PETO, was identified as a novel cytochrome b(6)f subunit in Chlamydomonas reinhardtii. The PETO ge ne product is a bona fide subunit, subunit V, of the cytochrome b(6)f compl ex, because (i) it copurifies with the other cytochrome b(6)f subunits in t he early stages of the purification procedure, (ii) it is deficient in cyto chrome b(6)f mutants accumulating little of the complex, and (iii) it coloc alizes with cytochrome f, which migrates between stacked and unstacked memb rane regions upon state transition. Sequence analysis and biochemical chara cterization of subunit V shows that it has a one transmembrane alpha-helix topology with two large hydrophilic domains extending on the stromal and lu menal side of the thylakoid membranes, with a lumenal location of the N ter minus. Subunit V is reversibly phosphorylated upon state transition, a uniq ue feature that, together with its topological organization, points to the possible role of subunit V in signal transduction during redox-controlled s hort term and long term adaptation of the photosynthetic apparatus in eukar yotes.