The chloroplast gene ycf9 encodes a photosystem II (PSII) core subunit, PsbZ, that participates in PSII supramolecular architecture

We have characterized the biochemical nature and the function of PsbZ, the protein product of a ubiquitous open reading frame, which is known as ycf9 in Chlamydomonas and ORF 62 in tobacco, that is present in chloroplast and cyanobacterial genomes. After raising specific antibodies to PsbZ from Chla mydomonas and tobacco, we demonstrated that it is a bona fide photosystem I I (PSII) subunit, PsbZ copurifies with PSII cores in Chlamydomonas as well as in tobacco. Accordingly, PSII mutants from Chlamydomonas and tobacco are deficient in PsbZ, Using psbZ-targeted gene inactivation in tobacco and Ch lamydomonas, we show that this protein controls the interaction of PSII cor es with the light-harvesting antenna; in particular, PSII-LHCII supercomple xes no longer could be isolated from PsbZ-deficient tobacco plants. The con tent of the minor chlorophyll binding protein CP26, and to a lesser extent that of CP29, also was altered substantially under most growth conditions i n the tobacco mutant and in Chlamydomonas mutant cells grown under photoaut otrophic conditions. These PsbZ-dependent changes in the supramolecular org anization of the PSII cores with their peripheral antennas cause two distin ct phenotypes in tobacco and are accompanied by considerable modifications in (1) the pattern of protein phosphorylation within PSII units, (2) the de epoxidation of xanthophylls, and (3) the kinetics and amplitude of nonphoto chemical quenching. The role of PsbZ in excitation energy dissipation withi n PSII is discussed in light of its proximity to CP43, in agreement with th e most recent structural data on PSII.