THE CHLOROPLAST YCF8 OPEN READING FRAME ENCODES A PHOTOSYSTEM-II POLYPEPTIDE WHICH MAINTAINS PHOTOSYNTHETIC ACTIVITY UNDER ADVERSE GROWTH-CONDITIONS

We have engineered and analyzed a chlorplast mutant of Chlamydomonas r einhardtii that lacks ycf8, the chloroplast open reading frame 8, whic h is highly conserved in location and predicted amino acid sequence in land plants and C.reinhardtii. The ycf8 sequence was replaced with th e aadA cassette which confers resistance to spectinomycin when express ed in the chloroplast. Although the mutant is able to grow phototrophi cally, photosystem II function and cell growth are impaired under stre ss conditions such as high light intensity and diminished chloroplast protein synthesis induced by spectinomycin. Use of an antibody generat ed against the ycf8 product has revealed that this hydrophobic polypep tide is associated with photosystem II, based on its severely reduced levels in various photosystem II-deficient mutants and on its copurifi cation with photosystem II. This protein, therefore, appears to be (i) a novel photosystem II subunit and (ii) required for maintaining opti mal photosystem II activity under adverse growth conditions.