THE DELETION OF PETG IN CHLAMYDOMONAS-REINHARDTII DISRUPTS THE CYTOCHROME BF COMPLEX

The 4-kDa protein encoded by chloroplast petG copurifies with the cyto chrome bf complex of spinach and is found in a number of other photosy nthetic organisms, including the eukaryotic alga Chlamydomonas reinhar dtii. To determine whether petG is involved in the function or assembl y of the cytochrome bf complex, the gene was cloned from C. reinhardti i, excised from the DNA fragment, and replaced with a spectinomycin re sistance cassette. A petG deletion strain of C. reinhardtii was then o btained by biolistic transformation. The resulting homoplasmic petG de letion strains are unable to grow photosynthetically, and immunoblot a nalysis shows markedly decreased levels of cytochrome b(6), cytochrome f, the Rieske iron-sulfur protein, and subunit IV. To verify that thi s phenotype was due to the removal of petG, we also constructed a stra in with a deletion in the open reading frame (ORF56), which is found 2 5 base pairs downstream of petG. The ORF56 deletion strain grew photos ynthetically and had wild-type levels of the four major cytochrome bf subunits. We conclude that the absence of the PetG protein affects eit her the assembly or stability of the cytochrome bf complex in C. reinh ardtii.