THE CHLOROPLAST YCF7 (PETL) OPEN READING FRAME OF CHLAMYDOMONAS-REINHARDTII ENCODES A SMALL FUNCTIONALLY IMPORTANT SUBUNIT OF THE CYTOCHROME B(6)F COMPLEX

The small chloroplast open reading frame ORF43 (ycf 7) of the green un icellular alga Chlamydomonas reinhardtii is cotranscribed with the psa C gene and ORF58. While ORF58 has been found only in the chloroplast g enome of C.reinhardtii, ycf 7 has been conserved in land plants and it s sequence suggests that its product is a hydrophobic protein with a s ingle transmembrane alpha helix. We have disrupted ORF58 and ycf 7 wit h the aadA expression cassette by particle-gun mediated chloroplast tr ansformation. While the ORF58::aadA transformants are indistinguishabl e from wild type, photoautotrophic growth of the ycf 7::aadA transform ants is considerably impaired. In these mutant cells, the amount of cy tochrome b(6)f complex is reduced to 25-50% of wild-type level in mid- exponential phase, and the rate of transmembrane electron transfer per b(6)f complex measured in vivo under saturating light is three to fou r times slower than in wild type. Under subsaturating light conditions , the rate of tile electron transfer reactions within the b(6)f comple x is reduced more strongly in the mutant than in the wild type by the proton electrochemical gradient. The ycf 7 product (Ycf 7) is absent i n mutants deficient in cytochrome b(6)f complex and present in highly purified b(6)f complex from the wild-type strain. Ycf 7-less complexes appear more fragile than wild-type complexes and selectively lose the Rieske iron-sulfur protein during purification. These observations in dicate that Ycf 7 is an authentic subunit of the cytochrome b(6)f comp lex, which is required for its stability, accumulation and optimal eff iciency. We therefore propose to rename the ycf 7 gene petL.