Targeted inactivation of the smallest plastid genome-encoded open reading frame reveals a novel and essential subunit of the cytochrome b(6)f complex.
The smallest conserved open reading frame in the plastid genome, ycf6, potentially specifies a hydrophobic polypeptide of only 29 amino acids. In order to determine the function of this reading frame we have constructed a knockout allele for ycf6. This allele was introduced into the tobacco plastid genome by chloroplast transformation to replace the wild-type ycf6 allele. Homoplasmic Deltaycf6 plants display a photosynthetically incompetent phenotype. Whereas the two photosystems are intact and physiologically active, we found that the electron transfer from photosystem II to photosystem I is interrupted in Deltaycf6 plants. Molecular analyses revealed that this block is caused by the complete absence of the cytochrome b(6)f complex, the redox-coupling complex that interconnects the two photosystems. Analysis of purified cytochrome b(6)f complex by mass spectroscopy revealed the presence of a protein that has exactly the molecular mass calculated for the Ycf6 protein. This suggests that Ycf6 is a genuine subunit of the cytochrome b(6)f complex, which plays a crucial role in complex assembly and/or stability. We therefore propose to rename the ycf6 reading frame petN.
Hager
M
M
Institut für Biologie III, Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany.
Biehler
K
K
Illerhaus
J
J
Ruf
S
S
Bock
R
R
D000595:Amino Acid Sequence; D003573:Cytochrome b Group / Q000737:chemistry / Q000235:genetics*; D045346:Cytochrome b6f Complex; D004579:Electron Transport; D005453:Fluorescence; D018506:Gene Expression Regulation, Plant; D018390:Gene Targeting; D017343:Genes, Plant; D018745:Genome, Plant; D008969:Molecular Sequence Data; D016366:Open Reading Frames / Q000235:genetics*; D010641:Phenotype; D045322:Photosynthetic Reaction Center Complex Proteins / Q000235:genetics; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D010947:Plants, Toxic; D018087:Plastids / Q000235:genetics*; D012150:Polymorphism, Restriction Fragment Length; D017386:Sequence Homology, Amino Acid; D013057:Spectrum Analysis; D014026:Nicotiana / Q000235:genetics*; D014170:Transformation, Genetic
D003573:Cytochrome b Group; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D045346:Cytochrome b6f Complex