Targeted inactivation of the smallest plastid genome-encoded open reading frame reveals a novel and essential subunit of the cytochrome b6f complex

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Abstract
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 Δycf6 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 Δycf6 plants. Molecular analyses revealed that this block is caused by the complete absence of the cytochrome b6f complex, the redox‐coupling complex that interconnects the two photosystems. Analysis of purified cytochrome b6f 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 b6f complex, which plays a crucial role in complex assembly and/or stability. We therefore propose to rename the ycf6 reading frame petN.

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