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Institut für Biologie III, Universität Freiburg, D-79104 Freiburg, Germany
The chloroplast genome of all higher plants
encodes, in its large single-copy region, a conserved
open reading frame of unknown function (ycf3), which
is split by two group II introns and undergoes RNA editing in monocotyledonous plants. To elucidate the
function of ycf3 we have deleted the reading frame
from the tobacco plastid genome by biolistic transformation. We show here that homoplasmic 
ycf3 plants
display a photosynthetically incompetent phenotype. Molecular analyses indicate that this phenotype is not
due to a defect in any of the general functions of the
plastid genetic apparatus. Instead, the mutant plants
specifically lack detectable amounts of all photosystem
I (PSI) subunits analyzed. In contrast, at least under
low light conditions, photosystem II subunits are still
present and assemble into a physiologically active complex. Faithful transcription of photosystem I genes as
well as correct mRNA processing and efficient transcript loading with ribosomes in the ycf3 plants suggest a posttranslational cause of the PSI-defective phenotype. We therefore propose that ycf3 encodes an
essential protein for the assembly and/or stability of
functional PSI units. This study provides a first example
for the suitability of reverse genetics approaches to
complete our picture of the coding capacity of higher
plant chloroplast genomes.
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