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© The Rockefeller University Press, 0021-9525/1997//95 $5.00
The Journal of Cell Biology, Volume 139, Number 1, , 1997 95-102

Article

Targeted Inactivation of a Tobacco Intron–containing Open Reading Frame Reveals a Novel Chloroplast-encoded Photosystem I–related Gene



Stephanie Ruf, Hans Kössel{dagger}, and Ralph Bock

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 {Delta}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 {Delta}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.

Abbreviations used in this paper: PSI and PSII, photosystem I and II; RFLP, restriction fragment–length polymorphism.

This paper is dedicated to the memory of our late teacher Hans Kössel who passed away on December 24, 1995.

This research was supported by grants from the Deutsche Forschungsgemeinschaft (BO 1482/1-1); the SFB 388; the Land Baden-Württemberg (Graduiertenförderung); and the Human Frontiers Science Program Organization (RG-437/94 M).

Address all correspondence to Ralph Bock, Institut für Biologie III, Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany. Tel.: 49-761-2032721. Fax: 49-761-2032745. e-mail: bock{at}sun1.biologie.uni-freiburg.de


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