The cytochrome-b 6 f complex, a key component of the photosynthetic electron transport chain, contains a number of very small protein subunits whose functions are not well defined. Here we have investigated the function of the 31-amino acid PetL subunit encoded in the chloroplast genome in all higher plants. Chloroplast-transformed petL knock-out tobacco plants display no obvious phenotype, suggesting that PetL is not essential for cytochrome b 6 f complex biogenesis and function (Fiebig, A., Stegemann, S., and Bock, R. (2004) Nucleic Acids Res. 32, 3615-3622). We show here that, whereas young mutant leaves accumulate comparable amounts of cytochrome b 6 f complex and have an identical assimilation capacity as wild type leaves, both cytochrome b 6 f complex contents and assimilation capacities of mature and old leaves are strongly reduced in the mutant, indicating that the cytochrome b 6 f complex is less stable than in the wild type. Reduced complex stability was also confirmed by in vitro treatments of isolated thylakoids with chaotropic reagents. Adaptive responses observed in the knockout mutants, such as delayed down-regulation of plastocyanin contents, indicate that plants can sense the restricted electron flux to photosystem I yet cannot compensate the reduced stability of the cytochrome b 6 f complex by adaptive up-regulation of complex synthesis. We propose that efficient cytochrome b 6 f complex biogenesis occurs only in young leaves and that the capacity for de novo synthesis of the complex is very low in mature and aging leaves. Gene expression analysis indicates that the ontogenetic down-regulation of cytochrome b 6 f complex biogenesis occurs at the post-transcriptional level.The Cyt-bf 2 is the smallest of the three thylakoid membraneintrinsic multiprotein complexes of photosynthetic electron transport. Cyt-bf functions as a dimer with a molecular mass of 220 kDa and consists of nine subunits per monomer (1). It catalyzes the rate-limiting step of linear electron transport, oxidizing PQ generated by PSII and reducing PC. PC then diffuses through the thylakoid lumen toward PSI and reduces the photooxidized chlorophyll a dimer in the PSI reaction center, P 700 . The PQ-PC oxidoreductase activity of the Cyt-bf is coupled to the translocation of at least two protons per transported electron into the thylakoid lumen, thus contributing to the proton motif force required for ATP synthesis (2).The Cyt-bf is the predominant point of flux control of linear electron transport (3-6); its amount varies strongly in response to growth conditions and the developmental state of the plant, thus adjusting photosynthetic electron flux to the metabolic demand for ATP and NADPH. The mechanisms underlying these adjustments of Cyt-bf concentration are currently not understood. There are some indications that Cyt-bf is a relatively stable complex with lifetimes at least in the range of several days (7), but how Cyt-bf biogenesis is regulated during plant development is completely unknown. Cyt-bf biogenesis is a highly com...

The functions of several small subunits of the large photosynthetic multiprotein complex PSI (Photosystem I) are not yet understood. To elucidate the function of the small plastome-encoded PsaJ subunit, we have produced knockout mutants by chloroplast transformation in tobacco (Nicotiana tabacum). PsaJ binds two chlorophyll-a molecules and is localized at the periphery of PSI, close to both the Lhca2- and Lhca3-docking sites and the plastocyanin-binding site. Tobacco psaJ-knockout lines do not display a visible phenotype. Despite a 25% reduction in the content of redox-active PSI, neither growth rate nor assimilation capacity are altered in the mutants. In vivo, redox equilibration of plastocyanin and PSI is as efficient as in the wild-type, indicating that PsaJ is not required for fast plastocyanin oxidation. However, PsaJ is involved in PSI excitation: altered 77 K chlorophyll-a fluorescence emission spectra and reduced accumulation of Lhca3 indicate that antenna binding and exciton transfer to the PSI reaction centre are impaired in DeltapsaJ mutants. Under limiting light intensities, growth of DeltapsaJ plants is retarded and the electron-transport chain is far more reduced than in the wild-type, indicating that PSI excitation might limit electron flux at sub-saturating light intensities. In addition to defining in vivo functions of PsaJ, our data may also have implications for the interpretation of the crystal structure of PSI.

In higher plants, the plastome-encoded photosystem I-subunit PsaI is specifically required for photosystem I stability during high-light and chilling stress and leaf senescence.

Pure word deafness is a rare disorder dramatically impairing comprehension of spoken language, while auditory functions remain relatively intact. We present a 71-year-old woman with a slowly progressive disturbance of speech perception due to pure word deafness. MRI revealed degeneration of the temporal lobes. A magnetoencephalographic investigation using alternating single tone stimulation showed that N100 was followed by a second transient response and was abnormally prolonged up to 600-700 ms. We conclude that auditory processing is disturbed at long latency ranges following the N100, which may result in the clinical presentation of pure word deafness.