Evolution of the fast dynamic interaction between cytochrome c6
or plastocyanin and photosystem I
Drepper, F.1, Ralfs, C.1, Gelhaus, C.1, Grzybek, S.1, Ziegler, K.2, Lockau, W.2, Haehnel, W.1
1Biochemie der Pflanzen, Institut für Biologie II, Albert-Ludwigs-Universität, Schänzlestrasse 1, D-79104 Freiburg, Germany
2Biochemie der Pflanzen, Humboldt-Universität Berlin, Chausseestrasse 117,
D-10115 Berlin, Germany
Electron transfer between the cytochrome b6f complex and photosystem I (PSI) is accomplished by diffusable electron carriers, the copper protein plastocyanin or the heme protein cytochrome c6. Higher plants use plastocyanin whereas many green algae or cyanobacteria can use either, depending on growth conditions. In eucaryots, the positively charged subunit PsaF of photosystem I is needed in order to attract the acidic donor protein and to form a temporary electron transfer complex [1,2]. In several cyanobacteria, the analogous reaction was found to proceed in a collisional mechanism between negatively charged proteins and the rate to be independent of the presence of subunit PsaF [2,3]. Since the electron transfer to photosystem I can be triggered by short flashes of light, single-turnover kinetics of the binding and electron transfer can be measured with high precision by using time-resolved flash absorption spectroscopy.
Now we report two different mechanisms of binding and electron transfer between PSI and plastocyanin and/or cytochrome c6 in two cyanobacteria. In Anabaena variabilis the binding of the positively charged donor to PSI is dominated by attractive electrostatic interaction. In Synechococcus elongatus a complex is found only at high salt concentrations necessary to screen the repulsive electrostatic interactions. In both cyanobacteria we resolved the microsecond electron transfer within a preformed complex that is formed without the help of subunit PsaF. Results obtained with heterologous reaction partners suggest a common contact region at the surface of the cyanobacterial donors leading to the formation of a specific complex with PSI.
These results are compared to the function and properties of cytochrome c6 and plastocyanin in eucaryotic organisms. In spite of the different binding mechanisms we propose similar relative orientations of plastocyanin and cytochrome c6 when docked to its reaction partner. This is indicative of convergent evolution of the two proteins which has occured in a common ancestor as well as in each organism.
[1] Drepper, F., Hippler, M., Nitschke, W., & Haehnel, W. (1996) Biochemistry 35: 1282-1295
[2] Hippler, M., Reichert, J., Sutter, M., Zak, E., Altschmied, L., Schröer, U., Herrmann, R.G., & Haehnel, W. (1996) EMBO J. 15: 6374-6384
[3] Hervás, M., Navarro, J. A., Díaz, A., & De la Rosa, M. A. (1996) Biochemistry 35: 2693-2698
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