METABOLISM AND BIOENERGETICS
Electron Transfer and Stability of the Cytochromeb6 f Complex in a Small Domain Deletion Mutant of Cytochrome f *

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The lumen segment of cytochrome fconsists of a small and a large domain. The role of the small domain in the biogenesis and stability of the cytochromeb6 f complex and electron transfer through the cytochrome b6 f complex was studied with a small domain deletion mutant in Chlamydomonas reinhardtii. The mutant is able to grow photoautotrophically but with a slower rate than the wild type strain. The heme group is covalently attached to the polypeptide, and the visible absorption spectrum of the mutant protein is identical to that of the native protein. The kinetics of electron transfer in the mutant were measured by flash kinetic spectroscopy. Our results show that the rate for the oxidation of cytochrome f was unchanged (t = 12 = ∼100 μs), but the half-time for the reduction of cytochrome f is increased (t 12 = 32 ms; for wild type,t 12 = 2.1 ms). Cytochromeb6 reduction was slower than that of the wild type by a factor of approximately 2 (t 12 = 8.6 ms; for wild type, t 12 = 4.7 ms); the slow phase of the electrochromic band shift also displayed a slower kinetics (t 12 = 5.5 ms; for wild type,t 12= 2.7 ms). The stability of the cytochromeb6 f complex in the mutant was examined by following the kinetics of the degradation of the individual subunits after inhibiting protein synthesis in the chloroplast. The results indicate that the cytochromeb6 f complex in the small domain deletion mutant is less stable than in the wild type. We conclude that the small domain is not essential for the biogenesis of cytochromef and the cytochromeb6 f complex. However, it does have a role in electron transfer through the cytochromeb6 f complex and contributes to the stability of the complex.

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Published, JBC Papers in Press, April 24, 2001, DOI 10.1074/jbc.M010721200

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This work was supported in part by National Institutes of Health Grant GM-20571.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.