Abstract
The smallest molecular weight subunit (subunit IV), which contains no redox prosthetic group,is the only supernumerary subunit in the four-subunit Rhodobacter sphaeroides bc 1 complex.This subunit is involved in Q binding and the structural integrity of the complex. When thecytochrome bc 1 complex is photoaffinity labeled with [3H]azido-Q derivative, radioactivity isfound in subunits IV and I (cytochrome b), indicating that these two subunits are responsiblefor Q binding in the complex. When the subunit IV gene (fbcQ) is deleted from the R.sphaeroides chromosome, the resulting strain (RSΔIV) requires a period of adaptation beforethe start of photosynthetic growth. The cytochrome bc 1 complex in adapted RSΔIVchromatophores is labile to detergent treatment (60–75% inactivation), and shows a four-fold increasein the K m for Q2H2. The first two changes indicate a structural role of subunit IV; the thirdchange supports its Q-binding function. Tryptophan-79 is important for structural andQ-binding functions of subunit IV. Subunit IV is overexpressed in Escherichia coli as a GSTfusion protein using the constructed expression vector, pGEX/IV. Purified recombinant subunitIV is functionally active as it can restore the bc 1 complex activity from the three-subunit corecomplex to the same level as that of wild-type or complement complex. Three regions in thesubunit IV sequence, residues 86–109, 77–85, and 41–55, are essential for interaction withthe core complex because deleting one of these regions yields a subunit completely or partiallyunable to restore cytochrome bc 1 from the core complex.
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Yu, L., Tso, SC., Shenoy, S.K. et al. The Role of the Supernumerary Subunit of Rhodobactersphaeroides Cytochrome bc 1 Complex. J Bioenerg Biomembr 31, 251–258 (1999). https://doi.org/10.1023/A:1005423913639
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DOI: https://doi.org/10.1023/A:1005423913639