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Cytochrome c3 of Desulfovibrio desulfuricans strain G20 is an electron carrier for uranium (VI) reduction. When D. desulfuricans G20 was grown in medium containing a non-lethal concentration of uranyl acetate (1 mM), the rate at which the cells reduced U(VI) was decreased compared to cells grown in the absence of uranium. Western analysis did not detect cytochrome c3 in periplasmic extracts from cells grown in the presence of uranium. The expression of this predominant tetraheme cytochrome was not detectably altered by uranium during growth of the cells as monitored through a translational fusion of the gene encoding cytochrome c3 (cycA) to lacZ. Instead, cytochrome c3 protein was found tightly associated with insoluble U(IV), uraninite, after the periplasmic contents of cells were harvested by a pH shift. The association of cytochrome c3 with U(IV) was interpreted to be non-specific, since pure cytochrome c3 adsorbed to other insoluble metal oxides, including cupric oxide (CuO), ferric oxide (Fe2O3), and commercially available U(IV) oxide. [ABSTRACT FROM AUTHOR]