Complementation of the MR-1 mutant without MtrC and OmcA by UndAHRCR-6. (A) Ferrihydrite reduction. Kinetics of Fe(II) formation are shown after reduction of 10 mM ferrihydrite by MR-1, HRCR-6, the ΔmtrC ΔomcA double mutant with empty vector, and the double mutant complemented with UndAHRCR-6 (Comp.1) or MtrC and OmcA (Comp.2). Heat-killed MR-1 cells were used as a negative control. The values reported are the means and standard deviations from triplicate biological measurements. (B) U(VI) reduction. Transmission electron microscopy examination of reduced UO2 after reduction of 250 μM U(VI) for 48 h by the MR-1 wild type (I), HRCR-6 (II), the ΔmtrC ΔomcA double mutant with empty vector (III), and the double mutant complemented with UndAHRCR-6 (IV). The outer membrane-associated (OMA) and extracellular polymeric substance (EPS)-associated UO2 are indicated by arrows. Bar = 200 nm.