Proton translocation rates of the four stop codon substitutions in nuoK. The reactions were initiated with deamino-NADH (dNADH) to 250 μM final concentration. The fluorescence of ACMA (1 mM) was followed for several minutes. After about 2 minutes, the uncoupler FCCP was added (1 mM) to collapse the generated proton gradient. The wild type strain is shown for comparison (black, trace 1). The G100stop is shown in green, trace 2. The traces for R99stop, M98stop, and E97stop were identical (traces 3,4,5), and lacked any proton pumping activity. The traces shown are representative of 2–3 experiments.