Table 1.

Characteristics of R. capsulatus mutants

Strains	Ps phenotype*	Steady-state activity, %†	Electron transfer QH2 → cytc, %‡	Em7 [2Fe-2S], mV§
Wild-type	Ps+	100	100	310
+1Ala	Psslow	120	35	370
+2Ala	Ps−	5	2	410
+3Ala	Ps−	2	2	nd
A46T¶	Ps+	70	65	386
Y147A∥	Ps−	13	6	310

^*Ps⁺ and Ps⁻ indicate photosynthetic competence and incompetence, respectively. 

^† Steady-state bc₁ complex activity was determined by measuring the 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzohydroquinone:cyt c reductase activity (17) and is expressed as a percentage of the wild-type activity, which was, in this particular instance, 3.4 μmol cyt c reduced min⁻¹⋅mg of membrane protein⁻¹. 

^‡ QH₂ to cyt c electron transfer rates were determined by recording cyt c rereduction kinetics at 550–540 nm and fitting them to a single exponential equation (22). The rates are expressed as a percentage of the wild-type rate, which was 300 s⁻¹, and reflect single turnover bc₁ complex activity. Note that under these conditions, the electron transfer activities from QH₂ → cyt b are not significantly different than those from QH₂ → cyt c shown here. 

^§ The E_m7 values were obtained after fitting the amplitude of the EPR g_y signal during potentiometric titration of the [2Fe-2S] cluster as described in Materials and Methods. nd, not determined. 

^¶ A 46 T mutation is located in the Fe-S subunit, and the data are taken from ref. 21. 

^∥Y147A mutation is located in cyt b subunit, and the data are taken from ref. 23.