Functional Incorporation of Chimeric b Subunits into F 1 F o ATP Synthase

F1 Fo ATP synthases function by a rotary mechanism. The enzyme's peripheral stalk serves as the stator that holds the F1 sector and its catalytic sites against the movement of the rotor. InEscherichia coli , the peripheral stalk is a homodimer of identicalb subunits, but photosynthetic bacteria have open reading frames for two differentb -like subunits thought to form heterodimericb /b ′ peripheral stalks. Chimericb subunit genes have been constructed by substituting sequence from theThermosynechococcus elongatus b andb ′ genes in theE. coli uncF gene, encoding theb subunit. The recombinant genes were expressed alone and in combination in theE. coli deletion strain KM2 (Δb ). Although not all of the chimeric subunits were incorporated into F1 Fo ATP synthase complexes, plasmids expressing either chimericb E39-I86 orb ′E39-I86 were capable of functionally complementing strain KM2 (Δb ). Strains expressing these subunits grew better than cells with smaller chimeric segments, such as those expressing theb ′E39-D53 orb L54-I86 subunit, indicating intragenic suppression. In general, the chimeric subunits modeled on theT. elongatus b subunit proved to be more stable than theb ′ subunit in vitro. Coexpression of theb E39-I86 andb ′E39-I86 subunits in strain KM2 (Δb ) yielded F1 Fo complexes containing heterodimeric peripheral stalks composed of both subunits.