Recent electron microscopy and NMR studies have revealed a distinctive 20o bend in the

b dimer within the tether domain (27, 136, 196). The research presented here suggests

the possibility of straightening or further bending of the two b subunits within the

peripheral stalk and lends support to the concept of a flexible peripheral stalk.

 This raises the question, why should the tether domain be so flexible as to allow

insertions, deletions and dimerization of b subunits of unequal lengths? If one views the

peripheral stalk to be a rope-like structure linking Flto Fo, then its position holding Fl

against the rotation of the central stalk would not be expected to be the same for

counterclockwise and clockwise rotation. Flexibility of the tether domain might facilitate

reorienting the peripheral stalk to act as a stator for rotation in either direction during

ATP synthesis and ATP hydrolysis.

 The ability to generate and purify F1Fo complexes with two genetically different b

subunits provides a potentially useful experimental tool. It is now feasible to specifically

label a single b subunit within a purified complex. This will facilitate biochemical

modification experiments and the use of physical methods.