  F1-ATPase is the soluble portion of the membrane-embedded enzyme FoF1-ATP
synthase that catalyzes the production of adenosine triphosphate in eukaryotic
and eubacterial cells. In reverse, the F1 part can also hydrolyze ATP quickly
at three catalytic binding sites. Therefore, catalysis of 'non-productive' ATP
hydrolysis by F1 (or FoF1) must be minimized in the cell. In bacteria, the
epsilon subunit is thought to control and block ATP hydrolysis by mechanically
inserting its C-terminus into the rotary motor region of F1. We investigate
this proposed mechanism by labeling F1 specifically with two fluorophores to
monitor the C-terminus of the epsilon subunit by F\"orster resonance energy
transfer. Single F1 molecules are trapped in solution by an Anti-Brownian
electrokinetic trap which keeps the FRET-labeled F1 in place for extended
observation times of several hundreds of milliseconds, limited by
photobleaching. FRET changes in single F1 and FRET histograms for different
biochemical conditions are compared to evaluate the proposed regulatory
mechanism.
