Characteristics of FRET trajectories when one vs. two Cy5-SSB tetramers are bound to 3′dT-Cy-(dT)140 DNA. (A) Histogram of FRET states from Cy3-DNA molecules bound with Cy5-SSB (n = 83). The DNA was incubated with 4 nM Cy5-SSB(A122C) tetramer for 5 min and then washed with 10 column volumes of buffer. (B) Example single molecule trace showing the transition from two Cy5-SSB tetramers bound to a single DNA to one Cy5-SSB tetramers bound. The first 38 s of the trace shows a persistent high FRET state with no fluctuations when two Cy5-SSB tetramers are bound, whereas the second half shows the expected fluctuating FRET signal when a single Cy5-SSB is bound and can diffuse. Solution conditions: 30 mM Tris⋅HCl, pH 8.1, 100 mM NaCl, 5 mM MgCl2, 1 mM DTT, 0.1 mg/mL BSA, 0.5% (wt/vol) dextrose, 3 mM Trolox, 1 mg/mL glucose oxidase, and 0.4 mg/mL catalase, 25 °C.