Extended Data Fig. 6: SAGA binding responds to the number and character of activation domains.

a. Gal4-SNAP^DY649-VP16 fluorescence intensity, a correlate of activator number, was measured for each spot within all micrograph images of the 3 and 10 nM activator experiments in Fig. 3. The probability density function (pdf) of intensities was plotted for all spots when SAGA was absent (gray) or present (purple). Because SAGA binding tended to occur at later times, a time-matched subset of SAGA-absent intensities was also plotted (black). b. Cumulative time-to-initial-binding distributions for Spt7-SNAP^DY549 on non-chromatinized DNA templates. Gal4 DNA binding domain derivatives fused to either the VP16 (light green, top panel) or Rap1 (dark green, bottom panel) activation domains were used. Gray curves illustrate off-target background binding to slide surface, and dashed lines indicate curve fits. Note that these plots are derived from the full data set represented by the 100 randomly chosen DNAs used for the rastergrams in Fig. 6c. c. Three models for how activators may ‘recruit’ SAGA HAT activity to promoters. In all cases, SAGA is initially tethered to the UAS/enhancer via interactions with the activation domain. In Model 1, SAGA transfers to nearby naked DNA and is released from the activator. Acetylation occurs at nucleosomes flanking the naked DNA. In Model 2, SAGA remains tethered to the activator while the HAT activity targets nearby nucleosomes without any DNA contact. In Model 3, SAGA remains tethered to the activator, but also contacts nearby naked DNA to target the flanking nucleosomes. See Discussion for details.