Full-length PU.1 is required for access to closed chromatin. (A–D) UCSC Genome Browser tracks comparing binding of endogenous PU.1, full-length exogenous PU1WTHA, and PU1ENGHA to open and closed regions in primary DN2b/3 CD25⁺ cells and Scid.adh.2C2 cells. Shown are the genes: (A) Cd34; (B) Robo1; (C) Syk; (D) Ffar2. (E) Comparison of PU1ENGHA binding with that of PU1WTHA and baseline binding of endogenous PU.1 in primary DN2b cells. Analysis as in Figure 4B. (Green bars) Open sites in primary DN2b-DN3 cells; (gray bars) closed sites; (dashed line) background. (F) Comparison of exogenous PU1WTHA with PU1ENGHA or PU1ETSHA binding in transduced Scid.adh.2C2 cells at open and closed promoter and nonpromoter sites. PU1WTHA αHA counts in open (red) and closed (blue) regions are plotted against PU1WTHA αPU.1 (left), PU1ENGHA αHA (middle), or PU1ETSHA αHA (right) tag counts in promoters (top three panels) or nonpromoter genomic sites (bottom three panels). Peaks at open promoters: n = 4498; peaks at closed promoters: n = 810; peaks at open nonpromoter elements: n = 8719; peaks at closed nonpromoter elements: n = 33158. Pearson's r shown. Dashed lines represent the tag threshold for peaks considered bound.