In addition to the ETS motifs, common promoter-proximal PU.1-bound regions were highly enriched for motifs that have previously been shown to be generally enriched in promoters (Xie et al., 2005) (Figure 2A). Both promoter-proximal and promoter-distal commonly PU.1-bound regions exhibited enrichment for a PU.1-IRF composite, consistent with the ability of PU.1 and IRF4/8 to form a ternary complex on this class of composite sites in both cell types (Eisenbeis et al., 1993; Pongubala et al., 1992). Additionally, promoter-distal regions bound by PU.1 in both cell types were enriched for a CTCF motif, likely because CTCF-bound sites generally coincide with accessible chromatin as measured by DNase I hypersensitivity (Xi et al., 2007).
