Stable co-occupancy of transcription factors and histones at the HIV-1 enhancer.

To investigate mechanisms yielding DNase I-hypersensitive sites (DHSs) at gene regulatory regions, we have initiated a biochemical analysis of transcription factor binding and nucleosome remodeling with a region of the human immunodeficiency virus 1 (HIV-1) 5' long terminal repeat (LTR) that harbors constitutive DHSs in vivo. In vitro reconstitution of an HIV-1 5' LTR fragment into nucleosome core particles demonstrates that Sp1, NF-kappaB1, LEF-1, ETS-1 and USF can gain access to their binding sites in HIV-1 nucleosomal DNA. The factor-bound mononucleosomes resist histone displacement from the DNA by the chromatin remodeling activity, SW1-SNF, or the histone chaperone, nucleoplasmin, suggesting that the binding of these factors to nucleosomal HIV-1 sequences forms a stable complex that includes the underlying histones. However, when the HIV-1 5' LTR fragment is incorporated into a nucleosomal array, Sp1 and NF-kappaB1 binding produce regions of enhanced DNase I sensitivity specifically at the HIV-1 nucleosome. These regions resemble the observed in vivo DHSs, yet the HIV-1 nucleosome remains intact even in the presence of nucleoplasmin. Thus, the constitutive DHSs identified at the HIV-1 enhancer in native chromatin may reflect the presence of a ternary complex composed of transcriptional activators, histones and DNA.