Rapid and selective remodeling of a positioned nucleosome during the induction of IL-12 p40 transcription.

Nucleosomes are important for gene regulation, but comprehensive studies of nucleosome positioning, remodeling, and transcription factor binding at inducible mammalian promoters have not been reported. We have analyzed the IL-12 p40 promoter, which is induced in macrophages by bacterial products. High-resolution micrococcal nuclease analyses revealed that a positioned nucleosome, nucleosome 1, spans the promoter, with three positioned nucleosomes further upstream. Upon activation, nucleosome 1 was rapidly and selectively remodeled in a protein synthesis-dependent manner. In primary macrophages, IFNgamma synergistically enhanced p40 expression, but little effect on remodeling or promoter occupancy was observed. These results suggest that remodeling complexes are selectively targeted to a single, promoter-encompassing nucleosome and that IFNgamma influences an event that is independent or downstream of remodeling.