NUCLEOSOME ASSEMBLY ON THE HUMAN C-FOS PROMOTER INTERFERES WITH TRANSCRIPTION FACTOR-BINDING

cAMP-responsive-element (CRE)-binding factors interaction with nucleos omal DNA has been investigated in vitro on the human c-fos promoter. A nalysis of nucleosome reconstitution of this promoter shows a preferen tial nucleosome positioning on the proximal promoter sequences, includ ing the CRE centered at -60 relative to the start site of transcriptio n, CRE-binding protein (CREB) and modulator protein (CREM) are unable to interact with their recognition site incorporated in a nucleosome. However, competition between transcription factor binding and nucleoso me assembly allows CREM binding and induces important modifications in the nucleosomal structure suggesting the displacement of nucleosomes, These findings imply that binding of transcription factors to the CRE prior to cAMP induction might be required to prevent the incorporatio n of this element in a nucleosome.