C/EBP induces chromatin opening at a cell-type specific enhancer

Institut für Biochemie, Westfälische-Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 2, D-48149 Münster, Germany, and Division of Experimental Haematology, LIMM, University of Leeds, St James's University Hospital, Leeds LS9 7TF, UK

^* To whom correspondence should be addressed. Email: klempna{at}uni-muenster.de.

	Abstract

We have used the chicken mim-1 gene as a model to study the mechanisms by which transcription factors gain initial access to their target sites in compacted chromatin. Expression of mim-1 is restricted to the myelomonocytic lineage of the hematopoietic system where it is regulated synergistically by Myb and C/EBP factors. Myb and C/EBP cooperate at two distinct cis-elements of mim-1, the promoter and a cell-type specific enhancer, both of which are associated with DNase I hypersensitive sites in myelomonocytic cells but not in mim-1 non-expressing cells. Previous work has shown that ectopic expression of Myb and C/EBP activates the endogenous mim-1 gene in a non-hematopoietic cell type (fibroblasts), where the gene is normally completely silent. Here we investigated the molecular details of this finding and show that activation of mim-1 occurs by two independent mechanisms: in the absence of Myb, C/EBP triggers the initial steps of chromatin opening at the mim-1 enhancer without inducing transcription of the gene. Mim-1 transcription occurs only in the presence of Myb and is associated with chromatin opening at the promoter. Our work identifies a novel function of C/EBP in the initial steps of localized chromatin opening at a specific, physiologically relevant target region.