DNA conformation driven by AP-1 triggers cell-specific expression via a strong epithelial enhancer

We report here the characterization of the regulatory region of the human L AMA3 gene, coding for the alpha 3A chain of laminin-5. A 202 bp fragment is sufficient to confer epithelial-specific expression to a thymidine kinase promoter through the cooperative effect of three AP-1 binding sites. Remark ably, removal of the sequences located between the AP-1 sites does not modi fy the promoter activity in keratinocytes but allows strong expression in f ibroblasts. Replacement of the deleted sequences by non-homologous ones ful ly restores the restricted enhancement in keratinocytes. Functional analysi s and mutagenesis experiments demonstrate that a minimal distance between t he AP-1 sites is required for the enhancer DNA fragment to adopt a particul ar conformation driven by the binding of Jun-Fos heterodimers. In non-permi ssive cells, this conformation leads to the anchorage of non-DNA-binding fi broblastic cofactors to form an inhibitory ternary complex. Therefore, our results describe for the first time an unusual conformation-dependent epith elial-specific enhancer.