A composite DNA element in the promoter of the polymeric immunoglobulin receptor regulates its constitutive expression

The polymeric immunoglobulin receptor (pIgR), which is constitutively expressed on the basolateral surface of secretory epithelial cells, mediates external translocation of polymeric IgA and pentameric IgM (collectively called pIg) to exocrine secretions. A high level of synthesis must be maintained because the receptor is continuously cleaved to release bound secretory component (SC) in secretory IgA and secretory IgM, as well as free SC from unoccupied receptor. We have isolated the promoter of the pIgR gene and identified a short activating region that is required for the expression of pIgR promoter-driven reporter genes. This region contained an E-box and an inverted repeat sequence (IRS). Gel electrophoresis mobility shift assays with nuclear extracts from different pIgR-expressing epithelial cell lines demonstrated proteins that bind independently to both the E-box and the IRS sequence of the pIgR promoter. In addition, a DNA probe that contained both the E-box and the IRS gave rise to a larger complex that could not be competed by either element on its own. Binding was confirmed by DNase I footprinting of the E-box and IRS sequences with nuclear extracts, and by dimethyl sulfide footprinting in living HT-29 epithelial cells. Finally, a mutation in the pIgR promoter that inhibited protein binding to the E-box and the formation of the larger complex, abolished activated transcription from the reporter gene.