Isolation and genomic analysis of the rat polymeric immunoglobulin receptor gene terminal domain and transcriptional control region

The polymeric immunoglobulin receptor (pIgR) transports IgA and IgM across secretory epithelial cells and is essential in external immunity maintenance, We report here the structural characterization of the single-copy rat gene distributed over 30 kb of chromosomal DNA and analysis of its transcriptional control region, RNA sequencing and genomic analysis show a 5' terminal region originates at a major (+1) and a minor site producing an unusual 124-bp nontranslated exon I separated from a small 96-bp initiator ATG coding exon II by a 7,5-kb intron. The pIgR 5' region comprises a structured promoter with abundant helix-loop-helix (bHLH) cis elements positioned within an equivalent internal -70, -290, -528, and three centered at -745. The three latter bHLH elements each occur within 30-bp repeats at -690 to -780, Transient expression assays show a 1,3-kb 5' region is sufficient to drive expression in rat primary hepatocyte monolayer cultures, transformed human hepatic (HepG2) cells, and a mammary epithelial tumor cell line MCF-7, but is inactive in the rodent fibroblast 3T3 cell line, A minimal transcriptional promoter domain was deduced from sequentially deleted vectors revealing a +40 to -922 sequence to be sufficient for full activity. Further deletions within this region yield incremental losses in cis activity, indicating that multiple subregions comprise an extended transcriptional control region.