A novel downstream regulatory element of the mouse H‐2Kb class I major histocompatibility gene.

The H‐2Kb gene equipped with a minimal promoter (5′ deletion up to −61) was fully expressed in transfected fibroblasts, but inactive in transfected embryonal carcinoma cells. A strong transcriptional regulatory element (H2DRE) was identified when a fragment spanning the second exon and second intron was used to activate transient expression of the reporter chloramphenicol acetyltransferase (CAT) gene in mouse Ltk‐ or NIH3T3 fibroblasts. Its activity was twice that of a construct where the CAT gene was driven by the H‐2Kb 5′ enhancer region (H2TF1/KBF1 site) and comparable to that of pRSVCAT construct carrying the strong Rous sarcoma virus LTR enhancer. In accord with regulated transcriptional activity of the intact H‐2Kb gene, the H2DRE did not activate the CAT expression in P19 mouse embryonal carcinoma cells. The H2DRE did not function as a typical enhancer since its activity was strongly position dependent. Consistent with its anticipated role in transcription regulation, H2DRE displayed more than five target sites for specifically interacting nuclear factors, two of them being present in H‐2 positive fibroblasts, but not in H‐2 negative teratocarcinoma cells. None of them was cross‐competed by sequences of the 5′ enhancer. The results of deletion experiments show that H2DRE is the only regulatory region that can activate transcription from the 5′ enhancerless H‐2Kb gene in mouse L fibroblasts.