A tissue-specific transcriptional enhancer is found in the body of the HLA-DR alpha gene

We mapped cis-acting regulatory elements in the HLA-DR alpha gene, which encodes the monomorphic subunit of the HLA-DR heterodimer. Genomic fragments of HLA-DR alpha were placed 5' or 3' to the chloramphenicol acetyltransferase reporter gene, the transcription of which was initiated from the Herpes simplex thymidine kinase promoter. In transient expression assays, fragments from the body of the HLA-DR alpha gene were able to increase chloramphenicol acetyltransferase activity in a position-, orientation-, and promoter-independent yet tissue-specific fashion. These HLA-DR alpha cis-acting regulatory elements contain previously identified DNase I-hypersensitive sites and DNA sequences homologous to those found in other eukaryotic transcriptional enhancers.