Transcription of the HS2 enhancer toward a cis-linked gene is independent of the orientation, position, and distance of the enhancer relative to the gene

The locus control region (LCR) regulates transcription of the downstream β-like globin genes 10 to 50 kb away. Among hypersensitive sites HS4, -3, - 2, and -1, which define the LCR in erythroid cells, HS2 possesses prominent enhancer function. The mechanism by which the HS2 enhancer and other functional components of the LCR act over the distance is not clear. We have used reverse transcription-PCR and RNase protection assays to analyze the transcriptional statuses of both the endogenous and the transfected HS2 enhancer in erythroid K562 cells. A novel pattern of HS2 enhancer transcription was observed. The endogenous HS2 enhancer was transcribed predominantly in the direction toward the downstream globin genes. The HS2 enhancer in transfected recombinant chloramphenicol acetyltransferase (CAT) plasmids was also transcribed predominantly toward the CAT gene, regardless of whether the enhancer was placed (i) in the genomic or reverse genomic orientation, (ii) in a position 5' or 3' to the gene, or (iii) at various distances up to 6 kb from the gene. The orientation, position, and distance independence in gene-tropic transcription of the HS2 enhancer correlates with the observed orientation, position, and distance independence of HS2 enhancer function and suggests that enhancer transcription may play a role in enhancer function.