The HS2 Enhancer of the β-globin Locus Control Region Initiates Synthesis of Non-coding, Polyadenylated RNAs Independent of a cis-linked Globin Promoter

Abstract

The HS2 enhancer in the beta-globin locus control region (LCR) regulates transcription of the globin genes 10-50 kb away. Earlier studies show that a transcription mechanism initiated by the HS2 enhancer through the intervening DNA in the direction of the cis-linked promoter and gene mediates long-range enhancer function. Here, we further analyzed the enhancer-initiated RNAs and their mode of transcription from the HS2 enhancer in the endogenous genome of erythroid K562 cells, in plasmids integrated into K562 cells and in purified DNA used as template in in vitro transcription reactions. We found that the HS2 enhancer was able to initiate transcription autonomously in the absence of a cis-linked globin promoter. The enhancer-initiated, intergenic RNAs were different from the mRNA synthesized at the promoter in several aspects. The enhancer RNAs were synthesized not from a defined site but from multiple sites both within and as far as 1 kb downstream of the enhancer. The enhancer RNAs did not appear to contain a normal cap structure at the 5' ends. They were polyadenylated at multiple sites within 3 kb downstream of their initiation sites and were therefore shorter than 3 kb in lengths. The enhancer RNAs remained in discrete spots within the nucleus and were not processed into mRNA or translated into proteins. These particular features of enhancer-initiated transcription indicate that the transcriptional complex assembled by the enhancer was different from the basal transcription complex assembled at the promoter. The results suggest that in synthesizing non-coding, intergenic RNAs, the enhancer-assembled transcription complex could track through the intervening DNA to reach the basal promoter complex and activate efficient mRNA synthesis from the promoter.