In the human genome, the erythroidspecfic hypersensitive site HS2 enhancer regulates the trsiption of the downstream 1-like globin genes 10-50 kilobses away. The mechanism of HS2 eancr fuction Is not known. The present study employs RNA protection assays to analyze the transcriptional status of the HS2 enancer in trneced recombinant chloramphenicol acetltrnsferase (CAT) pasmids. In erythroid K562 cells in which the HS2 enhancer is active, the HS2 sequence directs the synthesis of long enhancer transcripts that are initiated apparently from within the enhancer and elongated through the intervening DNA into the cis-linked CAT gene. In nonerythroid HL-60 cells in which the HS2 enhancer is inactive, long enhancer tra rpt are not detectable. Splitting the HS2 encer between two tandem Apl sites abolishes the synthesis of a group of log enhancer transcripts and results in loss of enhancer function and transcriptional silencing ofthe cis-linked CAT gene. In directing the synthesis ofRNA through the intervening DNA and the gene by a tracking and transcription mechan , the HS2 enhaner may (i) open up the chromatin structure of a gene domain and (it) deliver enhancer binding proteins to the promoter sequence where they may stimulate the transcription of the gene at the cap site.In seeking to identify the cis regulatory elements of the human (-like globin genes ( this laboratory (1) and others (2, 3) have mapped four erythroid-specific and developmentally stable DNase I-hypersensitive sites HS1, -2, -3, and -4 in the locus control region (LCR) between 50 and 70 kilobases (kb) upstream of the (3-globin gene. HS2 at -11 kb 5' of the embryonic E-and thus at -54 kb 5' of the ,B-globin gene has been shown to possess an erythroidspecific and developmentally stable enhancer function (4-6). It is capable of stimulating the transcription of embryonic E-, fetal -, and adult (B-globin genes in erythroid cells (7-10). In y6f3thalassemia, the deletion of HS2 and more upstream DNA is associated with transcriptional silencing of the far downstream f3-globin gene (11-13) and possibly of the whole (-like globin gene domain.How the HS2 enhancer cooperates with distant globin promoter sequences to stimulate the E-, y-, and (-globin genes at the respective developmental stage is not clear. Looping (14) and tracking (15) mechanisms have been proposed to explain how a distant enhancer may communicate with the promoter of a cis-linked gene (14,16,17). In the present study, the detection of HS2 enhancer transcripts in erythroid cells suggests that the tandem Apl sites and other sequence motifs in the enhancer may provide entry sites for transcription factors associated with the transcriptional machinery that track the DNA and synthesize long strands of RNA through the intervening DNA into the cis-linked gene.The possible biological significance of such long transcripts in enhancer function will be discussed. MATERIALS AND METHODSConstruction ofRecombinant Constructs. HS2-eP-CAT and EP-CAT have been described (5). 5'-HS2-eP-CAT was made by ...

The locus control region (LCR) of the human ␤-globin gene domain is defined by four erythroid-cell-specific DNase I-hypersensitive (HS) sites, HS1, -2, -3, and -4, located 50 to 70 kb upstream of the ␤-globin gene in a transcriptional direction: 5Ј HS4-HS3-HS2-HS1-//-ε-G ␥-A ␥-␦-␤ 3Ј (17,20,37,50). The LCR has been shown by studies of natural deletions of ␥␦␤-thalassemia (8,11,24) and by gene transfer experiments (14,18,20,51) to be indispensable for erythroid-cell-specific and high-level transcription of cis-linked globin genes and transgenes. The mechanism by which the LCR regulates transcription of the distant embryonic ε-, fetal ␥-, and adult ␤-globin genes at the respective developmental stages is not fully understood.In previous studies examining the mechanism of LCR function, a number of laboratories have investigated the ability of the individual HS sites to activate the transcription of cislinked genes in cell lines and transgenic mice. Among the LCR HS sites, HS2 has been found to possess developmental-stageindependent enhancer function (52). It is capable of stimulating the transcription of embryonic ε-, fetal ␥-, and adult ␤-globin genes in erythroid cells at the corresponding developmental stages (9,28,34,42,45,47) and may therefore constitute a major functional component of the LCR. However, a recent targeted deletion study shows that deletion of the HS2 enhancer from the murine LCR generated only mild effects on the expression of the ␤-like globin genes (15), suggesting that HS2 is not essential for LCR function and that LCR function may be due to the contribution of the other HS sites. Surprisingly, similar targeted deletion of HS3 also did not cause significant changes in the expression of the ␤-like globin genes (22). These findings suggest that LCR function is due not to the contribution of any specific HS site but to a series of interactions among its functional components, including the enhancer elements underlying the HS sites.In the above-described targeted deletion studies, deleting the HS2 or the HS3 sequence from the endogenous murine LCR and inserting in its place an independent transcription unit-a neomycin-resistant gene driven by a strong promoter-has been found to disrupt LCR function and cause severe anemia in and the deaths of homozygous transgenic mice (15,21). In the human ␤-LCR, inserting an independent transcription unit at a location between the HS2 enhancer and the downstream globin genes also disrupted LCR function (23) and caused the distantly downstream ␤-globin gene to be transcriptionally turned off. The mechanism by which the transcription of a foreign gene within the LCR might disrupt LCR function is not clearly understood (21).In an attempt to delineate the functional mechanism of the HS2 enhancer and ultimately of the LCR, we have previously analyzed the transcriptional status of the HS2 enhancer in transfected recombinant chloramphenicol acetyltransferase (CAT) plasmids (53). This earlier study showed that the transfected HS2 enhancer is itself transcribed in eryt...

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