We have identified an abundant transcript initiated upstream from the canonical cap site of human α1 globin gene in bone marrow cells and in COS-7 cells transfected with an α1 globin gene-containing plasmid. Similar to the major α1 globin transcript, this upstream RNA is present almost exclusively in the cytoplasm of the transfected COS-7 cells. It is also synthesized efficiently in vitro by RNA polymerase II in the nuclear extracts prepared from a Hela cell line and an erythroleukemia cell line, K562. RNAs isolated from these cell lines, however, do not contain this upstream transcript. The putative 5′ end of the α1 globin upstream RNA is mapped by primer extension to base -45, which is located in between the CCAAT and TATA boxes. The synthesis of this RNA in vitro and in vivo, and the close proximity of its 5′ end to the promoter of the α1 globin gene suggest a common mechanism regulating the transcriptional initiation of both the upstream and the major α1 globin RNAs.
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