Thechicken a-globin genedomainistranscribed intoa17-kilobase polycistronic RNA

The5'start sites andthe3'endsofgiant transcripts ofthe-20-kilobase (kb)-long chicken a-globin gene domainwereidentified byreverse transcription withspecific primers andbynuclease S1mapping using cloned andse- quenced restriction fragments ofthedomain. Atranscriptional unit of-17kbwasfound that includes all three embryonic and adult genes ofthecluster. Thelargest transcript initiates 8kb upstream ofthegene, within acluster ofA+T-rich sequences placed upstream ofamatrix attachment point, atoneofseveral CAA(A)T boxes framing acluster offour TATAboxes. The5' endsofa groupof2.5-, 5-,and12-kbglobin transcripts accumulating inavianerythroblastosis virus-transformed cells, whichtranscribe globin genesabortively, maptothe sequence ATATATAATAA1kbupstream oftheembryonic iT-globin gene. Thissequence might correspond toasite ofRNA processing orofalternative transcription initiation. Transcrip- tion ofthedomain endsabout 2kbdownstream ofthelast gene ofthecluster, downstream ofanenhancer andimmediately upstream ofaCR1repetitive element inanA+T-rich sequence that includes amatrix attachment site. These dataindicate that full-domain transcripts including embryonic aswell asadult a-globin genes exist, andthat theregion transcribed isframed byA+T-rich linkers andmatrix attachment points.