The chicken alpha-globin gene domain is transcribed into a 17-kilobase polycistronic RNA

The 5' start sites and the 3' ends of giant transcripts of the approximately 20-kilobase (kb)-long chicken alpha-globin gene domain were identified by reverse transcription with specific primers and by nuclease S1 mapping using cloned and sequenced restriction fragments of the domain. A transcriptional unit of approximately 17 kb was found that includes all three embryonic and adult genes of the cluster. The largest transcript initiates 8 kb upstream of the gene, within a cluster of A + T-rich sequences placed upstream of a matrix attachment point, at one of several CAA(A)T boxes framing a cluster of four TATA boxes. The 5' ends of a group of 2.5-, 5-, and 12-kb globin transcripts accumulating in avian erythroblastosis virus-transformed cells, which transcribe globin genes abortively, map to the sequence ATATATAATAA 1 kb upstream of the embryonic pi-globin gene. This sequence might correspond to a site of RNA processing or of alternative transcription initiation. Transcription of the domain ends about 2 kb downstream of the last gene of the cluster, downstream of an enhancer and immediately upstream of a CR1 repetitive element in an A + T-rich sequence that includes a matrix attachment site. These data indicate that full-domain transcripts including embryonic as well as adult alpha-globin genes exist, and that the region transcribed is framed by A + T-rich linkers and matrix attachment points.