The transcription unit of the Drosophila engrailed locus: an unusually small portion of a 70,000 bp gene.

Transcripts from the engrailed gene of Drosophila melanogaster have been characterized by Northern, S1 nuclease sensitivity, and primer extension analyses. The engrailed gene encodes three poly(A)+ transcripts (3.6 kb, 2.7 kb, and 1.4 kb) that derive from a 3.9-kb portion of the genome. No other transcribed regions were found up to 16 kb downstream and 48 kb upstream of the engrailed transcription unit, the portion of the genome to which engrailed mutations have been mapped. The structures of the engrailed transcripts are unaffected by lethal engrailed mutations that break the locus at points in the transcriptionally silent regions. Transcripts expressed by 145 kb of DNA that surround the engrailed locus were also identified by Northern analysis. In contrast to the large portion of the engrailed gene that is transcriptionally inactive, most of the surrounding regions are 10-fold more densely populated with transcripts. We presume that the unusually large silent region at the periphery of the engrailed transcription unit signify the presence of special mechanisms that regulate its expression.