The structure of the Ultrabithorax promoter of Drosophila melanogaster.

The sequence of 4118 nucleotides upstream of the putative initiator codon of the D. melanogaster Ultrabithorax protein was determined. The transcription initiation site for the corresponding mRNA was identified by S1 nuclease mapping and primed extension. It appears that all embryonic RNA products that encode the first protein exon are initiated approximately one kilobase upstream of the initiator methionine, suggesting a unique Ultrabithorax promoter. The unusually long mRNA leader is unspliced. Upstream of the initiator codon occur two additional methonine codons; the first one is followed by an open reading frame encoding a putative polypeptide of 69 amino acids. We discuss the role of the leader and 5' flanking sequences with respect to transcriptional and posttranscriptional control of Ultrabithorax gene expression.