Alternative RNA products from the Ultrabithorax domain of the bithorax complex.

The homeotic gene, Ultrabithorax (Ubx) is involved in specifying the identities of several segments in the fly Drosophila melanogaster. The structures of over 60 independent Ubx cDNAs have been examined. There are two major species of transcripts, 3.2 and 4.3 kb in length, which are produced by alternate sites of polyadenylation. Differential splicing gives rise to at least five variant Ubx proteins. The variant forms share common 5' and 3' exons but differ in their small internal 'micro' exons. Additional variation is generated by two separate splice donor sites at the end of the common 5' exon, situated 27 bp apart. Northern hybridization and S1 nuclease protection studies of RNA from various developmental stages and tissue types reveal that the alternate splicing and the choice of polyadenylation site are each differentially regulated in both a temporal and a tissue specific manner. Additional transcripts were found just downstream of the Ubx transcription unit, which may be products of the lethal left of bithorax gene (llb).