Structure and expression of a family of Ultrabithorax mRNAs generated by alternative splicing and polyadenylation in Drosophila.

The 77-kb primary transcript of the homeotic Ultrabithorax (Ubx) gene is alternatively spliced to yield at least five different coding regions. Each is restricted to either a 3.2- or a 4.3-kb size class generated by alternative polyadenylation. The pathways for splicing and polyadenylation are therefore coordinately regulated, and because the relative abundance of the respective mRNAs varies throughout development, these pathways also appear to be developmentally regulated. Translation of these mRNAs yields a family of Ubx proteins characterized by constant amino- and carboxy-proximal regions of 247 and 99 amino acid residues, respectively. Members of this family are distinguished by a short variable region that links the constant regions and consists of different combinations of three optional elements of 9, 17, and 17 residues. Only four amino acid residues separate this variable region from the 60-residue homeo domain of the carboxy-terminal constant region. This proximity suggests that functional differences among the Ubx proteins derive from the differential effects of their variable regions on the DNA-binding capacity of the homeo domain. An argument is made that these functional differences are tissue specific.