EVOLUTIONARY CONSERVATION OF THE STRUCTURE AND EXPRESSION OF ALTERNATIVELY SPLICED ULTRABITHORAX ISOFORMS FROM DROSOPHILA

In Drosophila melanogaster, alternatively spliced mRNAs from the homeo tic gene Ultrabithorax (Ubx) encode a family of structurally distinct homeoprotein isoforms. The developmentally regulated expression patter ns of these isoforms suggest that they have specialized stage- and tis sue-specific functions. To evaluate the functional importance of UBX i soform diversity and gain clues to the mechanism that regulates proces sing of Ubx RNAs, we have investigated whether the Ubx RNAs of other i nsects undergo similar alternative splicing. We have isolated and char acterized Ubx cDNA fragments from D. melanogaster, Drosophila pseudoob scura, Drosophila hydei and Drosophila virilis, species separated by a s much as 60 million years of evolution, and have found that three asp ects of Ubx RNA processing have been conserved. (1) These four species exhibit identical patterns of optional exon use in a region adjacent to the homeodomain. (2) These four species produce the same family of UBX protein isoforms with identical amino acid sequences in the option al exons, even though the common amino-proximal region has undergone s ubstantial divergence. The nucleotide sequences of the optional exons, including third positions of rare codons, have also been conserved st rongly, suggesting functional constraints that are not limited to codi ng potential. (3) The tissue- and stage-specific patterns of expressio n of different UBX isoforms are identical among these Drosophila speci es, indicating that the developmental regulation of the alternative sp licing events has also been conserved. These findings argue for an imp ortant role of alternative splicing in Ubx function. We discuss the im plications of these results for models of UBX protein function and the mechanism of alternative splicing.