FUNCTIONAL DIFFERENCES BETWEEN ULTRABITHORAX PROTEIN ISOFORMS IN DROSOPHILA-MELANOGASTER - EVIDENCE FROM ELIMINATION, SUBSTITUTION AND ECTOPIC EXPRESSION OF SPECIFIC ISOFORMS

The homeotic selector gene Ultrabithorax (Ubx) specifies regional iden tities in multiple tissues within the thorax and abdomen of Drosophila melanogaster. Ubx encodes a family of six developmentally specific ho meodomain protein isoforms translated from alternatively spliced mRNAs . The mutant allele Ubx(195) contains a stop codon in exon mil, one of three differential elements, and consequently produces functional UBX protein only from mRNAs of type IVa and IVb, which are expressed main ly in the central nervous system. Although it retains activity for oth er processes, Ubx(195) behaves like a null allele with respect to deve lopment of the peripheral nervous system, indicating that UBX-IVa and IVb alone do not contribute detectable Ubx function for this tissue. T he mutant allele Ubx(MX17) contains an inversion of exon mII. We find that this allele only produces mRNAs of type IVa, but the expression p attern of the resulting UBX-IVa protein is indistinguishable from that of total UBX protein expression in wild-type embryos. The phenotype o f homozygous UbX(MX17) embryos indicates that UBX-IVa cannot substitut e functionally for other isoforms to promote normal development of the peripheral nervous system. This functional limitation is confirmed by a detailed analysis of the peripheral nervous system in embryos that express specific UBX isoforms ectopically under control of a heat shoc k promoter. Additional observations suggest that UBX isoforms also dif fer in their ability to function in other tissues.