Trans-acting factors required for inclusion of regulated exons in the Ultrabithorax mRNAs of Drosophila melanogaster

Alternatively spliced Ultrabithorax mRNAs differ by the presence of interna l exons mi and mil. Two approaches were used to identify trans-acting facto rs required for inclusion of these cassette exons. First, mutations in a se t of genes implicated in the control of other alternative splicing decision s were tested for dominant effects on the Ubx alternative splicing pattern. To identify additional genes involved in regulation of Ubx splicing, a lar ge collection of deficiencies was tested first for dominant enhancement of the haploinsufficient Ubx haltere phenotype and second for effects on the s plicing pattern. Inclusion of the cassette exons in Ubx mRNAs was reduced s trongly in heterozygotes for hypomorphic alleles of hrp48, which encodes a member of the hnRNP A/B family and is implicated in control of P-element sp licing. Significant reductions of mi and mil inclusion were also observed i n heterozygotes for loss-of-function alleles of virilizer, fl(2)d, and croo ked neck. The products of virilizer and fl(2)d are also required for Sxl au toregulation at the level of splicing; crooked neck encodes a protein with structural similarities to yeast-splicing factors Prp39p and Prp42p. Deleti on of at least five other loci caused significant reductions in the inclusi on of mi and/or mII. Possible roles of identified factors are discussed in the context of the resplicing strategy for generation of alternative Ubx mR NAs.