Abdominal-B protein isoforms exhibit distinct cuticular transformations and regulatory activities when ectopically expressed in Drosophila embryos.

The Drosophila homeotic gene Abdominal-B includes two genetically distinct elements, a morphogenetic (m) activity and a regulatory (r) activity. The proteins responsible for these activities were ectopically expressed in fly embryos. The larval cuticular transformations which result are consistent with the genetically defined role of each protein during normal embryogenesis. Both ABD-B proteins activate ectopic expression of transcripts encoding the m protein, but the levels of Antennapedia, Ultrabithorax and abdominal-A transcripts are differentially repressed. A structural and functional comparison of the ABD-B proteins and a chimeric DFD/ABD-B protein reaffirms that target specificity is largely determined by the homeodomain region and suggests protein domains outside of the homeodomain influence the activation or repression of target gene expression.