The homeotic gene Ultrabithorax (Ubx) specifies metameric identities in multiple tissues of the thorax and abdomen in Drosophila melanogaster. Alternatively spliced Ultrabithorax mRNAs encode five protein isoforms that differ in internal sequences immediately adjacent to a homeodomain DNA-binding motif. Each of these proteins is phosphorylated in vivo at multiple serine and threonine residues. An extensive panel of monoclonal antibodies was raised against the Ultrabithorax proteins, including antibodies specific for individual isoforms and antibodies that discriminated between different phosphorylation states. Characterization of these antibodies provided insights into shared and isoform-specific features of Ultrabithorax protein structure that may be functionally important. Immunohistochemical staining experiments demonstrated that each isoform is expressed in a different stage- and tissue-specific pattern and suggested that Ultrabithorax protein phosphorylation is also developmentally regulated. These results support the hypothesis that alternative splicing and phosphorylation modulate developmentally specific functions of the Ubx gene.
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