The Antennapedia and Abdominal-B homeodomains bind to TFIIEß, but the Even-skipped homeodomain does not. Using a two-hybrid assay performed in cultured cells, it can be shown that the interaction occurs in vivo. The Abdominal-B homeodomain is shown to activate transcription in vitro, and this activation can be blocked with anti-TFIIEß antibody without affecting basal transcription levels. How can an interaction between TFIIEß and the homeodomain contribute to transcriptionsal activation? At a biochemical level. TFIIE promotes the phosphorylation of the C-terminal domain (CTD) of RNA polymerase II by TFIIH, but also inhibits a helicase activity shown by TFIIH that may be required to unwind the DNA prior to transcription initiation. Since the phosphorylated RNA polymerase II is the form of the enzyme that actively elongates transcripts, the ability of a homeodomain to attract TFIIE to the initiation complex would serve to stimulate transcription by enhancing the kinase activity of TFIIH resulting in a completely phosphorylated CTD. Alternatively, since TFIIE inhibits the helicase activity of TFIIH, it has been proposed that TFIIE might be removed from the complex following the CTD phosphorylation, and this be facilitated by interaction with a homeodomain (Zhu, 1996 and references).

The Abdominal-B gene is unique among homeotic Drosophila genes as it encodes two proteins ( morphogenetic [m]and regulatory [r]) that confer different functions. The m protein corresponds to an r protein with a largeN-terminal extension; the two proteins contain the same homeodomain. There are two strong transcriptional activation domains in the part common to the two proteins. The m-specific exon contains additional transcriptional activation potential.Despite this, the m protein is a weaker transcriptional activator than the r protein. Apparently, thereare inhibitory sequences in the m-specific exon which, in the embryo, may have a role in masking rfunction in the intact m protein (Ali, 1991).

A heat-shock promoter/selector gene was constructed that encodes a Deformed/Abdominal-B chimera in which the Abdominal-B homeodomain is substituted for that of Deformed. Expression ofthis chimeric protein throughout the embryo causes morphological transformation of anteriorsegments toward more posterior identities. A number of other homeotic selector genes, all normallyrepressed by Abdominal-B, are ectopically activated by the chimeric protein. These results supportthe hypothesis that the target specificity of similar homeodomain proteins is largely determined bythe amino acid sequence of the homeodomain (Kuziora, 1990).

A structural and functional comparison of the r and m ABD-B proteins and a chimeric Deformed/ABD-B protein reaffirmsthat target specificity is largely determined by the homeodomain region and suggests proteindomains outside of the homeodomain influence the activation or repression of target geneexpression (Kuziora, 1993).

The DNA sequence preferences were compared ofhomeodomains encoded by four of the eight Drosophila HOM proteins. One of the four,Abdominal-B, binds preferentially to a sequence with an unusual 5' TTAT 3' core, whereas theother three prefer 5' TAAT 3'. Of these latter three, the Ultrabithorax and Antennapediahomeodomains display indistinguishable preferences outside the core while Deformed differs. Thus,with three distinct binding classes defined by four HOM proteins, differences in individual siterecognition may account for some but not all of HOM protein functional specificity. Amino acid residues within the N-terminal arm are responsible for the sequencespecificity differences between the Ultrabithorax and Abdominal-B homeodomains. Similarities anddifferences at the corresponding positions within the N-terminal arms are conserved in thevertebrate Abdominal-B-like HOM proteins, which play critical roles in limb specifications as wellas in regional specification along the anterior-posterior axis (Ekker, 1994).

Abdominal-B:Biological Overview | Evolutionary Homologs | Promoter Structure | Transcriptional Regulation | Targets of activity | Developmental Biology | Effects of Mutation | References

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