Cap 'n' collar B cooperates with a small Maf subunit to specify pharyngealdevelopment and suppress Deformed homeotic function in the Drosophila head

The basic-leucine zipper protein Cap 'n' collar B (CncB) suppresses the seg mental identity function of the Hox gene Deformed (Dfd) in the mandibular s egment of Drosophila embryos. CncB is also required for proper development of intercalary, labral and mandibular structures. In this study, we provide evidence that the CncB-mediated suppression of Dfd requires the Drosophila homolog of the mammalian small Maf proteins, Maf-S, and that the suppressi on occurs even in the presence of high amounts of Dfd protein. Interestingl y, the CncB/Maf-S suppressive effect can be partially reversed by overexpre ssion of Homothorax (Hth), suggesting that Hth and Extradenticle proteins a ntagonize the effects of CncB/Maf-S on Dfd function in the mandibular segme nt. In embryos, multimers of simple CncB/Maf-S heterodimer sites are transc riptionally activated in response to CncB, and in tissue culture cells the amino-terminal domain of CncB acts as a strong transcriptional activation d omain. There are no good matches to CncB/Maf binding consensus sites in the known elements that are activated in response to Dfd and repressed in a Cn cB-dependent fashion. This suggests that some of the suppressive effect of CncB/Maf-S proteins on Dfd protein function might be exerted indirectly, wh ile some may be exerted by direct binding to as yet uncharacterized Dfd res ponse elements. We also show that ectopic CncB is sufficient to transform v entral epidermis in the trunk into repetitive arrays of ventral pharynx. We compare the functions of CncB to those of its vertebrate and invertebrate homologs, p45 NF-E2, Nrf and Skn-1 proteins, and suggest that the pharynx s elector function of CncB is highly conserved on some branches of the evolut ionary tree.