A cap 'n' collar protein isoform contains a selective Hox repressor function

We have characterized a protein isoform (CncB) from the Drosophila cap 'n' collar locus that selectively represses cis-regulatory elements that are ac tivated by the Hox protein Deformed. Of the three Cnc protein isoforms, Cnc B is expressed in a localized pattern in mandibular and labral cells of the head during mid-stages of embryogenesis. When CncB protein is absent or re duced, mandibular cells are homeotically transformed toward maxillary ident ities. This transformation is associated with persistent Deformed expressio n in anterior mandibular cells, since the Deformed autoactivation circuit i s normally antagonized by CncB function in these cells. Heat-shock-induced ectopic expression of CncB in mid-stages of embryogenesis is sufficient to attenuate the activity of Dfd response elements in maxillary epidermal cell s, but appears to have no effect in trunk epidermal cells on either the fun ction or the response elements of other Hox proteins. CncB provides a mecha nism to modulate the specificity of Hox morphogenetic outcomes, which resul ts in an increase in the segmental diversity in the Drosophila head.