DEFINITION OF THE TRANSCRIPTIONAL ACTIVATION DOMAINS OF 3 HUMAN HOX PROTEINS DEPENDS ON THE DNA-BINDING CONTEXT

Hox proteins control developmental patterns and cell differentiation i n vertebrates by acting as positive or negative regulators of still un identified downstream target genes. The homeodomain and other small ac cessory sequences encode the DNA-protein and protein-protein interacti on functions which ultimately dictate target recognition and functiona l specificity in vivo, The effector domains responsible for either pos itive or negative interactions with the cell transcriptional machinery are unknown for most Hox proteins, largely due to a lack of physiolog ical targets on which to carry out functional analysis, We report the identification of the transcriptional activation domains of three huma n Hox proteins, HOXB1, HOXB3, and HOXD9, which interact in vivo,vith t he autoregulatory and cross-regulatory enhancers of the murine Hoxb-1 and human HOXD9 genes. Activation domains have been defined both in a homologous context, i.e., within a HOX protein binding as a monomer or as a HOX-PBX heterodimer to the specific target, and in a heterologou s context, after translocation to the yeast Gal4 DNA-binding domain, T ransfection analysis indicates that activation domains can be identifi ed in different regions of the three HOX proteins depending on the con text in which they interact with the DNA target. These results suggest that Hox proteins may be multifunctional transcriptional regulators, interacting with different cofactors and/or components of the transcri ptional machinery depending on the structure of their target regulator y elements.