PBX RAISES THE DNA-BINDING SPECIFICITY BUT NOT THE SELECTIVITY OF ANTENNAPEDIA HOX PROTEINS

We have used a binding site selection strategy to determine the optima l binding sites for Pbx proteins by themselves and as heterodimeric pa rtners with various Hox gene products, Among the Pbx proteins by thems elves, only Pbx3 binds with high affinity, as a monomer or as a homodi mer, to an optimal binding site, TGATTGATTTGAT, An inhibitory domain l ocated N terminal of the Pbx1 homeodomain prevents intrinsic Pbx1 bind ing to this sequence. When complexed with Hoxc-6, each of the Pbx gene products binds the same consensus sequence, TGATTTAT, which differs f rom the site bound by Pbx3 alone. Three members of the Antennapedia fa mily, Hoxc-6, Hoxb-7, and Hoxb-8, select the same binding site in conj unction with Pbx1, The affinities of these proteins as heterodimeric p artners with Pbx1 for the selected optimal binding site are similar, H owever, the binding specificity of Hox proteins for optimal binding si tes is increased, compared to nonspecific DNA, in the presence of Pbx proteins. Thus, while cooperative DNA binding involving heterodimers o f Pbx and Hox gene products derived from members within the Antennaped ia family does not increase binding site selectivity, DNA binding spec ificity of the Hox gene products is significantly enhanced in the pres ence of Pbx.