MEMBERS OF THE MEIS1 AND PBX HOMEODOMAIN PROTEIN FAMILIES COOPERATIVELY BIND A CAMP-RESPONSIVE SEQUENCE (CRS1) FROM BOVINE CYP17

The mammalian Pbx homeodomain proteins provide specificity and increas ed DNA binding affinity to other homeodomain proteins, A cAMP-responsi ve sequence (CRS1) from bovine CYP17 has previously been shown to be a binding site for Pbx1, A member of a second mammalian homeodomain fam ily, Meis1, is now also demon strated to be a CRS1-binding protein upo n purification using CRS1 affinity chromatography, CRS1 binding comple xes from Y1 adrenal cell nuclear extract contain both Pbx1 and Meis1, This is the first transcriptional regulatory element reported as a bin ding site for members of the Meis1 homeodomain family. Pbx1 and Meis1 bind cooperatively to CRS1, whereas neither protein can bind this elem ent alone. Mutagenesis of the CRS1 element indicates a binding site fo r Meis1 adjacent to the Pbx site, All previously identified Pbx bindin g partners have Pbx interacting motifs that contain a tryptophan resid ue amino-terminal to the homeodomain that is required for cooperative binding to DNA with Pbx. Members of the Meis1 family contain one trypt ophan residue amino-terminal to the homeodomain, but site-directed mut agenesis indicates that this residue is not required for cooperative C RS1 binding with Pbx, Thus, the Pbx-Meis1 interaction is unique among Pbx complexes. Meis1 also cooperatively binds CRS1 with the Pbx homolo gs extradenticle from Drosophila melanogaster and ceh-20 from Caenorha bditis elegans, indicating that this interaction is evolutionarily con served, Thus, CYP17 CRS1 is a transcriptional regulatory element conta ining both Pbx and Meis1 binding sites, which permit these two homeodo main proteins to bind and potentially regulate cAMP-dependent transcri ption through this sequence.