DISTINCT HOX N-TERMINAL ARM RESIDUES ARE RESPONSIBLE FOR SPECIFICITY OF DNA RECOGNITION BY HOX MONOMERS AND HOX-PBX HETERODIMERS

Dimerization with extradenticle or PBX homeoproteins dramatically impr oves DNA binding by HOX transcription factors, indicating that recogni tion by such complexes is important for HOX specificity. For HOX monom eric binding, a major determinant of specificity is the flexible N-ter minal arm. It makes base-specific contacts via the minor groove, inclu ding one to the Ist position of a 5'-TNAT-3' core by a conserved argin ine (Arg-5). Here we show that Arg-5 also contributes to the stability of HOX PBX complexes, apparently by forming the same DNA contact. We further show that heterodimers of PBX with HOXA1 or HOXD4 proteins hav e different specificities at another position recognized by the N-term inal arm (the 2nd position in the TNAT core). Importantly, N-terminal arm residues 2 and 3, which distinguish the binding of HOXA1 and HOXD4 monomers, play no role in the specificity of their complexes with PBX In addition, HOXD9 and HOXD10, which are capable of binding both TTAT and TAAT sites as monomers, can cooperate with PBX1A only on a TTAT s ite. These data suggest that some DNA contacts made by the N-terminal arm are altered by interaction with PBX.