Cooperative dimerization of the POU domain protein Brn-2 on a new motif activates the neuronal promoter of the human aromatic L-amino acid decarboxylase gene

The neuronal promoter of the human aromatic L-amino acid decarboxylase (AADC) gene contains a perfectly palindromic element (TB) that conforms to the structure of a POU domain protein binding site of the MORE+2 type. The TB motif (located at nts -900/-872 relative to the neuronal cap site) bears striking similarities with the dimeric Pit-1 binding site from growth hormone gene promoter (GH-1), and it enhanced the activity of the minimal tk promoter in transfected SK-N-BE neuroblastoma cells. In transfected COS-7 cells, the expression of a 3xTB-tk-luc was stimulated up to 11-fold by the overexpressed Brn-2 protein. In AADC gene neuronal promoter, we previously characterized a bipartite regulatory element (ONF for octamer-like/NF-Y, nts -86/-57) that binds Brn-2 and NF-Y proteins in a cooperative manner. We now show that both TB and ONF sites participate in the activation of the neuronal promoter by Brn-2. EMSA experiments showed that the recombinant Brn-2 POU domain dimerized on the TB element in a cooperative manner. By site directed mutagenesis of the POU domain of Brn-2, the dimerization interface on the TB element was localized to the hydrophobic pocket of the POU specific domain and the C-terminal part of the POU homeodomain.