A CARBOXYL-TERMINAL EXTENSION OF THE ZINC-FINGER DOMAIN CONTRIBUTES TO THE SPECIFICITY AND POLARITY OF PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR DNA-BINDING

Heterodimers of the peroxisome proliferator-activated receptors (PPAR) and the retinoid X receptors (RXR) recognize response elements (PPREs ) that exhibit the consensus sequence 5'-A(A/T)CT(A/G)GGNCAAAG(G/T)TCA -3', The consensus PPRE includes both a Fil-extension and a direct rep eat (DR1) comprised of two canonical core recognition sequences (under lined) for nuclear receptor zinc fingers separated by a single nucleot ide spacer. The extended binding site recognized by PPARs is very simi lar to sites that bind monomers of the nuclear receptors Rev-ErbA and ROR suggesting that the latter could bind to PPREs and affect gene tra nscription. However, Rev-ErbA and ROR bind weakly to naturally occurri ng PPREs relative to the consensus binding site, and significant effec ts on PPAR alpha transactivation of a CYP4A6-Z reporter were not obser ved. In contrast, PPAR/ RXR heterodimers bind to a DR2 element contain ing the conserved 5'-extended sequence that is recognized by dimers of ROR alpha or Rev-ErbA. PPAR alpha/RXR alpha positively regulate trans cription from this element, and co-expression of Rev-ErbA blocks this effect. The nuclear receptors NGFI-B and ROR utilize a carboxyl-termin al extension (CTE) of the zinc finger DNA binding domain in their inte ractions with the 5'-extension of a single zinc finger-binding site. D NA binding domains (DBD) of PPARs alpha, delta, and gamma that contain the zinc finger motif and a CTE display binding to core recognition s equences that is dependent on the 5'-extended sequence found in PPREs. Unlike DBDs of other nuclear receptors that form heterodimers with RX R, the PPAR-DBDs did not exhibit cooperative binding with the DBD of R XR and exhibit the opposite polarity for binding to the direct repeat motif. In contrast to the corresponding DBD of RXR, the PPAR-DBDs bind as monomers to a single extended binding site as well as to the conse nsus PPRE. A chimera linking the zinc finger domain of RXR alpha to th e CTE from PPAR alpha bound to a single extended binding site indicati ng a functional role for the CTE of PPARs in extended binding site rec ognition.