The leucine zipper of NRL interacts with the CRX homeodomain - A possible mechanism of transcriptional synergy in rhodopsin regulation

Photoreceptor-specific expression of rhodopsin is mediated by multiple cis- acting elements in the proximal promoter region. NRL (neural retina leucine zipper) and CRX (cone rod homeobox) proteins bind to the adjacent NRE and Ret-4 sites, respectively, within this region. Although NRL and CRX are eac h individually able to induce rhodopsin promoter activity, when expressed t ogether they exhibit transcriptional synergy in rhodopsin promoter activati on. Using the yeast two-hybrid method and glutathione S-transferase pull-do wn assays, we demonstrate that the leucine zipper of NRL can physically int eract with CRX Deletion analysis revealed that the CRX homeodomain (CRX-HD) plays an important role in the interaction with the NRL leucine zipper. Al though binding with the CRX-HD alone was weak, a strong interaction was det ected when flanking regions including the glutamine-rich and the basic regi ons that follow the HD were included. A reciprocal deletion analysis showed that the leucine zipper of NRL is required for interaction with CRX-HD, Tw o disease-causing mutations in CRX-HD (R41W and R90W) that exhibit reduced DNA binding and transcriptional synergy also decrease its interaction with NRL, These studies suggest novel possibilities for protein-protein interact ion between two conserved DNA-binding motifs and imply that cross-talk amon g distinct regulatory pathways contributes to the establishment and mainten ance of photoreceptor function.