ERX, A NOVEL RETINA-SPECIFIC HOMEODOMAIN TRANSCRIPTION FACTOR, CAN INTERACT WITH RET 1 PCEI SITES/

Our previous studies on the transcriptional regulation of rod opsin ge ne expression had defined a strikingly conserved element, Ret 1/PCEI, present in the upstream regulatory regions of opsin and other photorec eptor-specific genes. This element interacts with a 40 kDa, developmen tally regulated, retina-specific protein. In this study we report the cloning of the novel retina specific homeodomain protein Erx. Erx cont ains a homeodomain that is 79% homologous to that of Drosophila empty spiracles. This 40 kDa protein can interact with the Ret I element in electrophoretic mobility shift assays. Mutation of key residues in Ret 1 eliminates all Erx binding. Transient transfection of Y79 retinobal stoma cells with Erx leads to significant transcriptional activation o f a reporter gene via Ret 1 elements. We conclude that Erx is the Ret 1 binding activity. This is the first example of a Q50 homeodomain pro tein expressed in retinal photoreceptors. (C) 1998 Academic Press.