THE BASIC MOTIF LEUCINE-ZIPPER TRANSCRIPTION FACTOR NRL CAN POSITIVELY REGULATE RHODOPSIN GENE-EXPRESSION

The retinal protein Nrl belongs to a distinct subfamily of basic motif -leucine zipper DNA-binding proteins and has been shown to bind extend ed AP-1-like sequence elements as a homo- or heterodimer. Here, we dem onstrate that Nrl can positively regulate the expression of the photor eceptor cell-specific gene rhodopsin. Electrophoretic mobility-shift a nalysis reveals that a protein(s) in nuclear extracts from bovine reti na and the Y79 human retinoblastoma cell line binds to a conserved Nrl response element (NRE) in the upstream promoter region of the rhodops in gene. Nrl or an antigenically similar protein is shown to be part o f the bound protein complex by supershift experiments using Nrl-specif ic antiserum. Cotransfection studies using an Nrl-expression plasmid a nd a luciferase reporter gene demonstrate that interaction of the Nrl protein with the -61 to -84 region of the rhodopsin promoter (which in cludes the NRE) stimulates expression of the reporter gene in CV-1 mon key kidney cells, This Nrl-mediated transactivation is specifically in hibited by coexpression of a naturally occurring truncated form of Nrl (dominant negative effect). Involvement of Nrl in photoreceptor gene regulation and its continued high levels of expression in the adult re tina suggest that Nrl plays a significant role in controlling retinal function.