Characterization of a bovine homeodomain-zinc finger protein (BHZ) that interacts with the rhodopsin promoter and enhancer regions

Purpose. We described previously a partial length cDNA for a bovine homeodomain-zinc finger protein (BHZ) was identified by an expression cloning approach utilizing a.-, bait the -163 to -123 bp fragment from the bovine rhodopsin proximal promoter region (ARVO, 1996). The purpose of the present study was to characterize its cDNA structure, DNA binding properties, and retinal expression pattern. Methods. In situ hybridization and biochemical analyses were performed by established methods. Results. BHZ is homologous to NÜ-2A and the deltacrystalline enhancer-binding protein delta EF1, both of which are repressors of E2 box-mediated gene aciivation. Full length BHZ clones were obtained by a modified RT-PCR approach, and there appear to be two differentially spliced forms, one of which lacks the 5'-zinc finger domain. Purified His-tag-BHZ fusion protein shows specific binding to regions of rhodopsin promoter and enhancer including the sequence CACCT, but not to mutated or heterologous DNA fragments. On going in stilt hybridization studies of chick embryo retina sections with the chick BHZ ortholog demonstrate:) that BHZ transcript is present diffusely at E5. except in the mitotic: layer adjacent to the RPE. BHZ is present in precursors of the inner nuclear layer at E8 and in ganglion cells and some inner nuclear layer cells at El8 and P2 stages, but is not expressed in photoreceptor cells at any of these stages. Conclusion. BHZ can bind specifically to several sites in the rhodopsin promoter and enhancer regions and appear as a candidate to tunction as a negative regulator of rhodopsin expression in nonDhotoreceplor retinal cells.