Reporter gene expression in cones in transgenic mice carrying bovine rhodopsin promoter/lacZ transgenes.

Rhodopsin gene expression has been used as a model system to study the mechanisms regulating photoreceptor gene expression. Previous transgenic experiments using rhodopsin promoter/lacZ fusion constructs identified some of the cis-acting DNA elements responsible for photoreceptor cell-specific expression. However, the issue of rod specificity vs. photoreceptor (rod and cone) specificity of the elements was not resolved. To address this issue, the specificity of reporter gene expression in the retinas of transgenic mice carrying bovine rhodopsin promoter/lacZ (beta-galactosidase) fusion genes was assessed using X-gal staining and electron microscopy. Two independent transgenic lines, one carrying a rhodopsin promoter fragment extending from -2174 to +70 base pairs (bp) relative to the messenger RNA start site and another line carrying a fragment from -222 to +70 bp, both showed reporter gene expression in cones as well as rods, although the level of staining appeared to be less in the cones than in the rods. These results demonstrate that the -2174 to +70 bp and -222 to +70 bp bovine rhodopsin promoter fragments are not rod-specific in transgenic mice and indicate that the existence of rod promoter mediated-expression in cones must be considered when interpreting results from transgenic experiments utilizing the rhodopsin promoter.