PARALLEL REGULATION OF FETAL GENE-EXPRESSION IN DIFFERENT PHOTORECEPTOR CELL-TYPES

Transcriptional induction for multiple bovine rod-specific photorecept or genes has been shown to coincide with the first emergence of rod ou ter segments (two-thirds gestation) suggesting a coordinate regulation of these genes. Since cone visual transduction proteins are encoded f or by distinct genes and mammalian cone cell genesis has been found to precede rod cell genesis it is important to determine when induction of photoreceptor-specific gene expression occurs in cone cells and how the timing of this event compares to that in rods. RNase protection a ssays specific for each cell type-specific gene were used to compare m RNA levels for bovine rod and blue cone gamma subunits of cGMP phospho diesterase and rhodopsin and red cone opsin from before transcriptiona l induction (4 months gestation) to the adult. Both pairs of rod and c one visual transduction mRNAs exhibit indistinguishable transcriptiona l induction points at around 6 months of gestation. Before this time, transcripts of all four genes are present at low, but detectable level s. Thus, although rod and cone cells are distinct cell types and cone cells may be committed well before rod cells early in development, dur ing the final trimester when elaboration of outer segments begins, the re appears to be a coordinated regulation of both rod- and cone-specif ic visual transduction genes.