Regulation of rhodopsins: Single nucleotides are critical for photoreceptor subtype-specific expression. Jens Rister, Claude Desplan. New York University, Department of Biology, 100 Washington Square East, New York, NY.

   Cis-regulatory elements (CREs) control where, when, and how strongly genes are expressed. We are interested in the regulatory mechanisms that generate the complex expression patterns of rhodopsins in specific photoreceptor subtypes involved in color vision. Rhodopsins are particularly suited for a cis-regulatory analysis, as their expression is mostly transcriptional, while compact CREs of less than 300 base pairs are sufficient to reproduce their endogenous expression patterns. Yet, it is still unclear how the combinatorial input of transcriptional activators and repressors (that is integrated in the CREs) controls spatiotemporal rhodopsin expression. We performed an extensive dissection of the rhodopsin promoters. They all contain an 11 base pair rhodopsin core sequence I (RCSI, consensus: TAATYNRATTN), which is necessary for rhodopsin expression. Multimerization of a generic RCSI (TAATYNRATTA) drives reporter expression in all photoreceptors. It was therefore suggested that the RCSI plays a role in the general activation of rhodopsins. However, each rhodopsin contains a preferred, highly conserved RCSI that differs from the consensus in 1-2 base pairs. Multimerization of each of these individual RCSI motifs did not drive reporter expression in all photoreceptors, as had been observed with the generic RCSI, but in subsets of photoreceptors. Moreover, point mutations affecting the single RCSI base pair differences in the wildtype promoter context led to an expansion of reporter expression into other photoreceptor subtypes. Depending on the respective RCSI, we found that the de-repression was either due to disruption of repressor sites or due to the generation of activator sites. Thus, these data suggest that subtle, but highly conserved differences in the RCSI are critical for subtype-specific rhodopsin expression. Single base pair changes therefore appear to be a major driving force in the evolution of mutually exclusive rhodopsin expression, a prerequisite for color vision.