Elucidating the transcriptional regulation of zebrafish odorant receptor genes

A large family of G-protein coupled receptors are responsible for the detection of odorants at the periphery. Previous work in mouse and zebrafish indicates that each olfactory receptor neuron expresses only one of a large repertoire of these odorant receptor genes. In addition, those neurons expressing the same receptor project to common glomeruli in the olfactory bulb in mouse, and this precise pattern of projection requires the expression of a functional odorant receptor protein. Thus, the choice of receptor is important to both receptor neuron identity and neuronal connectivity. In view of the observation that OR genes are tightly clustered in the genomes of all organisms examined, examination of these clusters may reveal regulatory motifs required for OR gene expression. Our lab has previously characterized 140 kb of zebrafish sequence containing 20 OR genes. Expanding this contig to ∼210 kb revealed four additional olfactory receptor genes, three of which belong to a previously uncharacterized subfamily. An additional 60 kb unlinked BAC clone containing a single OR gene was also analyzed. A 13 bp imperfect palindromic cis-acting motif correlated with the early onset of receptor expression, identified in the analysis of the original 140 kb contig, may be located upstream of this additional unlinked gene. Interestingly, this motif overlaps a known sequence similar to an olfactory-specific transcription factor binding site, and may be involved in the transcriptional regulation of these genes. A one hybrid screen was initiated to identify proteins that could potentially bind to this 13 bp motif yielded three false positives; this screen is currently in progress. Other studies focused on transgenic approaches to identify the cis-acting regions required to recapitulate the endogenous expression pattern of an OR gene. Each of these approaches aim to elucidate the mechanisms underlying the exquisite transcriptional regulation of odorant receptor genes.