Analysis of promoter activity of 5'-upstream regions of zebrafish olfactory receptor genes.

The vertebrate olfactory system receives and discriminates a great variety of odorants. Many lines of evidence suggest that individual olfactory neuron expresses a single type or limited types of the olfactory receptor genes. However, the mechanism of selection of a single gene in the olfactory receptor family remains unclear. In the present study, we utilized zebrafish to identify the promoter element of the olfactory receptor genes in their 5'-upstream regions. First, we isolated a number of zebrafish olfactory receptor genes. These olfactory receptor genes were specifically expressed in the olfactory tissue as visualized by whole mount in situ hybridization analysis. Time of onset of the expression of each receptor clone varied from 24 h to 48 h postfertilization. Then, we injected various constructs containing the 5'-upstream regions of the olfactory receptor genes connected to β-galactosidase reporter gene into fertilized zebrafish embryos. Constructs from two independent olfactory receptor genes exerted β-galactosidase (promoter) activity that is specifically upregulated in the olfactory tissue. Use of either longer or deleted constructs of these two genes diminished the promoter activity in the olfactory tissue. From these results we discuss the mechanism of the transcription of the olfactory receptor genes in the olfactory neurons.