The genomic structure of human olfactory receptor genes

The genomic and cDNA structures were studied for eight human olfactory receptor (OR) genes within the chromosome 17p13.3 cluster. A common gene structure was revealed, which included an approximately 1-kb intronless coding region terminated by a signal for polyadenylation and a variable number of upstream noncoding exons. The latter were found to be alternatively spliced, giving rise to different isoforms of OR mRNA. While the initial exons mostly agreed with previous computer predictions and were conserved within OR subfamilies, other upstream exons were novel and idiosyncratic. In some cases, repetitive sequences were involved in the generation of splice sites and putative transcription control elements. Such gene structure is consistent with early repertoire enhancement by retrogene generation, which was likely followed by extensive genomic duplication. Each OR gene had a unique signature of transcription factor elements, consistent with a combinatorial expression control mechanism.