CHARACTERIZATION OF THE PACIFIC SALMON GONADOTROPIN-RELEASING-HORMONEGENE, COPY NUMBER AND TRANSCRIPTION START SITE

Multiple forms of gonadotropin-releasing hormone (GnRH) have been show n to exist in all vertebrates examined except recently-evolved placent al mammals. To study the origin and regulation of the GnRH genes in a Pacific salmon (Oncorhynchus nerka), we isolated and sequenced the sal mon form of GnRH. The Southern blot shows a single band that strongly hybridizes to a probe for the gene reported here and weaker bands that may represent genes for related forms of GnRH. There is strong conser vation of sequence in the hormone coding region and of the gene organi zation between fish and mammals. However, the GnRH-associated peptide (GAP) shows very little sequence identity with the mammalian GAPs, que stioning its physiological role. We also show for the first time the t ranscriptional start site for a GnRH gene in a non-mammalian species, Interestingly, a large segment of 1152 nucleotides in the promoter reg ion of the Pacific salmon GnRH gene is missing compared with the Atlan tic salmon (Salmo salar) gene, These gene rearrangements suggest that these two salmonid species, which have been geographically separated f or 8-15 million years, have evolved promoters with different mechanism s for control and transcription of GnRH.