2 DIFFERING PRECURSOR GENES FOR THE SALMON-TYPE GONADOTROPIN-RELEASING-HORMONE EXIST IN SALMONIDS

Salmon gonadotropin-releasing hormone (sGnRH) is considered to have an important role in the control of reproduction in salmonid fish. As a basis for understanding the physiological functioning of sGnRH at the molecular level, we characterized the nucleotide sequences of two type s of cDNAs encoding the precursors of sGnRH in sockeye salmon (ss), On corhynchus nerka, by a cloning strategy based on reverse transcription -PCR. The two types of cDNAs are referred to as ss-pro-sGnRH-I and -II , and consisted of 435 and 481 bases respectively. Both precursors are predicted to contain a signal peptide, the hormone and a GnRH-associa ted peptide that is attached to the hormone via a Gly-Lys-Arg sequence . The presence of two types of mRNAs hybridizing with either cDNA was confirmed by Northern blot analysis of brain RNA from sockeye salmon, masu salmon, O. masou, and rainbow trout, O. mykiss. The ss-prosGnRH-I cDNA had 97.2% and 82.8% overall identity with sGnRH cDNA from masu s almon and putative sGnRH cDNA deduced from the gene of the Atlantic sa lmon, Salmo salar respectively, whereas the ss-pro-sGnRH-II cDNA had 8 0.0% and 91.2% overall identity with the former and the latter respect ively. The nucleotide sequences of ss-sGnRH-I and -II cDNAs showed les s similarity (79.3%). These results indicated that each salmonid speci es possesses two differing sGnRH genes. The results of Southern blot a nalysis using genomic DNA extracted from individuals support this evid ence in sockeye salmon, masu salmon and rainbow trout.