Regulation and expression of gonadotropin-releasing hormone gene differs in brain and gonads in rainbow trout

Abstract

The GnRH gene is transcribed in both the brain and gonads. GnRH in the brain is critical for reproduction, but the function and importance of GnRH in the ovary and testis is not clear. In this study we examine whether regulation of the GnRH gene is distinct in the brain and gonads, whether the regulation of the GnRH gene in the gonads is altered after genome duplication, and whether the regulatory region of the GnRH gene is tightly conserved in vertebrates. From ovary and testis, we isolated and sequenced for the first time two different genes and their complementary DNAs that encode the identical peptide known as salmon GnRH. Rainbow trout were selected because they are tetraploid due to genome duplication. A downstream promoter is used in the brain and gonads by salmon GnRH messenger RNA1 (mRNA1) and mRNA2, but mRNA2 also uses an upstream promoter only in the gonads. Two types of long mRNA2 transcripts in ovary and testis both use an alternative start site at position 323; one of these types also retains intron 1. This long 5'-untranslated region is a likely site for distinct regulation of mRNA in the gonad. Additional evidence for separate regulation is that a different expression pattern exists in brain and gonads for GnRH mRNAs during development and maturation. Gene duplication did not alter the encoded peptide, but changed the expression pattern and resulted in complete divergence of the promoter sequence from position -215. A comparison of the mammalian and trout GnRH genes reveals that the promoters are without sequence identity except for a few consensus sites in both regulatory regions. The duplicated trout genes provide a model to study a critical gene whose product controls reproduction in all vertebrates.