Hydropathy profiles of predicted thyrotropin-releasing hormone precursors are highly conserved despite low similarity of primary structures

Two types of cDNAs encoding thyrotropin-releasing hormone (TRH) precursors (TRH-A and TRH-B) were amplified from hypothalamic mRNA of sockeye salmon by reverse transcriptase-polymerase chain reaction (RT-PCR). The amplification was achieved using two primers which correspond to TRH progenitor sequence (Lys/Arg-Arg-Gln-His-Pro-Gly-Lys/Arg-Arg). A full length cDNA encoding TRH-A was obtained by 5'- and 3'-RACE methods. It has a length of 1324 base pairs (bp) that contains sequences of 5' and 3' untranslated regions and an open reading frame of 259 codons. The sockeye salmon TRH-A deduced from the nucleotide sequence tandemly contains 8 copies of TRH progenitor sequences. Another cDNA which encodes a part of TRH-B consists of 242 bp, and the sequence homology between TRH-A and -B cDNAs is 90%. The result of Southern blot analysis of sockeye and masu salmon genomic DNAs supported the evidence that there are at least two TRH genes in the salmonid. A RT-PCR analysis of TRH gene expression in various tissues of sockeye salmon showed that strong expression was observed only in the brain. The primary structure of the sockeye salmon TRH-A shares low similarity to those of human, rat and Xenopus TRH precursors (35, 27 and 44%, respectively). However, their hydropathy profiles were almost the same with each other. The profile of sockeye salmon TRH-A showed the presence of two discrete hydrophobic regions, one in the N-terminal region which corresponds to the signal peptide and the other in the C-terminal region. All of the repetitive TRH progenitor sequences are included in three hydrophilic regions easily recognizable. The present results thus suggest that the three-dimensional structures of TRH precursors are highly conserved, although the primary structures of TRH precursors have diverged through the evolutionary pathway of vertebrates.