Identification of a Single Melanin‐Concentrating Hormone Messenger Ribonucleic Acid in Coho Salmon: Structural Relatedness with 7SL Ribonucleic Acid

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Abstract
Melanin‐concentrating hormone (MCH) is a cyclic neuropeptide possessing antagonistic function to α‐melanocyte‐stimulating hormone and corticotropin‐releasing factor in the control of melanosome dispersion within melanophores and adrenocorticotropin release in fish. We have isolated and characterized MCH cDNAs from coho salmon (Oncorhyncus kisutch). The precursor protein predicted by the longest cDNA consists of 132 amino‐acids with a characteristic signal peptide at the N‐terminus and the biologically active salmon MCH (sMCH) peptide at the C‐terminus. The coho sMCH mRNA and protein sequences are very similar but not identical to the previously reported chum or chinook salmon counterparts, suggesting the existence of species polymorphism. Sequence similarities were revealed between α‐melanocyte‐stimulating hormone and part of the C‐terminal domain of sMCH precursor. Two sMCH genes were found in coho salmon. By contrast to other salmon species, only one major sMCH mRNA was detected in coho species suggesting that differential MCH gene expression might occur in salmon. In addition, under low stringency oligoprobes complementary to the sMCH RNA recognize a 0.3 kb RNA which was identified as the 7SL RNA. The regions conserved between those RNAs fold in a similar secondary structure. These similarities might reflect common ancestry which may have functional significance.