Greater conservation of somatolactin, a presumed pituitary hormone of the growth hormone/prolactin family, than of growth hormone in teleost fish

From pituitary cDNA libraries of Atlantic cod and chum salmon, cDNA clones coding for somatolactin (SL), a presumed pituitary hormone belonging to the growth hormone (GH)/prolactin (PRL) family, were isolated and characterized. The 1.3-kb cod SL mRNA was composed of a greater than 0.25-kb 5' untranslated region, a coding region for the precursor of 235 amino acids (aa), a 0.14-kb 3' untranslated region, and a poly(A) tail. The 2.5-kb salmon SL mRNA had a less than 0.1-kb 5' untranslated region, a precursor (233 aa) coding region, a 1.6-kb 3' untranslated region, and a poly(A) tail. A signal peptide of 26 and 24 aa was found in the SL precursor of cod and salmon, respectively. Thus, the mature SLs of these fish are composed of 209 aa. Two potential N-glycosylation sites were identified in cod SL, whereas no site could be found in the salmon. A comparison of amino acid sequences of the three SLs so far isolated indicated six Cys residues to be in homologous positions to those in GH and PRL, and one Cys residue to be characteristically present in SL. Among cod, salmon, and flounder, greater colinearity of amino acid sequences was noted in SLs than in GHs. The identities of the SL amino acid sequences were between 73 and 81% as compared to 58-62% for the corresponding GHs, indicating greater conservation of SL than GH.