CLONING AND CHARACTERIZATION OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) SOMATOLACTIN CDNA AND ITS EXPRESSION IN PITUITARY AND NONPITUITARY TISSUES

A cDNA clone encoding rainbow trout (Oncorhynckus mykiss) somatolactin (rtSL) has been isolated from a rainbow trout pituitary cDNA library. This 2329-bp cDNA clone includes a very short 7-bp 5'-untranslated re gion, a coding region of 702 bp, and a long 3'-untranslated region of 1620 bp. The deduced amino acid sequence of rtSL shows a polypeptide o f 233 amino acid residues which consists of a 24-amino-acid putative s ignal peptide followed by a 209-amino-acid mature polypeptide. This ma ture polypeptide has a molecular weight of 24 kDa. The rtSL shares 99% amino acid identity with chum salmon SL (csSL) and approximately 53-7 7% amino acid identity with SLs in other fishes, including the 7 conse rved cysteine residues. Although a glycosylation site has been identif ied in SL of other fish species, none is observed in rtSL polypeptide. The level of rtSL mRNA in a single pituitary gland was determined by RNA blot hybridization. Results showed that levels of SL mRNA in pitui tary glands of 2-year-old fish were 4- to 7-fold higher than those of 1-year-old fish. The tissue distribution of SL gene expression in adul t fish was determined by reverse transcription-polymerase chain reacti on (RT-PCR) and DNA blot hybridization. In addition to the pituitary g land, SL mRNA was detected in all tissues examined including brain, gi ll, heart, kidney, liver, skeleton muscle, spleen, ovary, testis, and immature oocytes. The extrapituitary expression of the SL gene was als o detected in embryos and fry. The PCR products which contained the re gion coding for mature SL from heart and kidney were cloned and charac terized. Nucleotide sequence analysis showed that the SL mRNAs in hear t and kidney were identical to that in the pituitary gland. These resu lts suggest that, although the pituitary gland is the predominant tiss ue for producing SL, it is not the only tissue that SL gene is express ed in, and the extrapituitary expression of rtSL gene starts from very early developmental stages. (C) 1997 Academic Press.