Cloning and phylogenetic relationship of red drum somatolactin cDNA and effects of light on pituitary somatolactin mRNA expression

The nucleotide sequence for red drum somatolactin (SL) cDNA was determined and the expression of pituitary SL mRNA was examined in red drum kept under various light conditions. A full length of SL cDNA (1629 bp) was isolated and characterized from a red drum pituitary cDNA library. The SL cDNA has a n open reading frame of 696 nucleotides which encodes a 24-amino-acid signa l peptide and a 207-amino-acid mature peptide. Red drum SL shares 58-87% am ino acid sequence identity and 56-85% nucleotide sequence identity with oth er teleost SLs. The characteristic seven cysteine residues and one N-glycos ylation site of SL are well conserved in the red drum SL mature peptide. Ph ylogenetic analysis shows that red drum SL is closely related to seabream S L and is also closely related to lumpfish, flounder, halibut, and sole SLs, whereas SLs of Atlantic cod, chum salmon, rainbow trout, and eel are more distantly related to those of the more advanced teleosts. Two SL transcript s, designated as SL I at 1.8 kb and SL II at 1.3 kb, are expressed in red d rum pituitaries and correspond to two polyadenylation signal sites in red d rum SL cDNA at nucleotide positions 1554 and 1270. Levels of the SL I mRNA were 2- to C-fold higher in pituitaries of blind red drum and intact fish k ept under constant darkness for 1 week than those in control fish sampled d uring the light phase of the light-dark cycle. Similarly, pituitary levels of SL II mRNA were 9-fold higher in blind fish and 1.6- to 4-fold higher in intact fish kept under constant darkness than in the control fish. Further more, these changes in mRNA levels in pituitaries were accompanied by more than 10-fold increases in SL protein concentrations in plasma. The finding that the absence of light perception for extended periods leads to dramatic increases in SL mRNA expression as well as SL secretion in red drum provid es further evidence that illumination levels and SL physiology are intimate ly related in this species. (C) 1999 Academic Press.