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| Two cDNAs encoding growth hormone receptor (GHR) were isolated from Nile tilapia (Oreochromis niloticus) liver tissue. The two cDNAs, one consisting of 1 908 bp and the other of 1 728 bp, encoding for putative 635- and 575-amino acid GHR (designated ntGHR1 and ntGHR2, respectively), shared 34.4% identity in nucleotide sequence and 29.6% in deduced amino acid sequence. ntGHR1 and ntGHR2 showed conserved structural characteristics of GHR, including the FGEFS motif, the box1 and box2 regions, and extracellular cysteine residues and intracellular tyrosine residues. On the other hand, ntGHR1 and ntGHR2 showed distinctly different features in structure, ntGHR2 lacked one pair of extracellular cysteines (C5, C6) and 4 intracellular tyrosine residues (Y1, Y3, Y6, and Y7) which were conservative in GHR1. Northern blot analysis indicated a single 6.0-kb transcript of ntGHR1 and a single 4.0-kb transcript of ntGHR2 in liver. Real-time RT-PCR analysis showed that both ntGHR1 and ntGHR2 mRNAs were presented in all tissues tested and expressed extremely highly in liver. Changes of ntGHRs mRNA in gonad were detected during reproductive cycle. Significantly higher expressions of ntGHRs in ovary were detected at sexual matured stage while expressions of GHRs in testis were significantly higher at sexual recrudescent stage. |
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| Keywords: growth hormone receptor; Nile tilapia (Oreochromis niloticus); Northern blot; real-time RT-PCR; reproductive cycle; tissue distribution. |
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