A novel prolactin-like protein (PRL-L) gene in chickens and zebrafish: cloning and characterization of its tissue expression.
In this study, a full-length cDNA encoding a prolactin-like protein (PRL-L) was cloned from chicken brain tissues using RT-PCR. This putative PRL-L precursor has 225 amino acids in length and shares 30-35% amino acid sequence identity with prolactin (PRL) of chicken, zebrafish, Xenopus, rat and human. Using RT-PCR, the mRNA expression of PRL-L in chicken tissues was further examined. Unlike the predominant expression of PRL in pituitary, PRL-L was found to be widely expressed in adult chicken extra-pituitary tissues with only minimal expression detected in pituitary. In day-7 chicken embryos, the expression of PRL-L, but not PRL, was also detected in all extra-pituitary tissues examined. In line with this finding, the 5-flanking region of chicken PRL-L (cPRL-L) gene, but not PRL gene, displayed a strong promoter activity in cultured DF-1 cell (a chicken embryo fibroblast cell line), suggesting that the basal expression of PRL-L gene is controlled by a transcriptional regulatory mechanism different from that of PRL gene. As the same findings in chickens, PRL-like protein(s), which share high amino acid sequence (42-86%) identity with chicken PRL-L, was identified in several non-mammalian vertebrate species including zebra finch, tiger puffer, green puffer and zebrafish. RT-PCR assay demonstrated that zebrafish PRL-L, similar to chicken PRL-L, is expressed in extra-pituitary tissues including brain, gill, muscle, ovary and testis. Taken together, these findings strongly suggest that a novel PRL-like protein exists in some non-mammalian vertebrates and may play an important role in target tissues, such as extra-pituitary tissues of chickens and zebrafish.