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Identification of a Novel First Exon of Prolactin Receptor Gene Expressed in the Rat Brain

Department of Biochemistry, Faculty of Medicine (M.T., T.I., N.N., M.S., N.N., K.N.), and Department of Animal Science, Faculty of Bioresources (Y.H.), Mie University, 2-174 Edobashi, Tsu, Mie 514-8507, Japan

Address all correspondence and requests for reprints to: Dr. Minoru Tanaka, Department of Biochemistry, Mie University Faculty of Medicine, 2-174 Edobashi, Tsu, Mie 514-8507, Japan.

A novel first exon, E1₄, whose sequence was distinct from those of the three known first exons, E1₁, E1₂, and E1₃, of the rat PRL receptor (PRL-R) gene was identified by cDNA cloning for the 5'-end region of PRL-R mRNA expressed in the rat brain. Sequence analysis revealed the presence of two different length E1₄ cDNAs. The longer cDNA contained the 243-bp E1₄ sequence, and the shorter cDNA lacked the 139-bp sequence at the 5'-end of the longer one. Neither E1₄ cDNA has a second exon sequence, indicating that the E1₄ first exon is extensively spliced to the third exon. E1₄-containing PRL-R mRNAs were detected only in the brain by RT-PCR and ribonuclease protection assay. The longer E1₄ mRNA was expressed as the major PRL-R mRNA species in the brain and was greatly increased in pregnant (d 18) and lactating (d 5) rats. A genomic clone containing the E1₄ first exon together with its 5'- and 3'-flanking regions was isolated from a rat kidney genomic library. Ribonuclease protection assay revealed that the position corresponding to the 5'-end of the shorter E1₄ cDNA is the major transcription start point for the E1₄ exon. The 5'-flanking region of E1₄ contained a TATA box-like element 23 bp upstream of the major transcription start point. Other putative transcription factor-binding sites, such as CCAAT, Sp1, and glucocorticoid-responsive elements, were observed at further upstream regions. These results suggest that PRL-R gene expression in rat brain is controlled by the promoter for the E1₄ first exon.

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