Identification of a novel first exon of prolactin receptor gene expressed in the rat brain.

Abstract

A novel first exon, E1(4), whose sequence was distinct from those of the three known first exons, E1(1), E1(2), and E1(3), 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 E1(4) cDNAs. The longer cDNA contained the 243-bp E1(4) sequence, and the shorter cDNA lacked the 139-bp sequence at the 5'-end of the longer one. Neither E1(4) cDNA has a second exon sequence, indicating that the E1(4) first exon is extensively spliced to the third exon. E1(4)-containing PRL-R mRNAs were detected only in the brain by RT-PCR and ribonuclease protection assay. The longer E1(4) 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 E1(4) 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 E1(4) cDNA is the major transcription start point for the E1(4) exon. The 5'-flanking region of E1(4) 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 E1(4) first exon.