A placenta-specific 5' non-coding exon of human prolactin.

Expression of human prolactin (hPRL) is restricted to the anterior pituitary and the placenta, strictly, the decidualized endometrium of the uterus. We have cloned and sequenced placental hPRL cDNAs and also the 5' part of the hPRL gene, and found that the placental mRNA consisted of six exons including a 5' non-coding exon which was not present in the pituitary mRNA known to consist of five exons. The placenta-specific exon, designated exon 0, was located 5.3 kb upstream of the first exon of the pituitary mRNA. We have also located the transcriptional start site of the hPRL gene in the placenta to be 21 bp upstream from the 5' end of the longest cDNA clone we obtained using mRNA from human placenta of 22 weeks of gestation. These results suggest that the hPRL gene contains a placenta-specific promoter other than the pituitary-specific promoter.