Molecular cloning and expression of rat placental lactogen-I complementary deoxyribonucleic acid

A full-length cDNA clone for rat placental lactogen I (rPL-I) has been isolated from a phage lambda gt11 library containing cDNA synthesized from day 11 rat placental mRNA. By Northern blot analysis the rPL-I cDNA clone hybridizes to a 1.0-kilobase placental mRNA and appears as early as day 10 of gestation. Maximal expression of this mRNA was observed in day 11 and 12 placenta, and faint hybridization of the rPL-I cDNA was also detected in day 18 to term placenta. In contrast, the mouse clone hybridized to mRNA for mouse PL-I (mPL-I) only in day 10 mouse placenta (9). In vitro translation of rPL-I mRNA produced by transcription of the cDNA template yielded a 27-kDa polypeptide the size of the expected precursor protein which was immunoprecipitated by a monoclonal antibody to rPL-I. The rPL-I cDNA nucleotide sequence has been determined. The sequence is very similar to that for mPL-I and contains an open reading frame encoding a polypeptide of 230 amino acids compared to 224 for mPL-I. Comparison of the predicted primary translation product of rPL-I mRNA with that of mPL-I mRNA revealed that rPL-I shares 73% identity to mPL-I at the amino acid level. The predicted rPL-I protein shares 41% amino acid identity with rPL-II precursor, 24% with rat prolactin-like protein A, 26% with rat prolactin-like protein B, and 31% with rat PRL. In situ hybridization studies indicated that mRNA for rPL-I was present in a few rapidly dividing cells as early as day 8 of gestation, and by day 9 could be localized to giant cells which surround the conceptus.