Structure of the bovine placental lactogen gene and alternative splicing of transcripts

Preliminary evidence for heterogeneity among bovine placental lactogen (bPL) transcripts prompted characterization of additional cDNA clones and isolation of the bPL gene. Nucleotide replacements were detected among sequenced cDNAs isolated from different animals at a total of 11 positions. Four of these predict amino acid substitutions, which are generally conservative in nature. In addition, truncated forms of bPL are predicted by the sequences of two cDNAs in which alternative splicing is evident. In one case, exon III is deleted with no effect on reading frame. However, in the other instance, a shifted reading frame resulting in a novel carboxyl terminus is generated by use of an alternative 5' splice donor site within exon IV. Nuclease protection analysis demonstrated that these variant transcripts comprise about 13% of the total bPL mRNA present in the midgestation placenta. Characterization of the bPL gene revealed that it is similar in structure to other members of this gene family. Sequence analysis demonstrated that the 5'-flanking region of the bPL gene has diverged considerably from the bovine prolactin and growth hormone genes, but shares homology with the previously characterized gene corresponding to bovine prolactin-related cDNAI (bPRCI). Primer extension as well as nuclease protection analysis indicated that a single transcription start site was utilized in the fetal placenta at midgestation. Exact matches to the consensus sequences for response elements for thyroid hormone and transcription factor AP-2 were located 50 and 70 bp, respectively, upstream from the transcription start site in cloned genomic 5'-flanking sequences. We conclude that the bovine placenta may express more than a single placental lactogen product, raising the possibility of alternative hormones with distinct biological properties, and that the bPL gene may share regulatory elements with the gene for the distinct prolactin-related product, bPRCI, based on similarities in the 5' regions of the corresponding genes.