DEMONSTRATION OF TISSUE-SPECIFIC PROMOTERS IN NONPRIMATE SPECIES THAT EXPRESS AROMATASE P450 IN PLACENTAE

Conversion of androgens to estrogens is catalyzed by aromatase P450 (P450(arom); the product of the CYP19 gene). Regulation of tissue-specific expression of P450(arom) in humans is due, in part, to alternative transcriptional start sites that arise as a consequence of the use of tissue-specific promoters. We used the rapid amplification of cDNA ends (RACE) technique to characterize RNA extracted from ovarian granulosa cells and placental tissue from cows, horses, and pigs (ungulates) in order to determine whether these species, like the human, utilize tissue-specific promoters to drive P450(arom) expression. The majority of transcripts in the placenta have 5'-termini that differ from those in the ovary upstream of a common site of divergence, indicative of a splice junction. The use of tissue-specific promoters by the bovine CYP19 gene would produce these results, as it does in the case of the human CYP19 gene. A bovine genomic library was then screened with probes that hybridize to ovary- or placenta-specific transcripts. Two clones of approximately 15 kb each in length were isolated; one hybridized with the ovary-specific sequence and the other hybridized with the placenta-specific sequence. Whereas the former sequence was contiguous with the downstream sequence containing the translational start site, the latter was identical only with the sequence of the placental transcripts upstream of the putative splice junction, indicating that this was the distal sequence. Bovine and human ovary-specific genomic sequences share 77% bp identity, while bovine and human placenta-specific sequences demonstrated only 39% bp identity. These results mirror those obtained in comparisons of human, bovine, equine, and porcine ovarian and placental RACE cDNA 5'-termini. Thus, there is a high degree of sequence identity in the 5'-flanking region of the ovarian promoter for P450(arom) in spite of marked differences between species in the pattern of estrogen biosynthesis. Although the use of a distal promoter to drive placental CYP19 gene expression may be the norm in species in which estrogen biosynthesis occurs in the placenta, this may be a result of convergence, rather than divergence, in evolution.