TISSUE-SPECIFIC EXPRESSION OF THE BOVINE AROMATASE-ENCODING GENE USESMULTIPLE TRANSCRIPTIONAL START SITES AND ALTERNATIVE FIRST EXONS

Here we report on the genomic structure of the bovine aromatase cytoch rome P450-encoding gene (Cyp19) and its tissue-specific transcript var iants. The gene comprises at least 14 exons (1.1, 1.2a, 1.2b, 1.3, 1.4 , and 2-10) spanning more than 56 kilobases of genomic DNA. The coding area is confined to exons 2-10. Transcriptional start sites of Cyp19 were examined in granulosa cells, placenta, testis, adrenal gland, and brain, employing 5'-RACE (rapid amplification of complementary DNA en ds) and primer extension. The analysis of 5'-RACE clones revealed six Cyp19 transcript variants that were different within their 5'-untransl ated regions (5'-UTR). Yet, the coding region was identical in all clo nes. Although two of these 5'-UTR (the first 152 nucleotides of exon 2 and exon 1.4) are conserved among different species, four others (exo ns 1.1, 1.2a, 1.2b, and 1.3) did not show sequence homology to any oth er species. Transcription from exons 1.1 and 2 starts at several adjac ent sites. In granulosa cells and placenta, but not in brain, a fracti on of transcripts starting with exon 1.2a contains an additional untra nslated exon, 1.2b, due to alternative splicing. Transcript Variants c omprising exon 1.1, 1.2a, 1.2b, or 1.3 were mainly found in the placen ta, those with the 5'-UTR of exon 2 were predominant in granulosa cell s, and transcripts with exon 1.4 prevailed in the brain. Estimates of Cyp19 transcript concentrations in six different tissues revealed high levels in granulosa cells and placenta, intermediate levels in testis and brain, and low levels in adrenal gland and liver. Our experiments demonstrate that six transcript variants of the bovine Cyp19 gene, in cluding 9-11 exons, are expressed with tissue-specific preferences. Th ese transcripts are presumably generated using five different promoter regions and tissue-specific alternative splicing.