Existence and expression of the untranslated first exon of aromatase mRNA in the rat brain

Tissue-specific expression of aromatase activity and mRNA occurs by alternative utilization of multiple untranslated first exons and promoters in the human. The major transcript in the human brain contains the brain-specific first exon, “exon I-f”. However, few reports on the untranslated first exon of aromatase mRNA in the rat brain have been available so far. In the present study, we investigated the existence and expression of exon I-f in the rat brain to elucidate the mechanism of the tissue-specific expression of the brain aromatase. Total RNA extracted from amygdala (AMY) was subjected to a reverse transcription-polymerase chain reaction (RT-PCR). The nucleotide sequence of the RT-PCR product had 89.4% homology to the corresponding region of exon I-f of the human aromatase cDNA. It was indicated that the major transcript in the rat AMY contained exon I-f by the use of a rapid amplification of cDNA ends (RACE). Furthermore, in order to determine the distribution of the aromatase mRNA with exon I-f, total RNAs from the hypothalamus-preoptic area (HPOA), AMY, testis and ovary were analysed by RT-PCR using the primers specific for the mouse exon I-f and the primers for the rat exon III-V. Significant levels of PCR products were found in all tissues with the highest level being in the ovary, using the primers for exon III-V. On the other hand, using the primers for exon I-f, the levels of signals from HPOA and AMY were higher than those from the testis and ovary. These results suggest that tissue-specific expression of aromatase mRNA occurs by an alternative utilization of multiple promoters in the rat, as in the human. It should be noted that minor transcripts containing exon I-f were observed in the testis and ovary.