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Recently, we have isolated the untranslated first exon 'exon ON' of the rat estrogen receptor (ER) gene from the liver by the use of the 5'-rapid amplification of cDNA ends (5'-RACE) method. To investigate the existence of other untranslated first exon(s), we further analyzed the 5'-untranslated region (UTR) of ER mRNA in the rat liver in this study. Total RNA from the livers of 8-week-old male Wistar rats was subjected to 5'-RACE with the antisense primers located in exon 1 of the rat ER gene. The inserts of four clones (clones 3, 4, 7 and 8) were sequenced. The nucleotide sequences of the clones revealed the existence of a previously unidentified untranslated first exon (we termed it 'exon 0S') which was spliced onto exon 1 of the rat ER mRNA. The distribution of ER mRNA containing 'exon 0S' (ER mRNA (0S-I)) in several brain regions and various peripheral tissues of 8-week-old male and female Wistar rats was further analyzed by the use of the reverse transcription-polymerase chain reaction. ER mRNA (0S-l) was found to be widely distributed in the rat brain and peripheral tissues. The distribution of the message was different from that of ER mRNA containing exon 0 (the first reported 5'-UTR form of rat ER mRNA) or of ER mRNA with exon ON which was reported in our recent report. These results indicate that (1) 'exon 0S' is a novel untranslated first exon of the rat ER gene, (2) rat ER mRNAs possess at least three forms of 5'-UTRs which are exon 0, exon ON, and exon 0S, (3) the tissue specific expression of ER is regulated, at least in part, by the usage of differential promoters in the rat.

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