The novel untranslated first exon ''exon ON'' of the rat estrogen receptor gene

The 5'-untranslated region (UTR) of the estrogen receptor (ER) mRNA in the rat liver was analyzed by the use of the 5'-rapid amplification of the cDNA ends (5'-RACE) method. The nucleotide sequence of one of the positive RACE clones (clone 9) revealed that the existence of the novel untranslated first exon (termed ''exon ON'') being spliced onto the exon 1 of the rat ER mRNA. We further analyzed the distribution of the ER mRNA containing the ''exon ON'' (ER mRNA (ON-1)) and the ER mRNA containing the previously reported exon 0 (ER mRNA (0-1)) in the rat brain and peripheral tissues. In contrast to the wide distribution of the ER mRNA (0-1), the distribution of the ER mRNA (ON-1) was almost limited in the peripheral tissues. These results indicate that the ''exon ON'' is the novel untranslated first exon of the rat ER gene, and the tissue specific expression of the ER is regulated, at least in part, by differential promoter usage in the rat. (C) 1996 Academic Press, Inc.