Different usage of two polyadenylylation signals in transcription of the N-myc gene in rat tumor cells.

The complete nucleotide sequence of a rat genomic DNA fragment of 6.9 kbp containing the entire N-myc gene was determined. A unique structural feature of the rat N-myc gene is the presence of two polyadenylation signals in the exon 3 region resulting in formation of two poly(A)+ N-myc mRNAs of 2.9 and 2.2 kb length. Elevated expression of the 2.9 kb mRNA, which was not due to gene amplification, was observed in normal rat tissues such as brain, adrenal and testis, and in rat tumor cells such as ascites hepatoma AH130, AH70Btc and AH7974 cells, and pituitary tumor GH3 cells. In contrast, expression of 2.2 kb mRNA, for which transcription was terminated at the upstream polyadenylation site, was observed only in GH3 cells.