Expression of the gene for the beta subunit of mouse thyrotropin results in multiple mRNAs differing in their 5′-untranslated regions.

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The gene encoding the beta subunit of mouse thyrotropin (TSH beta) has been isolated from a mouse genomic library, and its nucleotide sequence has been determined. Blot hybridization analysis of restriction enzyme digests of mouse DNA indicates that there is a single mouse TSH beta gene. The gene is 4.8 kilobases in length and contains five exons, which are 27, 47, 41, 163, and 328 base pairs long. Exons 1, 2, and 3 encode only 5'-untranslated mRNA sequences and are separated by introns that are 150 and 380 base pairs long. The protein-coding mRNA sequences are found in exons 4 and 5 and are interrupted by a 460-base pair intron. The position of this intron, between the codons for amino acids 34 and 35, has been conserved in all the known glycoprotein hormone beta subunit genes. Exons 3 and 4 are separated by a large 3.2-kilobase intron. When primer extension analysis, using an oligonucleotide primer complementary to exon 4 sequences, was employed to locate the transcription start site, four products were obtained. Nucleotide sequencing of these products showed that they were derived from separate TSH beta mRNAs that differed in the lengths of their 5'-untranslated regions. These 5'-untranslated mRNA sequences are derived from different combinations of exons 1, 2, and 3, each spliced to exons 4 and 5. The longest 5'-untranslated sequence, 116 nucleotides long, includes exons 1, 2, and 3 and the first base of exon 4; the shorter 5'-untranslated regions, 75, 69, and 28 nucleotides long, arise by splicing out the second and/or the third exon sequences. In contrast to the mouse TSH beta gene, transcription of the rat TSH beta gene from the analogous start site has been reported to give only a single mRNA, with a 5'-untranslated region of 28 nucleotides. Divergence of the mouse and rat TSH beta gene sequences at RNA splice sites can account for the absence of exon 2, but not exon 3, sequences in rat TSH beta mRNA. Primer extension and RNase protection analyses also showed that the mouse TSH beta gene contains a second transcription start site, located 43 base pairs upstream of the first start site, in a position corresponding to that in the rat TSH beta gene. Each start site in the mouse gene is flanked by characteristic TATAA box and CAAT box sequences.(ABSTRACT TRUNCATED AT 400 WORDS)