Isolation and characterization of the rat thyrotropin beta-subunit gene. Differential regulation of two transcriptional start sites by thyroid hormone.

The gene encoding the beta-subunit of rat thyrotropin (TSH beta) has been isolated and characterized. Blot hybridization of restriction enzyme digests of rat genomic DNA suggests that the gene is present in a single copy. The transcriptional unit is 4.9 kilobases in size representing 3 exons interrupted by 2 introns of 3.9 and 0.4 kilobases. Its nucleotide sequence reveals that the locations of the exon/intron junctions are one nucleotide upstream from the translational start and between codons +34 and +35. The location of the second intron is apparently strictly conserved among the glycoprotein hormone beta-subunit genes being four codons downstream from a region encoding a consensus sequence: Cys-Ala-Gly-Tyr. Using S1 nuclease mapping and oligonucleotide-primed reverse transcription of normal and thyroidectomized rat pituitary mRNA, two transcriptional start sites were identified in the rat TSH beta gene that are 28 and 71 nucleotides upstream from the translational start site. The level of TSH beta mRNA containing the downstream site is altered by thyroidal status whereas the other mRNA utilizing the upstream cap site appears to be constitutively expressed. Characteristic promoter elements are present in the 5'-flanking region including TATAAA or Goldberg-Hogness consensus regions which are present 29 and 26 bases upstream from the respective starts of transcription. Also, several CAAT boxlike sequences are located between 95 and 300 bases upstream from the start of translation. Isolation and characterization of the gene encoding the TSH beta gene will facilitate the study of the molecular mechanisms by which hormones regulate TSH beta gene expression.