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Keywords:

  • thromboxane receptor;
  • isoforms;
  • splicing;
  • promoter;
  • 5′ untranslated region

In humans, thromboxane (TX) A2 signals through two TXA2 receptor (TP) isoforms, TPα and TPβ, that diverge within their carboxyl terminal cytoplasmic (C) tail regions and arise by differential splicing. The human TP gene contains three exons E1–E3; while E1 exclusively encodes 5′ untranslated region (UTR) sequence, E2 and E3 represent the main coding exons. An additional noncoding exon, E1b was identified within intron 1. Additionally, the TP gene contains two promoters P1 and P2 located 5′ of E1 and E1b, respectively.

Herein, we investigated the molecular basis of the differential expression of the TP isoforms by characterizing the 5′ UTR of the TP transcripts. While E1 and E1b were found associated with TP transcript(s), their expression was mutually exclusive. 5′ rapid amplification of cDNA ends (5′ RACE) established that the major transcription initiation (TI) sites were clustered between −115 and −92 within E1 and at −99 within E1b. While E1 and E1b sequences were identified on TPα transcript(s), neither existed on TPβ transcript(s). More specifically, TPα and TPβ transcripts diverged within E2 and the major TI sites for TPβ transcripts mapped to −12/−15 therein. Through genetic reporter assays, a previously unrecognized promoter, termed P3, was identified on the TP gene located immediately 5′ of −12. The proximity of P3 to the TI site of TPβ suggests a role for P3 in the control of TPβ expression and implies that TPα and TPβ, in addition to being products of differential splicing, are under the transcriptional control of distinct promoters.