The enzyme prostaglandin endoperoxide H synthase (PGHS) has a pivotal role in the prostanoid biosynthetic pathway because it catalyses the formation of prostaglandin H2 (PGH2), the common precursor of prostanoids. Two PGHS isoforms have been reported, PGHS-1 and PGHS-2, which have 61% identity (at the amino acid level) and 73% similarity (at the nucleotide level) between the two human enzymes. Transcription of the PGHS-1 gene leads to the formation of two transcripts (2.8 and 5.1 kb); two transcripts of 2.8 and 4.5 kb are produced from the PGHS-2 gene. By Northern blot analysis with the entire coding region of human PGHS-1, 2.8 and 5.1 kb transcripts as well as a novel 4.5 kb transcript were detected in the human megakaryoblastic cell line MEG-01. We designed a strategy to characterize the 4.5 kb PGHS transcript. Probes specific for each PGHS-1 and PGHS-2 were designed on the basis of the 3ʹ untranslated region (3ʹ UTR), where no similarity is present. The 4.5 kb transcript was detected only with the PGHS-1-specific 3ʹ UTR probes and not with the PGHS-2-specific 3ʹ UTR probe. To investigate the origin of the 4.5 kb PGHS-1 transcript, the remaining 947 bp of the 5.1 kb PGHS-1 transcript was generated by 3ʹ rapid amplification of cDNA ends (3ʹ RACE) and sequenced. A non-canonical polyadenylation signal (AAGAAA) located upstream of a potential cleavage site (CA) was found and could generate the 4.5 kb PGHS-1 transcript. Analysis of the sequence also produced several possible G/U-rich elements downstream of the potential cleavage site. An RNA dot-blot with 50 different human tissues was probed with the 4.5 and 5.1 kb PGHS-1-specific probes. A signal for the 4.5 kb PGHS-1 transcript was detected in the bladder and appendix. Signals of lower intensity were detected in the colon, bone marrow, small intestine, uterus, prostate, peripheral leucocyte, lymph node and stomach. In conclusion, our results suggest that the cell line MEG-01, the bladder and the appendix contain a new PGHS-1 transcript of 4.5 kb that can be produced from the PGHS-1 gene and we provide a better strategy for distinguishing PGHS-1 transcripts from PGHS-2.

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