In order to elucidate the regulatory mechanisms of expression of the human endothelin-A receptor (hET-AR) gene, we characterized hET-AR transcripts using reverse transcriptase (RT)-PCR analysis in a variety of human tissues. RT-PCR of lung mRNA using a set of primers from exons 2 and 5 showed two lower-molecular-mass transcripts in addition to the expected fragment. When RT-PCR with primers from exons 4 and 8 was performed, no transcripts other than the expected one were detected. PCR cloning utilizing a set of primers from exons 2 and 8 which covered the entire coding sequence revealed that the cDNA clones corresponding to the two novel transcripts contained deletions of 199 bp and 327 bp respectively compared with the previously described hET-AR cDNA. Comparison of their sequences with that of the hET-AR gene showed that the deleted sequences correspond exactly to exon 4 and exons 3 and 4 respectively, indicating that these lower-molecular-mass ET-AR transcripts result from alternative RNA splicing (designated ET-ARΔ4 and ET-ARΔ3,4 respectively). Alternative splicing of exon 4 results in a transcript which would be translated into a C-terminal truncated protein containing the first, second and third transmembrane domains, while the splicing out of exons 3 and 4 would produce a protein with five membrane-spanning domains but lacking the third and fourth domains present in the ET-AR protein. An RNase protection assay revealed that ET-ARΔ4 and ET-ARΔ3,4, as well as ET-AR, transcripts were observed in various human tissues, including the lung, aorta, atrium, kidney and placenta, which are known to express ET-AR abundantly. Thus we have isolated the cDNAs of novel transcripts of hET-AR which are generated by alternative RNA splicing, and these results suggest that this alternative RNA splicing might contribute to the regulation of ET-AR gene expression.
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