Two alpha 1b-adrenergic receptor mRNAS expressed in Sprague-Dawley rat liver have distinct 5'-regions

Previous studies have revealed two size classes of alpha 1b-adrenergic receptor mRNAs, 3.3 kb and 2.7 kb, in the Sprague Dawley rat that are transcribed from a single gene and are expressed in approximately equal amounts in liver. Only the 2.7 kb mRNA is expressed in heart. Both alpha 1b-adrenergic receptor mRNAs appear to share extensive regions of homology, therefore, we used oligonucleotide-directed ribonuclease H mapping to detect sequence differences between the two transcripts. Initial experiments using oligo (dT)-directed RNase H hydrolysis indicated that the two mRNAs have poly [A+] tails of identical length. By using region-specific cDNA probes, we determined that the sequence difference between the two alpha 1b-adrenergic receptor mRNAs lies in the 5' end, upstream from the known initiator AUG in the 2.7 kb transcript. In addition, results from ribonuclease protection assays and Northern blot analysis in which an oligonucleotide was used as the probe suggested that both alpha 1b-adrenergic receptor mRNAs are transcribed from the same DNA strand.