POSTTRANSCRIPTIONAL MESSENGER-RNA PROCESSING AS A MECHANISM FOR REGULATION OF HUMAN A(1) ADENOSINE RECEPTOR EXPRESSION

The human A(1) adenosine receptor gene contains six exons with exons 1, 2, 3, 4, and part of 5 representing 5' untranslated regions. Reverse transcription-PCR with exon-specific primers showed two distinct transcripts containing either exons 3, 5, and 6 or exons 4, 5, and 6, with exons 3 and 4 being mutually exclusive. No mature mRNAs containing exons 1 and 2 have been detected. All human tissues that express any A(1) receptors contain mRNA with exons 4, 5, and 6. Tissues which express high levels of A(1) receptors contain mRNA with exons 3, 5, and 6. Exon 4 contains two upstream ATG codons whereas exon 3 contains none. COS cells transfected with expression vectors containing exon 4 (exons 1-6, 3-6, or Ex4-6) express much lower levels of A(1) receptors than vectors without exon 4 (exons 3, 5, and 6). Mutation of upstream ATG codons in exon 4 leads to 3- to 7-fold increased A(1) receptor expression, up to the level seen with the construct containing exons 3, 5, and 6. Thus, in human tissues ''basal'' levels of A(1) receptors can be expressed by use of mRNA containing exons 4, 5, and 6, but when high levels are needed, alternative transcripts with exons 3, 5, and 6 are produced.