The 5' untranslated regions of the rat A(2A) adenosine receptor gene function as negative translational regulators

The rat A(2A) adenosine receptor (A(2A)-R) gene contains two promoters, P1 and P2, which produce transcript I and transcript 2, respectively. These transcripts differ in the lengths of their 5' untranslated regions (5'UTR1: 514 bp, initiated from P1; 5'UTR2: 221 bp, initiated from P2) but encode the same protein. In the present study, we demonstrate that transcript 2 is present in various tissues at different levels, whereas transcript I is found only in the striatum. In the striatum, the level of transcript 2 is ~300- fold higher than that of transcript 1. The 5'UTR of both transcripts suppresses the expression of A(2A)-R and a firefly luciferase reporter gene at the translational level; this suppression is not observed after mutational inactivation of an 'out-of-frame' upstream AUG codon. Translational suppression by the 5'UTR was also confirmed in cells using a bicistronic strategy. Collectively, these data suggest that P2 is the major promoter of the rat A(2A)-R gene. The 5'UTR of the rat A(2A)-R gene exerts an inhibitory effect on translation by an upstream open reading frame. Because the 5'UTR of the A(2A)-R gene possesses strong interspecies homology, translational suppression may be a general mechanism by which the expression of the A(2A)-R gene is regulated.