An internal ribosome entry site mediates the initiation of soluble guanylyl cyclase [beta]2 mRNA translation

The soluble guanylyl cyclases (sGC), the receptor for nitric oxide, are heterodimers consisting of an [alpha]- and [beta]-subunit. This study aimed to investigate the translational mechanism of the sGC [beta]2-subunit. Two mRNA species for sGC [beta]2 were isolated from human kidney. These transcripts had dissimilar 5'-untranslated regions (5'-UTRs). The most abundant sGC [beta]2 mRNA showed numerous upstream open reading frames (ORFs) and stable secondary structures that inhibited in vivo and in vitro translation. To evaluate whether these 5'-UTRs harbored an internal ribosome entry site (IRES) that allows translation by an alternative mechanism, we inserted these regions between the two luciferase genes of a bicistronic vector. Transfection of those genetic constructs into HeLa cells demonstrated that both sGC [beta]2 leaders had IRES activity in a cell-type dependent manner. Finally, the secondary structural model of the sGC [beta]2 5'-UTR predicts a Y-type pseudoknot that characterizes the IRES of cellular mRNAs. In conclusion, our findings suggest that sGC [beta]2 5'-UTRs have IRES activity that may permit sGC [beta]2 expression under conditions that are not optimal for scanning-dependent translation. [PUBLICATION ABSTRACT]