HUMAN BETA-GLOBIN MESSENGER-RNAS THAT HARBOR A NONSENSE CODON ARE DEGRADED IN MURINE ERYTHROID TISSUES TO INTERMEDIATES LACKING REGIONS OF EXON-I OR EXON-I AND EXON-II THAT HAVE A CAP-LIKE STRUCTURE AT THE 5' TERMINI

Previous studies have demonstrated that nonsense codons within beta-degrees-thalassemic or in vitro-mutagenized human beta-globin transgenes result in the production of mRNAs that are degraded abnormally rapidly in the cytoplasm of murine erythroid cells. As a consequence, three RNA degradative intermediates are formed that lack sequences from either exon I or exons I and II. We show here that the intermediates, like the full-length mRNA from which they derive and the endogenous murine beta(maj)-globin mRNA, bind to the anticap monoclonal antibody H-20 in a way that is competed by the cap analogue m7G and eliminated by prior exposure to tobacco acid pyrophosphatase. Furthermore, the intermediates, like the two full-length mRNAs, are resistant to a 5' --> 3' exonuclease activity isolated from HeLa cell nuclei that degrades uncapped but not capped ribopolymers. Based on these observations, the intermediates appear to possess a structure that is indistinguishable from the cap at the 5' end of mRNA, i.e. a methylated nucleoside that is linked to the RNA by a 5'-5' phosphodiester bond. Detection of the intermediates during murine development was concomitant with detection of full-length thalassemic mRNA. Intermediate production appears to be influenced by RNA structure as indicated by the products that derive from a beta-degrees-thalassemic beta-globin transgene harboring a structural alteration (a 4 bp deletion) that was larger than any of those previously studied.