β-Globin mRNA decay in erythroid cells:: UG site-preferred endonucleolytic cleavage that is augmented by a premature termination codon

Previous work showed that human beta-globin mRNAs harboring a premature termination codon are degraded in the erythroid tissues of mice to products that lack sequences from the mRNA 5' end but contain a 5' cap-like structure. Whether these decay products are the consequence of endonucleolytic or 5'-to-3' exonucleolytic activity is unclear. We report that this beta-globin mRNA decay pathway is recapitulated in cultured mouse erythroleukemia (MEL) cells and targets nonsense-free mRNA to a lesser extent than nonsense-containing mRNA. S1 nuclease mapping and primer extension demonstrated that 70-80% of decay product 5' ends contain a UG dinucleotide. Detection of upstream counterparts of these decay products indicates that they are generated by endonucleolytic activity. Both crude and partially purified polysome extracts prepared from MEL cells contain an endonucleolytic activity that generates decay products comparable to those observed in vivo. These data suggest that an endonuclease with preference for UG dinucleotides is involved in the degradation of nonsense-containing and, to a lesser extent, nonsense-free human beta-globin mRNAs in mouse erythroid cells.