Precursors of alpha and beta globin messenger RNAs

Abstract

Several structural properties of the pulse-labeled globin messenger RNA precursor molecules that sediment faster than steady-state (10 S) globin mRNA (Curtis & Weissmann, 1976; Ross, 1976; Kwan et al., 1977; Bastos & Aviv, 1977) are investigated. The radioactive, globin-specific RNA resolves into two peaks during electrophoresis in polyacrylamide gels prepared in 98% formamide. The molecular weights of the larger and the smaller RNAs are 600,000 and 280,000, respectively, as compared with the average molecule weight of cytoplasmic globin mRNA, 200,000. When RNA from cells labeled for 10 minutes is analyzed by formamide gel electrophoresis, a precursor larger than 600,000 M r is not detected. Most, if not all, of the molecules in the 600,000 and 280,000 M r RNA fractions are polyadenylated. The 600,000 M r RNA contains β globin mRNA sequences, but little, if any, a globin sequences; the 280,000 M r RNA contains both α and β sequences. At steady-state the 14-day-old mouse fetal liver erythroblast contains approximately 50 molecules of the 600,000 M r RNA, 1000 molecules of the 280,000 M r RNAs, and 60,000 molecules of mature globin mRNA. The half-lives of the 600,000 and 280,000 M r RNAs calculated using steady-state levels are 2 minutes and 17 minutes, respectively. These data demonstrate that β globin gene sequences are transcribed into a molecule (β-precursor) that is threefold larger than mature β globin mRNA. In contrast, the largest transcript of the α globin genes detected in these experiments is only 1.4-fold larger than mature α globin mRNA. The possibility that the 280,000 M r, β-specific RNA is a processing intermediate generated by cleavage of the 600,000 M r β-precursor is discussed, along with alternative hypotheses.