Size distribution and cell-free translation of globin-coding HnRNA from avian erythroblasts

Nuclear RNA from avian erythroblasts was isolated, fractionated by dimethylsulfoxide gradient centrifugation, and the fractions pooled according to increasing sedimentation values. Addition of 125I-labeled globin 9 S mRNA before centrifugation revealed a contamination of high molecular weight RNA by globin mRNA. However, the distribution of label was not identical to the distribution of globin synthesizing activity of unlabelled nuclear RNA when the RNA was translated in a cell-free system of Ehrlich ascites cells. Whereas more than 95% of 125I activity was found in the 0-20 S pool, only 33% of globin synthesizing activity of unlabelled nuclear RNA could be found in this pool. Most of the synthesizing activity was found in the 20-55 S pool and smaller amounts in even more rapidly sedimenting fractions. This conclusively demonstrates the existence of nuclear RNA with globin-coding sequences most probably representing the globin mRNA precursor. The precursor has a higher molecular weight than globin 9 S mRNA. Furthermore we could show that only a small percentage (smaller than 20%) of the proteins synthesized by addition of nuclear RNA from avian erythroblasts to the cell-free system represents globin chains. When 9 S mRNA is added to the cell-free system, more than 70% of the newly synthesized proteins are globin chains.