Synthesis and processing of nuclear precursor-messenger RNA in avian erythroblasts and HeLa cells

The kinetics of synthesis and turnover of animal cell nuclear precursor-mRNA fractions all of which, in the case of avian erythroblast RNA, are shown by specific complementary DNA hybridization to contain globin mRNA sequences, were analyzed by exponential polyacrylamide gel electrophoresis. Three metabolically distinct size-fractions were characterized: (1) nascent precursor-mRNA (apparent molecular weight 5 to 20 x 10(6), approximate half-life 30 min), (2) intermediate-size precursor-mRNA (molecular weight 1 to 5 x 10(6), approximate half-life 3 hr), (3) small precursor-mRNA (molecular weight 0.5 to 1.5 x 10(6), half-life more than 15 hr). Nascent precursor-mRNA behaves kinetically as a precursor to the smaller precursor-mRNAs that accumulate in the nucleus, as well as to cytoplasmic mRNA; however, no stringent proof can be given that the two smaller nuclear precursor-mRNA fractions are direct physical precursors of functional mRNA. In terms of total mass, more precursor-mRNA accumulates in the nucleus than there is translated mRNA in the cytoplasm. Globin mRNA of final size (9 S) does not accumulate in the nuclei of avian erythroblasts.