Shortening of the poly(A) region of mouse globin messenger RNA

Nucleated erythroid cells isolated from the spleens of anemic mice were used to investigate the processing of the polyadenylic acid region of globin mRNA. Cells were labeled in media containing [3H] adenosine and transferred to media containing no radioactive precursor and incubated further in the presence or absence of actinomycin D. After various times following the transfer of the cells, globin mRNA was isolated using a combination of oligo(dT)-cellulose affinity chromatography, sucrose density centrifugation, and globin cDNA (the complementary DNA copy of globin mRNA)-cellulose affinity chromatography. The size of the poly(A) region was determined by polyacrylamide gel electrophoresis of the T1 and pancreatic RNase-resistant fragments. The prelabeled poly(A) region which initially comprises approximately 150 adenylate residues was found to become shorter with time, both in cells incubated in medium containing no radioactive precursor and in the presence of actinomycin D. After 9 h of incubation in the presence of actinomycin D, two major size classes of poly(A) were observed, one containing 35 to 45 adenylic acid residues and the other containing 55 to 65 residues. These two size classes are similar to those found in circulating reticulocytes suggesting that the poly(A) shortening observed in these cell incubation studies is similar to that which occurs in vivo. Two protein synthesis inhibitors, emetine and cycloheximide, were investigated with respect to their effect on poly(A) shortening. Neither drug inhibited the shortening of the poly(A) region of globin mRNA, suggesting that protein synthesis is not required for this process to occur.