Elongation of the polyadenylate segment of messenger RNA in the cytoplasm of mammalian cells.

Chinese hamster and mouse sarcoma 180 ascites cells, treated with high levels of actinomycin D, still carry out limited poly(A) synthesis. The residual activity, which consists of poly(A) chain extension in the cytoplasm as well as in the nucleus, is essentially insensitive to cordycepin. Nuclear polyadenylation proceeds linearly and involves the gradual extension of unusually long poly(A) sequences. Cytoplasmic poly(A) labeling is initially more rapid than in the nucleus, but levels off within 5-10 min. It consists of addition of seven to eight AMP residues to the poly(A) sequences in preexisting mRNA molecules. The levelling off can be accounted for by a rapid turnover of the extremity of the poly(A) chain in the mRNA. Cytoplasmic poly(A) chain extension can be detected in cells not subjected to the actinomycin treatment. The rate of this process is of the same order of magnitude as that of new poly(A) transfer from nucleus to cytoplasm. It could serve to control the length of the poly(A) sequence in mRNA.