Synthesis and turnover of nuclear and cytoplasmic polyadenylic acid in mouse L cells

Abstract

A study was made of the kinetics of adenosine incorporation into the poly(A) segments of heterogeneous nuclear RNA, polyribosomal messenger RNA and total cytoplasmic RNA of mouse L cells. Special care was taken to ensure that the precursor pools used for incorporation remained at a constant specific activity over the entire course of an experiment. A comparison of the labeling of the poly(A) and the non-poly(A) portions of mRNA indicated that polyadenylation is a relatively late post-transcriptional step, occurring in the latter third of the interval between transcription of the mRNA precursor and appearance of the mRNA in cytoplasmic polyribosomes. The rates of labeling of messenger poly(A) and total cytoplasmic poly(A) reach a maximum after only a very brief lag and do not increase thereafter, in spite of that fact that the specific activity of nuclear poly(A) continues to increase for several hours. Such a result is not compatible with a simple precursor-product relation between all of the nuclear and all of the cytoplasmic poly(A), but rather, it suggests that there is a reasonably large intranuclear turnover of poly (A). The fact that nuclear poly(A) is not quantitatively converted to cytoplasmic poly(A) indicates that polyadenylation of a heterogeneous RNA molecule is not sufficient to ensure that it will be properly processed and transported to the cytoplasm.