Changes in RNA in relation to growth of the fibroblast. III. Posttranscriptional regulation of mRNA formation in resting and growing cells.

The proportion of hnRNA processed into cytoplasmic mRNA in resting and growing 3T3 and 3T6 cells has been investigated by measuring the efficiency of transfer of nuclear poly(A) to the cytoplasm. When nuclear poly(A) is either labeled continuously with precursor or pulse labeled and then chased with cordycepin, growing cells transfer approximately twice as much of the poly(A) from the nucleus to the cytoplasm as do resting cells. When cells undergo a serum-induced transition from the resting to growing state, the efficiency of poly(A) transfer is increased to that characteristic of growing cells by 3 hr after the addition of the serum. On the other hand, the proportion of hnRNA which is polyadenylated and the total nuclear poly(A) content are the same in resting and growing cells. It appears that the principal means by which the cell regulates its poly(A)+ mRNA content in the growing and resting states is by regulating the efficiency which nuclear poly(A)+ RNA is converted to cytoplasmic mRNA.