Changes in RNA in relation to growth of the fibroblast. IV. Alterations in theproduction and processing of mRNA and rRNA in resting and growing cells.

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Abstract
The concentration and production rate of rRNA and mRNA were greater in growing than in resting 3T6 fibroblasts. Studies on isolated nuclei indicated that rRNA production is apparently controlled at the level of transcription. In contrast, hnRNA, the putattive precursor of mRNA, appeared to be synthesized at the same rate in resting and growing cells. This finding was unexpected and was tested in several ways. The relative rate of production of mRNA compared to hnRNA was several-fold higher in growing than in resting cells, but the kinetics of processing of mRNA appear unchanged. This result suggests either that mRNA arises from a small subfraction of hnRNA or that the efficiency of processing of the hnRNA precursor is an important control mechanism which determines mRNA production in growing and resting states. Comparison of the initial rates of labeling of hnRNA and cytoplasmic message gives the efficiency with which the cytoplasmic mRNA is produced from nucleoplasmic RNA. The very low efficiency (3-4% in growing and 1-2% in resting cells) suggests that not every hnRNA molecule gives rise to a cytoplasmic message. In contrast to the similar kinetics of mRNA production in resting and growing states, processing of ribosomal RNA is much slower in the resting state and the emergence time for 28S RNA from nucleolus is greatly lengthened.