AbstractShortly after serum‐deprived BALB/c 3T3 fibroblasts are stimulated to grow in medium containing 10 calf serum, the RNA polymerase I activity in permeabilized cells shows a two‐fold increase over the values observed in either serum‐deprived or density‐inhibited resting cells. Inhibition of protein synthesis by pactamycin or cycloheximide specifically reduces the enhanced RNA polymerase I activity in serum‐stimulated cultures without affecting the values in resting cells. On the other hand, inhibition of rRNA processing by the nucleoside analogs 5‐fluoruridine and toyocamycin decreases the rate of 45S rRNA transcription in serum‐stimulated cells but has no effect on the values found in resting cultures. These data suggest that the regulation of rRNA transcription occurs by two different mechanisms, depending on the growth state of the cell. One mechanism, in serum‐stimulated cells, is dependent on a continuous protein synthesis and a correct 45S rRNA processing; the other, in resting cells, is independent of these
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