To extend our understanding of the mechanisms regulating ribosome biosynthesis during changes in cellular growth rate, the expression and subcellular distribution of
U3 snRNA and one of its associated
proteins,
fibrillarin, were examined in mouse 3T6 fibroblasts. Altering serum concentrations produces changes in the ribosome content of the cell as reflected by total
RNA levels. When exponentially growing 3T6 cells are induced to become quiescent by serum
starvation, a significant downshift in
U3 snRNA gene transcription occurs in parallel to a decrease in
pre-rRNA synthesis. Serum stimulation results in an increase in the rate of synthesis of both
U3 snRNA and
pre-rRNA. However,
U3 snRNA synthesis lags behind that of
pre-rRNA. Furthermore, in serum-starved fibroblasts, a significant portion of the total cellular
U3 snRNA appears in the cytoplasm. Following serum stimulation, a redistribution occurs and
U3 snRNA is localized predominantly in the nucleolus at a level similar to that observed in exponentially growing cells. This redistribution is inhibited when
RNA or
protein synthesis is repressed in serum-stimulated fibroblasts by
actinomycin D or
cycloheximide. In contrast, the level and subcellular distribution of
fibrillarin remain unchanged during serum
starvation. These results suggest that during changes in ribosome production, distinct pools of U3 snRNPs exist within the cell.