ALTERED SUBCELLULAR-DISTRIBUTION OF U3 SNRNA IN RESPONSE TO SERUM IN MOUSE FIBROBLASTS

To extend our understanding of the mechanisms regulating ribosome bios ynthesis during changes in cellular growth rate, the expression and su bcellular distribution of U3 snRNA and one of its associated proteins, fibrillarin, were examined in mouse 3T6 fibroblasts, Altering serum c oncentrations produces changes in the ribosome content of the cell as reflected by total RNA levels. When exponentially growing 3T6 cells ar e induced to become quiescent by serum starvation, a significant downs hift in U3 snRNA gene transcription occurs in parallel to a decrease i n pre-rRNA synthesis. Serum stimulation results in an increase in the rate of synthesis of both U3 snRNA and pre-rRNA. However, U3 snRNA syn thesis lags behind that of pre-rRNA, Furthermore, in serum-starved fib roblasts, a significant portion of the total cellular U3 snRNA appears in the cytoplasm. Following serum stimulation, a redistribution occur s and U3 snRNA is localized predominantly in the nucleolus at a level similar to that observed in exponentially growing cells. This redistri bution is inhibited when RNA or protein synthesis is repressed in seru m-stimulated fibroblasts by actinomycin D or cycloheximide. In contras t, the level and subcellular distribution of fibrillarin remain unchan ged during serum starvation, These results suggest that during changes in ribosome production, distinct pools of U3 snRNPs exist within the cell. (C) 1996 Academic Press, Inc.