Gene expression in a temperature-sensitive mutant of the cell cycle

The proliferative response of mammalian fibroblasts to growth stimulation requires gene expression, including an increase in RNA synthesis. Vimentin, a class III intermediate filament, has been shown to be encoded by a growth-regulated gene whose mRNA levels increase after serum stimulation of quiescent hamster fibroblasts. This induction is a consequence of increased vimentin-specific transcriptional activity and is followed by the accumulation of stable cytoplasmic transcripts which are translated into polymerized filaments. The expression of vimentin in a G1-specific temperature-sensitive cell-cycle mutant growth-stimulated at the restrictive temperature is strikingly different from the growth-regulated expression observed at the nonrestrictive temperature. While the vimentin gene is transcriptionally activated at the restrictive temperature, most of the transcripts accumulate in the nucleus and are not transported to the cytoplasm. Those transcripts that are cytoplasmic have a decreased stability and are not translated into protein. Vimentin gene expression in the parent cell line at both the restrictive and nonrestrictive temperatures is virtually identical, suggesting that the altered expression of vimentin in the cell-cycle mutant at the restrictive temperature is related to the G1-specific block and not to the elevated temperature. The expression of the other growth-regulated genes is currently being investigated in the temperature-sensitive cell cycle mutant to determine if this effect is vimentin-specific or specific for a subclass of growth-regulated genes.