Transfer RNA 4lys and cell cycle progression: Characterization of Ts-694 cells

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Lysine tRNA modification has been studied in mammalian ts-694 cells with respect to cell cycle progression in temperature downshift and upshift experiments. The modification of tRNA lys measured in temperature downshift experiments showed that tRNA 4 lys levels start to increase 6 h following the temperature shift, approximately 10–12 h prior to the cells entry into S phase. Ts-694 cells showed a gradual decrease in the level of tRNA 4 lys and the rates of DNA synthesis following a temperature upshift. The cells became growth arrested following incubation for 36–45 h at the rt. Cell cycle mapping of the temperature restriction point suggests a G 1 block prior to the serum deprivation restriction point. Depletion of cellular tRNA 4 lys by serum deprivation followed by simultaneously shifting cells to the rt and feeding medium containing 10% serum showed that cells with low tRNA 4 lys levels and no mechanism for the synthesis of tRNA 4 lys could not enter S phase and synthesize DNA. Blocking of ts-694 at the G 1 S boundary with aphidicolin indicates that cells that have passed through G 1 are capable of entering S phase and synthesizing DNA independent of the incubation temperature. These results indicate that tRNA 4 lys is not needed during S phase for DNA replication but suggests that tRNA 4 lys is required for cells to progress through G 1.