Lys-tRNA4 levels and cell division in mouse 3T3 cells

tRNA₄^lys is an isoaccepting tRNA^lys which has been proposed as a necessary requirement for cell division in mammalian cells. We have measured the levels of this tRNA^lys during the growth cycle of mouse 3T3 fibroblasts. High levels of tRNA₄^lys were seen throughout exponential growth. However, a marked decrease in tRNA₄^lys occurred 24 h before the cells became confluent. This decrease was observed in three different 3T3 cell lines, but was not seen in a transformed 3T3 cell line. Trypsinization and replating of contact-inhibited cells returned tRNA₄^lys to the levels characteristic of exponential cells. Data from these and other cell lines show a direct relationship between the levels of tRNA₄^lys and the growth rate of cells in culture.