Selective changes in tRNA methyltransferase activity in confluent monolayers of WI-38 cells stimulated to proliferate

In quiescent confluent monolayers of WI-38 cells, the specific activity of the tRNA methyltransferases falls to 20% of the level found in log phase cells. When the resting cells are stimulated to proliferate by a change to fresh medium, the enzyme show a rapid rise in specific activity which correlates with early increases in the rate of tRNA synthesis. The specific activity of the enzymes continues to rise throughout the period of DNA synthesis, at the end of which it is somewhat higher than that of log phase cells. The increases in enzyme activity could be blocked by exposure of the stimulated cells to Actinomycin D (2 microgram/ml). The increases in activity were not equivalent for the different base-specific enzymes. The contribution of the N2-methylguanine specific enzyme remained relatively constant, while that of the N2,N2-dimethyl-guanine specific and 1-methyladenine specific enzymes doubled and tripled, respectively, by late S phase. The contributions of the 1-methylguanine and the 7-methylguanine specific enzymes fell to a few percent of the total by late S phase. This indicates non-coordinate variations in the expression of the different base-specific enzymes after stimulation of resting cells and may be related to altered isoaccepting tRNA profiles observed in resting and growing cells.