INCREASING CONTENT OF POLY-A(+) MESSENGER-RNA OF SERUM-STIMULATED CELLS IN ABSENCE OF RIBOSOME SYNTHESIS

When 3T6 mouse fibroblast cells undergo a serum-induced transition from resting to growing state, the number of ribosomes and the amounts of mRNA and tRNA increase as the cells prepare for DNA synthesis. The effect of preventing ribosome synthesis during this transition was studied. When resting cells are stimulated to grow in the presence of 5-fluorouridine, mRNA accumulates normally during the first 8 h, though new ribosome formation is completely blocked by the drug. At later times mRNA continues to accumulate, but at a reduced rate. The ratio of poly A(+) mRNA to ribosomal RNA increases from the value characteristic of resting 3T6 (1.8%) to that of growing 3T6 (2.7%) by 5 h, and continues to increase to abnormally high values after this time. Although labeling of tRNA is not affected after brief exposure of cells to fluorouridine, the drug prevents the later accumulation of tRNA that ordinarily occurs following serum stimulation of resting cells. This failure of accumulation is not the result of increased lability of fluorinated tRNA, but is probably due to failure of the transcription rate of pre-tRNA to increase. It is possible that this effect might be due to a regulatory system coupling tRNA content to ribosome content. In cultures stimulated with serum in the presence of fluorouridine the rate of protein synthesis increases with poly A(+) mRNA content during the first 8 h; it then fails to increase further, possibly because ribosomes become rate-limiting.