Control of thymidylate synthase mRNA content and gene transcription in an overproducing mouse cell line

We studied the content and metabolism of thymidylate synthase mRNA in cultured mouse fibroblasts that were undergoing a serum-induced transition from the resting to growing state. The studies were performed with a 5-fluorodeoxyuridine-resistant 3T6 cell line (LU3-7) that over produces the enzyme and its mRNA about 50-fold and that regulates the expression of the thymidylate synthase gene in the same manner as the parental cell line. We have previously shown that the rate of synthesis of thymidylate synthase increases at least ninefold when the serum-stimulated cells traverse the S phase. Here we show, by Northern blot analysis, that thymidylate synthase mRNA increased 20- to 40-fold as cells progressed from resting to late S phase. About 85% of poly(A)+ thymidylate synthase mRNA was associated with polysomes at all times. The increase in thymidylate synthase poly(A)+ mRNA content was the result of an eightfold increase in the rate of production of this species, as shown by pulse-labeling studies. Pulse-chase analysis revealed that the half-life of thymidylate synthase poly(A)+ mRNA was similar in resting (9 h) and growing (7 h) cells. The rate of transcription of the thymidylate synthase gene, as determined in isolated nuclei, increased only by a factor of three to four during the S phase. Since the content of the message increased to a much greater extent than the rate of transcription of the gene, posttranscriptional controls must also play a role in regulating the content of thymidylate synthase mRNA under these conditions. Our results suggest that the cell may regulate the distribution of thymidylate synthase mRNA between a relatively stable poly(A)+ RNA species and a labile poly(A)- RNA species.