Fetal liver or bone marrow-derived T lymphocyte precursors undergo extensive, developmentally regulated proliferation in response to inductive signals from the thymic microenvironment. We have used neonatal mouse thymocytes size-separated by centrifugal elutriation to study the cell cycle stage-specific expression of several genes associated with cell proliferation. These include genes involved in the biosynthesis of deoxyribonucleotide precursors, such as dihydrofolate reductase (DHFR), thymidylate synthase (TS), and the M1 and M2 subunits of ribonucleotide reductase, as well as c-myc, a cellular oncogene of unknown function. Using nuclear run-on assays, we observed that the transcription rates for these genes, with the exception of TS, are essentially invariant not only throughout the cell cycle in proliferating cells, but also in noncycling (G0) cells. The TS gene showed a transient increase in transcription rate in cells which bordered between a proliferating and nonproliferating status. Studies of an elutriated T cell line, S49.1, yielded similar results, indicating that the process of immortalization has not affected the transcriptional regulation of these genes. Analysis of steady-state mRNA levels using an RNase protection assay demonstrated that the levels of DHFR and TS mRNA accumulate as thymocytes progress through the cell cycle. In contrast, only the M2 subunit of ribonucleotide reductase showed cyclic regulation. Finally, in contrast to cultured cell models, we observed an abrupt fivefold increase in the steady-state level of c-myc mRNA in the transition from G1 to S-phase. We conclude from these studies that the transcriptional regulation of specific genes necessary for cellular proliferation is a minor component of the developmental modulation of the thymocyte cell cycle.

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