tk-ts13 cells are G1-specific temperature-sensitive mutants of the cell cycle that arrest in G1 at the restrictive temperature. In these cells the mRNAs for early growth-regulated genes (for instance, c-myc) are inductible by serum at both permissive and restrictive temperatures. In contrast, the mRNAs for late growth-regulated genes [such as histones, proliferating cell nuclear antigen (PCNA), and DNA polymerase-α] are not detectable at the restrictive temperature, although they are normally induced by serum at the permissive temperature. Despite the absence of their mRNAs at the restrictive temperature, transcription rates for DNA polymerase-α, PCNA, and histone H3 are the same in serum-deprived cells and in cells that are serum stimulated at either the permissive or the restrictive temperature. Since the half-lives of the mRNAs are not substantially different at the two temperatures, the conclusion is that in tk-ts13 cells the mRNA levels of these late growth-regulated genes are regulated at a posttranscriptional level, presumably during hnRNA processing. When serum-deprived tk-ts13 cells carrying a stably integrated SV40 T antigen-coding gene (T-neo cells) are stimulated with serum, they are capable of one additional round of DNA replication at the restrictive temperature. At 20 h after stimulation of T-neo cells, the mRNAs for the late growth-regulated genes are detectable at the restrictive temperature in amounts not substantially different than those at the permissive temperature. Transcription rates in T-neo cells are increased for histone H3 (in comparison to tk-ts13 cells) but not for PCNA and DNA polymerase-α. The presence of the T antigen does not seem to seriously affect the half-lives of the mature mRNAs. The conclusion is that the presence of the SV40 T antigen in tk-ts13 cells promotes the appearance of mature mRNAs for DNA polymerase-α and PCNA. These experiments suggest that T antigen, in this instance, may intervene either directly or indirectly at a posttranscriptional level in the regulation of the steady state mRNA levels of certain cellular genes.


This work was supported by Grants GM 42383 from the NIH and CD214 from the American Cancer Society.

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