Growth-responsive expression from the murine thymidine kinase promoter: genetic analysis of DNA sequences

As a first step toward elucidating the biochemical basis of gene regulation at the G1-S boundary of the cell cycle, we have identified regions of the murine thymidine kinase (TK) promoter sufficient to confer appropriately growth-responsive expression to a heterologous gene. Using a series of TK promoter-chloramphenicol acetyltransferase (CAT) gene fusion constructs, we have identified sequences located between -174 base pairs upstream and +159 base pairs downstream of the TK translation initiation site that are sufficient to drive efficient S phase-specific expression of the CAT reporter gene in transfected murine fibroblasts. Both deletion analysis and site-specific mutagenesis experiments indicated that an Sp1 consensus binding site is critical to the activity of this promoter. Synchronized populations of BALB/c 3T3 cells stably transfected with either TK promoter-CAT fusion constructs or TK promoter-beta-globin fusion constructs expressed their respective reporter genes in an S phase-specific manner following serum stimulation. In each case, reporter gene expression was reduced during quiescence and G1 and rose upon entry of cells into S phase. The TK sequences included in these constructs therefore contained information sufficient to confer S phase-specific regulation to these two reporter genes. These results set the stage for a more detailed analysis of the sequences and trans-acting factors responsible for regulating murine TK gene expression and may lead to insights into the control of proliferation in normal and transformed cells.