Functional significance of Sp1, Sp2, and Sp3 transcription factors in regulation of the murine CTP : phosphocholine cytidylyltransferase alpha promoter

The transcription factor Sp1 has been implicated in regulation of the expre ssion of the murine CTP:phosphocholine cytidylyltransferase alpha (CT alpha ) gene, Ctpct (M, Bakovic, K. Waite, W. Tang, I. Tabas, and D. E. Vance, 19 99, Biochim. Biophys. Acta. 1438: 147-165). We have utilized transient tran sfections, mutation analysis, electromobility gel-shifts, and immunoblot an alysis to test the hypothesis that expression of the CT alpha gene is contr olled in part by the binding of three h-ans-acting nuclear factors, Sp1, Sp 2, and Sp3, Sp1 and Sp3 activate CT alpha gene transcription through sequen ce specific binding within three promoter domains. In Spl-mediated transcri ption, Sp3 acts as an activator in a dose-dependent manner and vice versa. Sp2 represses Sp1-and Sp3-driven transcription in Drosophila SL2 cells, but stimulates transcription in C3H10T1/2 mammalian cells. Our results suggest that the predominant action of Sp proteins is a direct function of local o rganization of three cis-acting elements in the regions A (-31/-9), B (-88/ -50), and C (-148/-128), The ability of distal C (-148/-128) and proximal A (-31/-9) regions to activate or repress transcription depends upon the cel lular background. The multiple binding elements at position B (-88/-50) con fer a positive regulation independent of the cell context. However, the eff ectiveness of Sp proteins at this site is strongly governed by neighboring sites A and C. The results suggest that the level of expression of the CT a lpha gene will depend on the cell. type, the availability of Sp proteins, a nd the structure and organization of three cis-acting elements.