The role of putative intragenic control elements in c-fos regulation by calcium and growth factor signalling pathways

Sequences in the transcribed region of the c-fos gene have been suggested t o control c-fos induction following exposure of cells to mitogens or stimul i that increase intracellular calcium concentrations. Using a mutational an alysis we show that putative regulatory elements present in the first intro n of the human c-fos gene and the fos-intragenic-regulatory-element (FIRE) are not required for c-fos regulation by growth factor and calcium signalli ng pathways in AtT20 and PC12 cells. Removal of the c-fos first intron and the FIRE did not increase the basal level of c-fos mRNA and only moderately reduced the magnitude of calcium-induced transcription mediated by either the entire c-fos promoter or the cAMP response element (CRE). Intragenic mu tations did not affect serum response element (SRE)-dependent gene expressi on induced by calcium signals but caused a superinduction of c-fos expressi on in nerve growth factor-stimulated PC12 cells. These results indicate tha t c-fos promoter elements, rather than intragenic sequences, are the princi pal targets of transcription-regulating signalling pathways. This suggests that CRE- and SRE-bound activators of transcription initiation may also enh ance, in a signal-dependent manner, c-fos transcript elongation beyond prom oter-proximal pause sites.