Studies of the Transcriptional Control of the Human c-fos Proto-Oncogene

The c-fos gene is the cellular counterpart of the transforming genes of several retroviruses. Its transcription and expression is increased in many types of different cells following extra-cellular stimuli, and this increase is both rapid and transient in most cases. Many studies have focussed on the transcriptional control of the c-fos proto-oncogene. These studies have characterised an element (the serum response element - SRE) that was initially found to increase transcription from the c-fos promoter following serum stimulation of quiescent cells and subsequently has been shown to activate transcription from the c-fos promoter following a variety of extra-cellular stimuli in a variety of different cell types. A computer-aided search of the human c-fos sequence revealed a sequence directly 3' of the SRE that shows similarity to the DNA binding sites for the transcription factors AP-1 and ATF. These proteins bind to their sequences and confer phorbol ester and cyclic AMP responsiveness upon adjacent promoters. The purpose of this project was to re-examine the role of the SRE in stimulating transcription from the c-fos promoter, to characterise the transcriptional properties of the fosATF/AP-1 binding site that lies adjacent to the SRE, and to investigate any interaction that may take place between these two sequences in controlling transcription from the c-fos promoter. Results obtained confirm that the SRE confers serum and phorbol ester responsiveness upon the c-fos promoter. They also show that the fosATF/AP-1 sequence forms a weaker complex with an AP-l/ATF protein than previously characterised AP-l/ATF binding sites, and that this sequence activates transcription from the c-fos promoter in growing cells. The fosATF/AP-1 binding site also confers serum and phorbol ester responsiveness upon the c-fos promoter. In quiescent cells the juxtaposed SRE and fosATF/AP-1 sequences contribute to a lower level of transcription from the c-fos promoter than does the SRE itself. This suggests an interaction between the proteins binding to these two sequences. Separating the SRE and fosATF/AP-1 sequences resulted in elevated levels of expression in quiescent cells and also resulted in higher levels of transcription from the c-fos promoter following stimulation of quiescent cells with serum or phorbol esters. It is concluded that there is a complex mechanism of interaction between the proteins that bind to the adjacent SRE and fosATF/AP-1 sequences and that there may be several proteins involved in this interaction