v-Raf activates transcription of growth-responsive promoters via GC-rich sequences that bind the transcription factor Sp1.

The serine/threonine kinase, Raf-1, is a component of intracellular signaling pathways that control responses to extracellular stimuli. Previously, we have shown that serum-induced transcription from the murine rep-3b and human mdr1 promoters is Raf-dependent and that the activated Raf kinase, v-Raf, induces transcription of mdr1 via a GC-rich element. We now demonstrate that GC-rich sequences in the rep-3b promoter are both necessary and sufficient for induction by v-Raf. The GC-rich, v-Raf-responsive elements of rep-3b and mdr1 bind the general transcription factor Sp1 in electromobility shift assays. Mutation of a minimal GC-rich element abolished inducibility by v-Raf and eliminated binding by the transcription factor Sp1. However, Sp1 binding activity following serum stimulation of quiescent NIH 3T3 cells was unchanged, suggesting that mitogenic signals may stimulate the transactivation potential of prebound Sp1.