The rate of c-fos transcription in vivo is continuously regulated at the level of elongation by dynamic stimulus-coupled recruitment of positive transcription elongation factor b.

In mammalian cells, multiple stimuli induce the expression of the immediate early gene c-fos. The specificity of c-fos transcriptional response depends on the activation of signaling protein kinases, transcription factors, and chromatin-modifying complexes but also on a regulated block to elongation in the first intron. Here we show by chromatin immunoprecipitation that finely tuned control of c-fos gene expression by distinct stimuli is associated with a dynamic regulation of transcription elongation and differential phosphorylation of the C-terminal domain of RNA polymerase II. Comparison of two stimuli of c-fos expression in the pituitary cell line GH4C1, namely the thyrotropin-releasing hormone versus depolarizing KCl, shows that both stimuli increase initiation, but only thyrotropin-releasing hormone is efficient to stimulate elongation and thus produce high transcription rates. To control elongation, the elongation factor P-TEFb is recruited to the 5'-end of the gene in a stimuli and time-dependent manner. Transition from initiation to elongation depends also on the dynamic recruitment of the initiation factors TFIIB and TFIIE but not TFIID, which remains constitutively bound on the promoter. It thus appears that tight coupling of signaling input to transcriptional output rate is achieved by c-fos gene-specific mechanisms, which control post-initiation steps rather than pre-initiation complex assembly.