Daniel Luk

Regulation of c-fos expression in transgenic mice requires multiple interdependent transcription control elements

Transcription control regions of eukaryotic genes contain multiple sequence elements proposed to function independently to regulate transcription. We developed transgenic mice carrying fos-lacZ fusion genes with clustered point mutations in each of several distinct regulatory sequences: the sis-inducible element, the serum response element, the fos AP-1 site, and the calcium/cAMP response element. Analysis of Fos-lacZ expression in the CNS and in cultured cells demonstrated that all of the regulatory elements tested were required in concert for tissue- and stimulus-specific regulation of the c-fos promoter. This implies that the regulation of c-fos expression requires the concerted action of multiple control elements that direct the assembly of an interdependent transcription complex.

Fos-IacZ transgenic mice: Mapping sites of gene induction in the central nervous system

Abstract A transgenic mouse line containing a fos-lacZ fusion gene was derived in which β-galactosidase activity identified cell populations expressing fos either constitutively or after stimulation. Seizures and light pulses induced nuclear lacZ activity in defined populations of neurons in vivo, and an array of neurotransmitters, including glutamate, induced the transgene in primary brain cultures. In unstimulated mice, the major sites of fos-lacZ expression were skin, hair follicle, and bone. fos-lacZ mice provide a new avenue for activity mapping studies based on gene expression.

Fos is a preferential target of glucocorticoid receptor inhibition of AP-1 activity in vitro.

Several regulatory interactions between the AP-1 and the nuclear hormone receptor families of transcription factors have been reported. However, the molecular mechanisms that underlie these interactions remain unknown, and models derived from transient-transfection experiments are contradictory. We have investigated the effect of the purified glucocorticoid receptor (GR) DNA-binding domain (GR residues 440 to 533 [GR440-533]) on DNA binding and transcription activation by Fos-Jun heterodimers and Jun homodimers. GR440-533 differentially inhibited DNA binding and transcription activation by Fos-Jun heterodimers. Inhibition of Jun homodimers required a 10-fold-higher concentration of GR440-533. An excess of Fos monomers protected Fos-Jun heterodimers from inhibition by GR440-533. Surprisingly, regions outside the leucine zipper and basic region were required for GR inhibition of Fos and Jun DNA binding. The region of GR440-533 required for inhibition of Fos-Jun DNA binding was localized to the zinc finger DNA-binding domain. However, inhibition of Fos-Jun DNA binding was independent of DNA binding by GR440-533. GR440-533 also differentially inhibited Fos-Jun heterodimer binding to the proliferin plfG element. Differential inhibition of DNA binding by different AP-1 family complexes provides a potential mechanism for the diverse interactions between nuclear hormone receptors and AP-1 family proteins at different promoters and in different cell types.

Transcriptional regulation by Fos and Jun in vitro: interaction among multiple activator and regulatory domains.

The proteins encoded by the proto-oncogenes c-fos and c-jun (Fos and Jun, respectively) form a heterodimeric complex that regulates transcription by interacting with the DNA-regulatory element known as the activator protein 1 (AP-1) binding site. Fos and Jun are members of a family of related transcription factors that dimerize via a leucine zipper structure and interact with DNA through a bipartite domain formed between regions of each protein that are rich in basic amino acids. Here we have defined other domains in the Fos-Jun heterodimer that contribute to transcriptional function in vitro. Although DNA-binding specificity is mediated by the leucine zipper and basic regions, Jun also contains a proline- and glutamine-rich region that functions as an ancillary DNA-binding domain but does not contribute directly to transcriptional activation. Transcriptional stimulation in vitro was associated with two regions in Fos and a single N-terminal activation domain in Jun. These activator regions were capable of operating independently; however, they appear to function cooperatively in the heterodimeric complex. The activity of these domains was modulated by inhibitory regions in Fos and Jun that repressed transcription in vitro. In the context of the heterodimer, the Jun activation domain was the major contributor to transcriptional stimulation and the inhibitory regions in Fos were the major contributors to transcriptional repression in vitro. Potentially, the inhibitory domains could serve a regulatory function in vivo. Thus, transcriptional regulation by the Fos-Jun heterodimer results from a complex integration of multiple activator and regulatory domains.

Fos and jun cooperate in transcriptional regulation via heterologous activation domains.

The products of c-fos and c-jun (Fos and Jun) function in gene regulation by interacting with the AP-1 binding site. Here we have examined the contribution of Fos and Jun toward transcriptional activity by using Fos and Jun polypeptides purified from Escherichia coli. Fos contained a transcriptional activation domain as well as a region which exerted a negative influence on transcriptional activity in vitro. Moreover, distinct activation domains in both Fos and Jun functioned cooperatively in transcriptional stimulation. Thus, regulation of gene expression by Fos and Jun results from an integration of several functional domains in a bimolecular complex.