Dynamics of transcription factors: Identification of two distinct populations of cofactors and general transcription factors that dependent on active transcription
ithin the nucleus, SAGA and ATAC coactivators are divided into two distinct populations, one interacting with the chromatin by modifications (i.e. H3K4me3) and therefore restricted, the other free and highly mobile.
July 19, 2017
Transcription initiation at gene promoters is a major regulatory step in eukaryotic gene expression. Transcription of protein encoding genes into mRNA is controlled by large multiprotein complexes, such as RNA polymerase II (Pol II), general transcription factors (GTF) and transcription cofactors. SAGA and ATAC are two distinct chromatin modifying coactivator complexes with distinct enzymatic activities involved in Pol II transcription regulation. The team of Laszlo Tora at the IGBMC was interested in the intracellular mobility and dynamics of GTFs and coactivators in live cells and demonstrated that in the nuclei of live cells the equilibrium between fast and slow population of GTFs, SAGA, or ATAC, complexes is regulated by active transcription via changes in the abundance of H3K4me3 on chromatin. These results are published 19 of July in the EMBO journal.
In order to visualize the behavior of transcription factors and cofactors and to measure their mobility in nuclei of cells, researchers from the Laszlo Tora team at IGBMC performed in vivo imaging measurements on single live cells. They first measured the rate at which molecules reconquer a "photobleached" zone of the nucleus where any pre-existing signal has been eliminated. This method enabled them to suggest that the tested GTFs and coactivators are highly dynamic, exhibiting transient associations with the nuclear environment, whereas RNA polymerase II exhibits more stable interactions with chromatin.
In collaboration with Pascal Didier (from the team of Yves Mély at the Laboratory of Biophotonics and Pharmacology, CNRS - University of Strasbourg), the researchers used fluorescence correlation spectroscopy (FCS). FCS is particularly useful for the analysis of diffusion properties of very mobile factors. Moreover, FCS has single molecule sensitivity and microsecond to a few seconds time resolution to measure the dynamics of molecules. The FCS analyses revealed that the mobile pool of the two coactivators (SAGA and ATAC), but also that of two GTFs (TFIID and TFIIB), can be subdivided in “fast” (free) and “slow” (chromatin interacting) populations. Inhibition of transcription elongation decreased H3K4 trimethylation and the “slow” population of SAGA, ATAC, TFIIB and TFIID. In addition, inhibiting histone H3K4 trimethylation reduced also the “slow” populations of SAGA and ATAC.
These results demonstrate that in the nuclei of living cells, the balance between the "fast" and "slow" population of the coactivators is regulated by active transcription and by changes in histone marks on the chromatin.
This will lead to a better understanding of mechanisms of regulation of the transcription, the first step of the process leading to protein synthesis.
The study was funded by the Foundation for Medical Research (FRM), the ANR and the European Council for Research (ERC).