Gene regulation mechanisms positive

The mode of action of Smad 4 clearly differs from that of the other members of the Smad family. Smad 4 binds to phosphorylated R-Smads and forms trimeric complexes composed of two R-Smad molecules and one Smad 4 molecule. These complexes translocate to the nucleus, where they bind to related DNA elements and activate the transcription of target genes. The mechanism of transcription regulation by Smads is complex and includes both positive and negative influences. Generally, Smad-dependent regulation of transcription requires the interaction with other transcription factors, such as members of the FoxH 1 family of forkhead transcription factors, the Vitamin D receptor and the c-Jun transcription factor, among others (review Attisano et al., 2001). Futhermore, Smads can interact with coactivators and corepressors of transcription and thereby recruit, e. g., histone acetylase activity or histone deacetylase activity to chromatin. 

Outline the roles of coactivators and corepressors in transcription complexes. Contrast mechanisms of positive and negative gene regulation in eukaryotes. 

I have used such an approach In an attempt to answer the question, why positive and negative mechanisms of gene regulation In what follows I shall not be concerned with the mathematical methods per se. Instead, my approach will be to outline the principal steps In the development of the theory and then emphasize the most Importamt results. 

Although it was considered as one possibility by Jacob and Monod (AA) in their formulation of a model for control of gene expression, regulation by positive control elements was considered a less likely mechanism for control of gene expression. There have been several examples described in which positive control... 

Figure lOJ Summary of secondary metabolite gene regulation via G protein-mediated signal transduction. The indicated positive and ne tive regulation (arrows and bars, respectively) has been found to be mediated transcriptionally, post-transcriptionally, or by both mechanisms, as described in the text. 

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