Mediator
Mediators are fairly new
discovery in terms of their supporting functionality to RNA transcription
machinery. Mediator is a modular complex that communicated between the
gene-specific activators and the core RNA transcription machinery, which
is described on our Website. Mediator achieves this by transducing both
positive and negative regulatory information between those two components.
Mediator complexes are now seen as “essential coactivators" in eukaryotic
systems (Woychik, 2002).
An yeast mediator, which was
observed in “low-resolution, 3-D image” in 1999 by Asturias et al. and
in 2000 by Dotson et al., shows an “elliptical structure,” which undergoes
a conformational change to a crescent shaped molecule, with three distinct
domains, head, middle, and tail. The quaternary structure of head domain
has been observed to be most closely in contact with RNAP II in its crescent-shaped
active state. The head domain is also most highly conserved region of the
mediator. It is important to note that though the mediator docks with the
RNAP II in several locations, it leaves most of the RNAP II accessible
to interact with other molecules involved in the transcription. The yeast
mediator is about 20 subunit modular complex that includes Srb proteins,
Med proteins, and other polypeptides (Woychik, 2002).
The following figure from CIMBio
Group http://cimbio.scripps.edu/groups/Asturias/
shows the similarities between mediators found in yeast, mouse, and
human.
The mediator is thought to
be required for the response to activators and repressors in RNA transcription
process. It forms an interface between the RNAP II + general transcription
factors complex and the activator and repressor proteins that respond to
regulatory DNA sequence. It is also believed to also “function through
the CTD” in some cases (Woychik, 2002) .
Mediator is a distinct molecule and not to be confused with RNAPII
molecule although it is in contact with RNAP II. However, the mediator
does not bind to a specific DNA sequence.
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