regulation (Allison and Ingles, 1989; Gerber et al., 1995; Scafe
et al., 1990).
The Med-PIC structure thereby provides a rationale for the ex-
istence of the linker, as well as indicating its minimum length. The
CTD path leading to TFIIK explains why Mediator enhances the
rate of phosphorylation by TFIIH in vitro (Kim et al., 1994). Further
consistent with the structure, splicing the CTD onto either Rpb4
or Rpb6 near its point of exit from the surface of the wild-type
enzyme rescued a lethal Rpb1-DCTD phenotype in yeast,
whereas splicing onto Rpb9, on the opposite side of the enzyme,
failed to rescue the lethal phenotype (Suh et al., 2013). These
CTD variants, which contained a modified linker region, showed
diminished levels of phosphorylation by TFIIK, possibly due to an
influence of linker length and topology on the kinetics of CTD
phosphorylation.
As regards the stimulation of transcription by Mediator, it is
notable what we do not observe: in many experiments, including
the example shown here (Figure 1D), we find no effect of Medi-
ator on the initiation of transcription by a fully formed PIC; and
we find no marked effect of Mediator on the structure of the
PIC, at the resolution of our analysis, except for a movement of
TFIIH toward the Middle module and an accompanying small
change in the angle of bend of the promoter DNA. The stimula-
tion of transcription by Mediator has been seen only in mixturesFigure 7. Tail Module Architecture and Dynamics
(A) The architectural model of free Mediator (colored subunit map) can be docked unambiguously into the Med-PIC structure (gray semi-transparent density)
showing the position of the three Mediator modules and a novel connection between the Middle and Tail (dashed red oval).
(B) Revised Tail module architectural map showing the position of Tail subunits and their colocalization with the Med14 C terminus at Tail interface 1and with
Med1 at Tail interface 2.
(C) Cross-links supporting the revised Tail module architecture.
(D) Alternate Tail module conformations. Whereas most particles show the Tail well separated from pol II (Tail-down), a subpopulation reveal a movement of the
Tail toward the upstream DNA and in close association of Tail with pol II (Tail-up). See alsoFigure S7.
Cell 166 , 1411–1422, September 8, 2016 1419