Mediator is a large multiprotein complex conserved in all eukaryotes, which has 
a crucial coregulator function in transcription by RNA polymerase II (Pol II). 
However, the molecular mechanisms of its action in vivo remain to be understood. 
Med17 is an essential and central component of the Mediator head module. In this 
work, we utilised our large collection of conditional temperature-sensitive 
med17 mutants to investigate Mediator's role in coordinating preinitiation 
complex (PIC) formation in vivo at the genome level after a transfer to a 
non-permissive temperature for 45 minutes. The effect of a yeast mutation 
proposed to be equivalent to the human Med17-L371P responsible for infantile 
cerebral atrophy was also analyzed. The ChIP-seq results demonstrate that med17 
mutations differentially affected the global presence of several PIC components 
including Mediator, TBP, TFIIH modules and Pol II. Our data show that Mediator 
stabilizes TFIIK kinase and TFIIH core modules independently, suggesting that 
the recruitment or the stability of TFIIH modules is regulated independently on 
yeast genome. We demonstrate that Mediator selectively contributes to TBP 
recruitment or stabilization to chromatin. This study provides an extensive 
genome-wide view of Mediator's role in PIC formation, suggesting that Mediator 
coordinates multiple steps of a PIC assembly pathway.