Med19(Rox3) regulates intermodule interactions in the Saccharomyces cerevisiale mediator complex

The Saccharomyces cerevisiae Mediator is a 25-subunit complex that facilitates both transcriptional activation and repression. Structural and functional studies have divided Mediator subunits into four distinct modules. The Head, Middle, and Tail modules form the core functional Mediator complex, whereas a fourth, the Cyc-C module, is variably associated with the core. By purifying Mediator from a strain lacking the Medl9(Rox3) subunit, we have found that a complex missing only the Med19(Rox3) subunit can be isolated under mild conditions. Additionally, we have established that the entire Middle module is released when the Amed19(rox3) Mediator is purified under more stringent conditions. In contrast to most models of the modular structure of Mediator, we show that release of the Middle module in the Amed]9(rox3) Mediator leaves a stable complex made up solely of Head and Tail subunits. Both the intact and Head-Tail Delta medl 9(rox3) Mediator complexes have defects in enhanced basal transcription, enhanced TFIIH phosphorylation of the CTD, as well as binding of RNA Pol II and the CTD. The largely intact Delta med19(rox3) complex facilitates activated transcription at levels similar to the wild type Mediator. In the absence of the Middle module, however, the Amed19(rox3) Mediator is unable to facilitate activated transcription. Although the Middle module is unnecessary for holding the Head and Tail modules together, it is required for the complex to function as a conduit between activators and the core transcription machinery.