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Abstract: Mediator is a general coactivator of RNA polymerase II (RNA pol II) bridging enhancer-bound transcriptional factors with RNA pol II. Mediator is organized in three distinct subcomplexes: head, middle, and tail modules. The head and middle modules interact with RNA pol II, and the tail module interacts with transcriptional activators. Deletion of one of the tail subunits SIN4 results in derepression of a subset of genes, including FLR1, by a largely unknown mechanism. Here we show that derepression of FLR1 transcription in sin4Δ cells occurs by enhanced recruitment of the mediator as well as Swi/Snf and SAGA complexes. The tail and head/middle modules of the mediator behave as separate complexes at the induced FLR1 promoter. While the tail module remains anchored to the promoter, the head/middle modules are also found in the coding region. The separation of the tail and head/middle modules in sin4Δ cells is also supported by the altered stoichiometry of the tail and head/middle modules at several tested promoters. Deletion of another subunit of the tail module MED2 in sin4Δ cells results in significantly decreased transcription of FLR1, pointing to the importance of the integrity of the separated tail module in derepression. All tested genes exhibited increased recruitment of the tail domain; however, only genes with increased occupancy of the head/middle modules also displayed increased transcription. The separated tail module thus represents a promiscuous transcriptional factor that binds to many different promoters and is necessary for derepression of FLR1 in sin4Δ cells. [Copyright &y& Elsevier]

While this Saccharomyces cerevisiae SIN4 gene product is a component of a mediator complex associated with RNA polymerase II, various studies suggest the involvement of Sin4 in the alteration of higher-order chromatin structure. Our previous analysis of a sin4 mutant suggested that the mechanisms of transcriptional repression by Sin4 (mediator) and the Tupl-Ssn6 complex (general repressor) are different. To elucidate the way in which these two repression systems are interrelated, we isolated mutants that exhibit enhanced transcription of a reporter gene harboring the upstream activation sequence (UAS), but still are subject to Tupl-Ssn6-mediated repression. Besides sin4, rgrl, tupl, and ssn6 mutants, we also obtained new mutants that enhance basal transcription even from a core promoter without UAS. Such mutants, designated rbt for regulator of basal transcription, can be classified into at least six complementation groups, ie., four single (rbtl, to rbt4,) and two apparently double (rbt5 rbt6 andrbt7 rbt8) mutations. The pheno-type of rbt mutants is dependent on the TATA box and not specific to the integration site or kind of core promoter. No significant difference in micrococcal nuclease (MNase) accessibility to the core promoter of test genes was observed between rbt mutants and the wild-type strain, indicating that the higher-order chromatin structure of the core promoter region is not significantly altered in these mutants. The rbtl to rbt4 mutations are suppressed by the Δgalll mutation as in the case of the sin4 mutation, but give rise to a different profile from the sin4 mutation with regard to the activity of some of the promoters. From these observations, we suggest that RBTgene product(s) could be novel mediators that act with or in close association with Sin4 but have a function distinct from that of Sin4. Moreover, the fact thatrbt mutations nullify Tupl-Ssn6 general repressor-mediated repression is consistent with the idea that the mechanisms of Rbt (mediator)- and Tupl-Ssn6 (general repressor)-mediated repression are interconnected but substantially different. [ABSTRACT FROM AUTHOR]