Abstract
Others have shown that yeast strains bearing a ts mutation in the Srb4 subunit of Mediator cease transcription of all mRNA at the restrictive temperature, in a manner virtually indistinguishable from a strain bearing a ts mutation in the largest subunit of RNA polymerase II. We find that srb4ts Mediator is defective for the stimulation of basal RNA polymerase II transcription at the restrictive temperature in vitro. Taken together, these findings lead to the suggestion that Mediator is required for basal RNA polymerase II transcription in vivo. On this basis, Mediator is identified as a general transcription factor, comparable in importance to RNA polymerase II and other general factors for the initiation of transcription. The possibility that Mediator serves as an anti-inhibitor, opposing the effects of global negative regulators, is largely excluded.
Highlights
Co-activators were reported in a human transcription system in 1991 but were believed at the time to be unrelated to yeast Mediator [7]
Temperature-sensitive mutants in two Mediator subunits, Srb4 and Srb6, exhibit a remarkable phenotype at a restrictive temperature, the rapid cessation of transcription of all genes tested [30]. This finding has been extended by microarray analysis to 5361 yeast genes, all but two of which showed as great an effect of the srb4ts mutant on transcription as that of a mutation in polymerase II (pol II) itself [31]
Transcription by srb4ts Mutant Cell Extract Correlates with the Mutant Phenotype, Implying a Role for Mediator in Basal Transcription in Vivo—Whole cell extracts were prepared from wild type and srb4ts yeast strains as described
Summary
Co-activators were reported in a human transcription system in 1991 but were believed at the time to be unrelated to yeast Mediator [7]. Attention focused instead on TAFs as responsible for the transmission of regulatory information in metazoan systems [8] It was only in 1998 that protein complexes in mammalian cells were identified as counterparts of yeast Mediator and shown to support transcriptional activation (9 –13). Temperature-sensitive mutants in two Mediator subunits, Srb and Srb, exhibit a remarkable phenotype at a restrictive temperature, the rapid cessation of transcription of all genes tested [30] This finding has been extended by microarray analysis to 5361 yeast genes, all but two of which showed as great an effect of the srb4ts mutant on transcription as that of a mutation in pol II itself [31]. They provide a test of the anti-inhibitor idea and illuminate the Mediator mechanism, especially in regard to the stimulation of basal transcription
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