The mediator complex in the RNA polymerase II holoenzyme is known to be involved in transcriptional activation. The role of the essential mediator component Srb7p has been difficult to investigate, since no conditional lethal allele has been available to date. While the expression of Srb7p under the control of a repressible promoter is not sufficient to reduce the level of Srb7p beneath the threshold for survival, we have been able to isolate a clone termed ts16 which confers a temperature sensitive phenotype. ts16 contains an insertion mutation that requires translational frameshifting for correct expression of Srb7p, leading to extremely low protein levels. Strains bearing the ts16 construct show mild defects in the transcription of constitutive genes like TDH1 but severely affect activated transcription, e.g. of the GAL1 gene. In contrast, CUP1, which is also independent of other holoenzyme components, is not affected by ts16.
