The interferon (IFN)-induced signal transduction and transcription activation 
complex, ISGF3, is assembled from three proteins, STAT1, STAT2, and IRF9. Of 
these components, STAT2 provides a fundamental and essential transcriptional 
activation function for ISGF3. In the present study, we show that ISGF3-mediated 
transcription is dependent on STAT2 interactions with DRIP150, a subunit of the 
multimeric Mediator coactivator complex. Other Mediator subunits, DRIP77 and 
DRIP130, were found either to bind STAT2 without augmenting ISGF3 
transcriptional activity or to enhance ISGF3 transcription without binding 
STAT2, but only DRIP150 both enhanced IFN-dependent transcription and 
coimmunoprecipitated with STAT2. Endogenous DRIP150 and STAT2 were able to 
interact in solution, and DNA affinity chromatography and chromatin 
immunoprecipitation assays demonstrated that DRIP150 binds to the mature, 
activated ISGF3-DNA complex and is recruited to target gene promoters in an 
IFN-dependent fashion. IFN-dependent recruitment of DRIP130 to an ISGF3 target 
promoter and SRB10-STAT2 coprecipitation suggest indirect association with a 
multisubunit Mediator complex. The site of STAT2 interaction was mapped to 
DRIP150 residues 188 to 566, which are necessary and sufficient for interaction 
with STAT2. Expression of this DRIP150 fragment, but not DRIP150 fragments 
outside the STAT2 interaction region, suppressed ISGF3-mediated transcriptional 
activity in a dominant-negative fashion, suggesting a direct functional role of 
this domain in mediating STAT2-DRIP150 interactions. These findings indicate 
that the IFN-activated ISGF3 transcription factor regulates transcription 
through contact with DRIP150 and implicate the Mediator coactivator complex in 
IFN-activated gene regulation.