It is largely unknown how specific histone modifications and the effector proteins they recruit are coordinated with assembly of the preinitiation complex (PIC) at gene promoters to achieve transcriptional activation. The chromatin remodeling protein CHD1, which is necessary for embryonic stem cell pluripotency, binds to nucleosomes that are trimethylated at histone H3 Lys4 (H3K4me3). To study the mechanism by which H3K4me3 contributes to active transcription, Carey and colleagues reconstituted H3K4me3- and CHD1-stimulated gene activation on chromatin templates. Proteomics analysis of PICs assembled on unmodified chromatin versus H3K4me3 chromatin showed the transcriptional activator–dependent recruitment of CHD1 to PICs on unmodified chromatin templates, and CHD1 recruitment to PICs was further enhanced by H3K4 trimethylation. Proteomics analysis also confirmed the activator-dependent recruitment of the mediator coactivator complex. Mediator, which controls PIC assembly, is in turn needed for CHD1 recruitment. CHD1 stimulated in vitro transcription in a Mediator-dependent manner, and previously reported genome-wide chromatin immunoprecipitation data revealed overlapping CHD1-binding– and Mediator-binding sites that correlate with active gene transcription in mouse embryonic stem cells. Together, these findings suggest that Mediator coordinates assembly of the PIC and recruitment of CHD1 and that the PIC and H3K4me3 together provide specificity in targeting CHD1 to active genes. (Genes Dev. doi:10.1101/gad.17554711, published online 6 October 2011)