A wealth of information correlates specific histone modifications with gene activity, but the molecular mechanisms by which the former influence transcription-factor function have remained elusive. Now, Roeder and colleagues provide the first insight into how histone H3 Lys4 trimethylation (H3K4me3), a promoter-specific histone modification associated with active transcription, selectively directs the expression of p53 target genes. They show that chromatin templates bearing H3K4me3 support higher levels of p53-dependent transcription in vitro than unmodified chromatin, by stimulating the rate of pre-initiation complex (PIC) assembly. Enhanced PIC formation and transcription is mediated by H3K4me3 interactions with the TAF3 subunit of the TFIID complex, as a PHD finger mutation that abolishes TAF3-H3K4me3 binding specifically reduces the stimulatory effect of H3K4me3. Importantly, the presence of H3K4me3 partially suppresses the effect of mutations in the promoter TATA box, suggesting that loss of TATA-box interactions can be compensated by TFIID interactions with chromatin via TAF3. Indeed, the authors show that TAF3-H3K4me3 interactions can act either in concert with the TATA box, to stimulate PIC formation and transcription, or independently, to enhance TFIID recruitment and transcription at promoters containing a mutant TATA box. In cells, depletion of H3K4me3 or disruption of its TAF3 interactions prevents the recruitment of TFIID and RNA polymerase II to p53-regulated cell-cycle promoters but not to proapoptotic p53 targets. Together, these observations establish a direct, gene-specific coactivator function for TAF3 within the p53 network and reveal the mechanism underlying H3K4me3's role in transcription. More generally, they demonstrate how chromatin structure and DNA sequence cooperate to modulate transcription factor activity at both TATA-dependent and TATA-less promoters. (Cell 152, 1021–1036, 2013)