Several previously characterized transcriptional adaptors and coactivators are now known to be histone acetyltransferases (HATs). Recent studies in Saccharomyces cerevisiae indicate that the Gcn5p HAT exists in large complexes containing several phenotypic classes of transcription factors. Genetic and biochemical studies of these transcription factors and their functions within HAT complexes suggest that acetylation of histones is one function of an integrated system of modular activities. These activities include interaction with activators, histone acetylation and interaction with basal factors. Coordination of these functions may well be an important component of gene activation in vivo.