The RNA polymerase II (Pol II) transcription initiation apparatus consists of several multisubunit complexes, including Pol II, general transcription factors and suppressor of RNA polymerase B (SRB) proteins. Recent evidence indicates that many of these components assemble into a large complex, called the RNA polymerase holoenzyme, the SRB components of which participate in the response to transcriptional regulators. We discuss these results and their implications for the regulation of gene expression.