Transcription activation of late genes in T4 bacteriophage requires the promoter specificity factor gp55, the coactivator gp33 and the sliding clamp gp45. Here, the authors provide structural insights into gp45- dependent transcription activation by determining the cryo-EM structures of a gp55-dependent RNA polymerase (RNAP)-promoter open complex and of an intact gp45-dependent transcription activation complex.