Transcriptional inhibitor. Inhibits sigma 70-directed transcription by weakening its interaction with the core of the host's RNA polymerase. This allows Gp55 to successfully compete for the core enzyme. Plays an important role during the prereplicative period of phage T4 development. Interacts with the host sigma factor RpoD, and thereby inhibits its interaction with the catalytic core of the host RNA polymerase.