The sigma-54 holoenzyme is an enhancer dependent form of the RNA polymerase. The AID is necessary for activator interaction [1]. In addition, the AID also inhibits transcription initiation in the sigma-54 holoenzyme prior to interaction with the activator [1].

1: Binding of transcriptional activators to sigma 54 in the presence of the transition state analog ADP-aluminum fluoride: insights into activator mechanochemical action. Chaney M, Grande R, Wigneshweraraj SR, Cannon W, Casaz P, Gallegos MT, Schumacher J, Jones S, Elderkin S, Dago AE, Morett E, Buck M; Genes Dev 2001;15:2282-2294. PMID:11544185