Fig 2.

Activation of bacterial transcription initiation (σ⁵⁴ family dependent). (A) σ⁵⁴ directs the RNA polymerase holoenzyme to bind at the −12 and −24 promoter elements. The interaction of the bacterial enhancer binding protein (bEBP) with the σ⁵⁴-RNAP holoenzyme is dependent on the binding of the activator (shown as an oligomer in green) to upstream activator sequences (UASs), 80 to 150 bp upstream of the transcriptional start site. (B) DNA looping occurs, often facilitated by other proteins such as integration host factor (IHF), enabling bEBP-σ⁵⁴ interactions. In σ⁵⁴-dependent transcription, the closed complex does not spontaneously undergo isomerization (melting of the double-stranded DNA). (C) Nucleotide hydrolysis by the activator promotes remodeling of the closed complex through a series of protein-protein and protein-DNA interactions that promote the formation of the open complex (30).