From Proteopedia
proteopedia linkproteopedia linkRNA polymerase (RNAP) is the enzymatic machinery responsible for transcription, a key regulatory step in gene expression. The prokaryotic RNAP is a highly conserved, "crab claw" shaped enzyme with a molecular mass of ~400kD. In order to recognize a promoter to begin transcription, the 5-subunit core enzyme (α,α,β,β’,ω) must bind to one of various sigma (σ) factors; this form of the enzyme is called the holoenzyme. Each of the different σ factors recognize different promoter elements upstream of genes allowing the cell to respond to various environmental cues.
Once holoenzyme binds the promoter, DNA downstream of this interaction is brought into the enzyme and melted to expose single-stranded DNA. This stage of transcription initiation is described here using a model of RNA polymerase-open-promoter-complex (RPo).
