RNAPolymeraseHoloenzym

RNAPolymeraseHoloenzym, commonly called the bacterial RNA polymerase holoenzyme, is the form of bacterial RNA polymerase that can initiate transcription at promoter sequences. It consists of a core RNA polymerase enzyme bound by a sigma factor. The core enzyme includes two α subunits, and one each of β, β', and ω; it carries out RNA synthesis but has limited promoter specificity on its own. The sigma factor guides the core to promoter DNA and facilitates the transition from initiation to elongation.

Promoter recognition involves conserved elements near the start of genes, such as the −10 and −35 regions in typical bacterial promoters. Upon binding, the DNA around the start site is unwound to form the open complex, allowing the first RNA nucleotides to be added. After several abortive initiation attempts, the complex achieves promoter clearance and productive elongation. The sigma factor generally dissociates from the core during early elongation, though some interactions may persist.

Different sigma factors provide promoter specificity and regulatory control. The primary σ factor governs housekeeping transcription, while alternative sigmas respond to environmental signals, stress, or developmental cues, altering which genes are transcribed by holoenzyme under given conditions.

Compared with eukaryotic RNA polymerases, bacterial holoenzyme concept emphasizes the combination of a catalytic core with a separate regulatory subunit that directs promoter recognition. In archaea and eukaryotes, RNA polymerases require general transcription factors to assemble a transcription-competent preinitiation complex, but the underlying principle of a regulated holoenzyme-like complex remains central to transcription initiation.