The DNA-dependent RNA polymerase (RNAP) is the principal enzyme of the transcription process, and is a final target in many regulatory pathways that control gene expression in all living organisms. In bacteria, RNAP is responsible for the synthesis of all RNAs in the cell (messenger RNA, ribosomal RNA, transfer RNA, and so on). <br> The binding of initiation factor σ to a RNAP core enzyme results in the formation of a holoenzyme, the active form of RNAP essential for transcription initiation. The atomic model of the σ70 subunit lacks the disordered N-terminal domain (σ1-73), which includes the conserved σ region 1.1. In the autoinhibited state, the polar surface of the region 2.2 helix is partially occluded by a loop within the nonconserved insert between regions 1.2 and 2.1, which masks the DNA-binding regions of σ before it binds the core. In the complex of σ with core RNAP, the loop is repositioned to expose the core RNAP binding determinants in region 2.2.