Region 2 of sigma-70 is the most conserved region of the entire protein. All members of this class of sigma-factor contain region 2. The high conservation is due to region 2 containing both the -10 promoter recognition helix and the primary core RNA polymerase binding determinant. The core binding helix, interacts with the clamp domain of the largest polymerase subunit, beta prime [1,2]. The aromatic residues of the recognition helix, found at the C-terminus of this domain are though to mediate strand separation, thereby allowing transcription initiation [1,2].

1: Structure of the bacterial RNA polymerase promoter specificity sigma subunit. Campbell EA, Muzzin O, Chlenov M, Sun JL, Olson CA, Weinman O, Trester-Zedlitz ML, Darst SA; Mol Cell 2002;9:527-539. PMID:11931761

2: Crystal structure of a sigma 70 subunit fragment from E. coli RNA polymerase. Malhotra A, Severinova E, Darst SA; Cell 1996;87:127-136. PMID:8858155