The domain is found in the primary vegetative sigma factor. The function of this domain is unclear and can be removed without loss of function.

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