 Solution

For a primary sigma factor of the ${\sigma }^{70}$ family, distinguish the following regions. Region 1.1 restrains inappropriate DNA binding by free sigma factor and must move out of the downstream-DNA channel during formation of the open RNA polymerase complex; region 1.2 contributes to recognition around the discriminator downstream of the $-10$ element. Domain 2 binds the RNA polymerase core, recognizes the $-10$ gene promoter element and helps melt the duplex and stabilize the exposed non-template strand. Domain 3 recognizes an extended $-10$ element; its region 3.2 projects toward the active-centre channel, helps position template and initiating substrates, and must be displaced as the nascent RNA grows. Domain 4 recognizes the $-35$ element through a helix-turn-helix motif and contacts the RNA polymerase beta flap and some transcription activators. Domains 2 and 4 supply the principal $-10$ and $-35$ sequence recognition, with the other regions controlling opening and productive initiation. This domain map concerns the ${\sigma }^{70}$ family, rather than every structurally distinct bacterial sigma factor.^[ref]