= 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>. \b[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>.<Structure of the bacterial RNA polymerase promoter specificity sigma subunit by Elizabeth A. Campbell and others>[Abstract and structural analysis of sigma domains 2, 3 and 4]{ref}
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