Regulation of transcription initiation is generally attributable to activator/repressor proteins that bind to specific DNA sequences. However, regulators can also achieve specificity by binding directly to RNA polymerase (RNAP) and exploiting the kinetic variation intrinsic to different RNAP-promoter complexes. We report here a previously unknown interaction with Escherichia coli RNAP that defines an additional recognition element in bacterial promoters. The strength of this sequence-specific interaction varies at different promoters and affects the lifetime of the complex with RNAP. Selection of rRNA promoter mutants forming long-lived complexes, kinetic analyses of duplex and bubble templates, dimethylsulfate footprinting, and zero-Angstrom crosslinking demonstrated that sigma subunit region 1.2 directly contacts the nontemplate strand base two positions downstream of the -10 element (within the "discriminator" region). By making a nonoptimal sigma1.2-discriminator interaction, rRNA promoters create the short-lived complex required for specific responses to the RNAP binding factors ppGpp and DksA, ultimately accounting for regulation of ribosome synthesis.