Gram‐negative bacteria tightly regulate intracellular iron, an essential nutrient. To ensure this strict control, some outer membrane TonB‐dependent transporters (TBDTs) responsible for iron import stimulate their own transcription in response to extracellular binding of ferric siderophore. This process is mediated by an inner membrane sigma regulator protein that transduces an unknown periplasmic signal from the TBDT to release an intracellular sigma factor from the inner membrane, which ultimately upregulates TBDT transcription. Here, we employ the Pseudomonas putida ferric‐pseudobactin BN7/BN8 sigma regulator, PupR, as a model system to understand the molecular mechanism of this conserved class of sigma regulators. We have determined the X‐ray crystal structure of the cytoplasmic anti‐sigma domain (ASD) of PupR to 2.0 Å. Size exclusion chromatography, small‐angle X‐ray scattering, and sedimentation velocity analytical ultracentrifugation (SV‐AUC) all indicate that, in contrast to other ASDs, the PupR‐ASD exists as a dimer in solution. Mutagenesis of residues at the dimer interface identified from the crystal structure disrupts dimerization and protein stability, as determined by SV‐AUC and thermal denaturation circular dichroism spectroscopy. Additionally, we have demonstrated that the helical content of the PupR‐ASD increases in the presence of trifluoroethanol, and are investigating the interaction of the PupR‐ASD with its cognate sigma factor, PupI. These combined results suggest that this type of inner membrane sigma regulator may utilize an unusual mechanism to sequester their cognate sigma factors and prevent transcriptional activation.Support or Funding InformationFunded by NSF ND EPSCoR DDA #FAR0025216 to JJ; NIH NINDS 1R03 NS090939 and NSF MCB‐1413525 to SS; NIH NIGMS 1R15 GM113227, NIH NCRR 2P20 RR015566, and NIH NIGMS P30 GM103332 to CC.