The Escherichia coli energy-sensing Aer protein initiates aerotaxis towards environments supporting optimal cellular energy. The Aer sensor is an N-terminal, FAD-binding, PAS domain. The PAS domain is linked by an F1 region to a membrane anchor, and in the C-terminal half of Aer, a HAMP domain links the membrane anchor to the signaling domain. The F1 region, membrane anchor, and HAMP domain are required for FAD binding. Presumably, alterations in the redox potential of FAD induce conformational changes in the PAS domain that are transmitted to the HAMP and C-terminal signaling domains. In this study we used random mutagenesis and intragenic pseudoreversion analysis to examine functional interactions between the HAMP domain and the N-terminal half of Aer. Missense mutations in the HAMP domain clustered in the AS-2 alpha-helix and abolished FAD binding to Aer, as previously reported. Three amino acid replacements in the Aer-PAS domain, S28G, A65V, and A99V, restored FAD binding and aerotaxis to the HAMP mutants. These suppressors are predicted to surround a cleft in the PAS domain that may bind FAD. On the other hand, suppression of an Aer-C253R HAMP mutant was specific to an N34D substitution with a predicted location on the PAS surface, suggesting that residues C253 and N34 interact or are in close proximity. No suppressor mutations were identified in the F1 region or membrane anchor. We propose that functional interactions between the PAS domain and the HAMP AS-2 helix are required for FAD binding and aerotactic signaling by Aer.