Intragenic complementation between mutant Aer proteins. A. Mutant Aer-PASM1 + Aer-HAMPM2 or Aer-proximal signallingM2 proteins were coexpressed from compatible plasmids in E. coli BT3400 (aer, tsr, recA). To test for aerotaxis, transformants were transferred to 30 mM succinate semi-soft agar and incubated at 30°C for 29 h. Representative examples are shown in which intragenic complementation restored (Aer-N34DM1 + Aer-V230DM2, and Aer-D68CM1 + Aer-G240RM2), partially restored (Aer-M21DM1 + Aer-D259HM2), or did not restore (Aer-G42CM1 + Aer-D259HM2) aerotaxis. The mutant Aer proteins formed non-aerotactic homodimers in BT3400 as shown. B. Cartoon of an Aer heterodimer summarizing missense mutations (X) in the PAS, HAMP and proximal signalling regions.