Aerotaxis elicited by Aer missense mutants in the receptorless E. coli strain BT3388 (A and B) or the Δaer Δtsr strain BT3312 (C and D). (A and B) Taxis on tryptone semisoft agar by Aer mutants in the receptorless strain induced with 20 μM IPTG after incubation at 30°C for 14 h (A), or 19 h (B). (A) Examples of Aer missense mutants demonstrating the different phenotypes observed. Upper row: WT Aer, plasmid vector pTrc99A, Aer-L20K (smooth swimming). Bottom row: Aer-L14H (increased tumbling frequency), Aer-L20C (constant tumbling), Aer-T19K (inverse response). (B) Enlargements showing details of the fuzzy, cotton ball edges produced by the constant tumbling of Aer-L20C in comparison with the more-regular edges of other nonaerotactic mutants such as Aer-L14H. (C and D) Aerotaxis on succinate semisoft agar by Aer mutants in the Δaer Δtsr strain after incubation at 30°C for 17 h. (C) Aer missense mutants from panel A. (D) Aer-T19S and Aer-M21S.