FIG 4.
Aer2 mutant phenotypes in temporal assays. (a) Effects of amino acid substitutions on Aer2-mediated behavior in E. coli BT3388. Signal-off mutants exhibited smooth-swimming behavior (∼2% tumbling) in both air and N2, whereas signal-on mutants tumbled constantly in both air and N2. Neither signal-off nor signal-on mutants responded to the introduction or removal of O2. Signal-off-biased mutants responded to the introduction of O2 but adapted, unlike WT Aer2 in BT3388, which remained signal-on in the presence of O2. Signal-on-biased mutants responded to the removal of O2, but at least 50% of the cells continued to tumble in N2. Residue substitutions marked by an asterisk resulted in receptors that could respond to CO, i.e., they directed cell tumbling in the presence of CO. CO responses could not be determined for signal-on-biased and signal-on mutants. (b) Alanine mutants mapped onto the cyanomet structure of Aer2. Original side chains are shown as color-coded sticks based on the O2 responses listed in panel a.