Phototaxis responses measured as swimming reversal frequency changes to photostimulus (arrow): 100 ms, 500 nm stimulus for wild-type SRII and SRII mutants and 100 ms, 550 nm stimulus for wild-type BR and BR-T. The swimming reversal frequency in a cell population is monitored by the quotient of the rate of change of direction (RCD) and linear swimming speed (SPD), which increase and decrease, respectively, during a swimming reversal. Cells contain the wild-type SRII–HtrII (a), T204A–HtrII (b), T204S–HtrII (c), T204C–HtrII (d), Y174F–HtrII (e), T79A–HtrII (f), and N105D–HtrII (g) complexes and the wild-type BR–HtrII (h) and BR-T–HtrII (i) complexes.