Fig. 4.

Mechanism of colour discrimination in H. halobium phototaxis. Photon absorption by sR₅₈₇, the dark-adapted form of sensory rhodopsin, generates a short-lived species S₆₈₀ which rapidly decays to S₃₇₃. The S₃₇₃ form can return to the sR₅₈₇ form by either of two paths: a purely thermal relaxation or a photoinduced path. If S₃₇₃ does not absorb a photon, it relaxes thermally to sR₅₈₇ completing the ‘one-photon’ photocycle. During its transit through the one-photon cycle, the sensory rhodopsin molecule generates an attractant signal to the flagellar motor. If S₃₇₃ absorbs a photon, it generates the intermediate ${\mathrm{S}}_{510}^{\mathrm{b}}$. The relaxation of ${\mathrm{S}}_{510}^{\mathrm{b}}$ to sR₅₈₇ completes a cycle containing two photon absorption steps. After absorption of the second photon in the ‘two-photon’ cycle, sensory rhodopsin generates a repellent signal to the flagellar motor. The attractant and repellent signals are integrated prior to or at their site of action on the flagellar motor³.