Unique Photochemistry Observed in a New Microbial Rhodopsin.

Light energy is first captured in animal and microbial rhodopsins by ultrafast photoisomerization, whose relaxation accompanies protein structural changes for each function. Here, we report a microbial rhodopsin, marine bacterial TAT rhodopsin, that displays no formation of photointermediates at >10 s. Low-temperature ultraviolet-visible and Fourier transform infrared spectroscopy revealed that TAT rhodopsin features all- to 13- photoisomerization like other microbial rhodopsins, but a planar 13- chromophore in the primary K intermediate seems to favor thermal back isomerization to the original state without photocycle completion. The molecular mechanism of the early photoreaction in TAT rhodopsin will be discussed.