The first steps in the photocycles of the archaeal photoreceptor proteins sensory rhodopsin (SR) I and II from Halobacterium salinarum and SRII from Natronobacterium pharaonis have been studied by ultrafast pump͞probe spectroscopy and steady-state fluorescence spectroscopy. The data for both species of the bluelight receptor SRII suggests that their primary reactions are nearly analogous with a fast decay of the excited electronic state in 300 -400 fs and a transition between two red-shifted product states in 4 -5 ps. Thus SRII behaves similarly to bacteriorhodopsin. In contrast for SRI at pH 6.0, which absorbs in the orange part of the spectrum, a strongly increased fluorescence quantum yield and a drastically slower and biexponential decay of the excited electronic state occurring on the picosecond time scale (5 ps and 33 ps) is observed. The results suggest that the primary reactions are controlled by the charge distribution in the vicinity of the Schiff base and demonstrate that there is no direct connection between absorption properties and reaction dynamics for the retinal protein family.S ensory rhodopsin (SR) I and II are members of the family of retinal-containing membrane proteins of archaea, which also includes the well-known ion pumps bacteriorhodopsin (BR) and halorhodopsin (HR). SRI and SRII act as receptors in phototaxis. In this process light absorption triggers movement of the flagellated cells toward favorable or away from unfavorable environments, depending on the wavelength of the absorbed light. Thus, the SRs provide a simple color-sensing mechanism (1) for haloarchaea like Halobacterium salinarum and other species. Attractant and repellent reactions of the cell are triggered by light absorption of specific photochromic states of these photoreceptors. In SRI, the dark-adapted state SRI 587 is converted into a blue-shifted intermediate S 373 by absorbing orange light around 590 nm (2), leading to positive signaling via the closely coupled transducer protein HtrI (3). Absorption of a UV photon in the S 373 state leads to a repellent stimulus, resulting in a photophobic response of the cell. SRII is a blue-light receptor with maximum ground-state absorption around 490 nm. The repellent signaling is initiated by the intermediate SRII 360 (M) in the course of the SRII photocycle (4).SRI and SRII are structurally very similar to BR and HR; sequence homology is especially striking in the chromophore environment (5, 6). Although 13 of 21 residues in a 5-Å shell around the retinal atoms are strictly conserved in BR, HR, and SRI of all species sequenced to date, SRII has three residues less conserved (7). This might be one reason for the blue-shifted absorption maximum of SRII compared with the other archaeal receptor proteins (8). Photoexcitation triggers a series of conformational changes initiated by trans-cis isomerization of the retinal chromophore in all retinal-containing membrane proteins (9). The first events lead to the formation of a red-shifted intermediate observed on the subnanosecond ...