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Abstract
Halobacterium halobium Flx mutants are deficient in bacteriorhodopsin (bR) and halorhodopsin
(hR). Such strains are phototactic and the light signal detectors are two additional
retinal pigments, sensory rhodopsins I and II (sR-I and sR-II), which absorb maximally
at 587 and 480 nm, respectively. A retinal-deficient Flx mutant, Flx5R, overproduces
sR-I-opsin and does not show any photochemical activity other than that of sR-I after
the pigment is regenerated by addition of all-trans retinal. Using native membrane
vesicles from this strain, we have resolved a new photointermediate in the sR-I photocycle
between the early bathointermediate S610 and the later intermediate S373. The new
form, S560, resembles the L intermediate of bR in its position in the photoreaction
cycle, its relatively low extinction, and its moderate blue shift. It forms with a
half-time of approximately 90 microseconds at 21 degrees C, concomitant with the decay
of S610. Its decay with a half-time of 270 microseconds parallels the appearance of
S373. From a data set consisting of laser flash-induced absorbance changes (300 ns,
580-nm excitation) measured at 24 wavelengths from 340 to 720 nm in a time window
spanning 1 microsecond to 8 s we have calculated the spectra of the photocycle intermediates
assuming a unidirectional, unbranched reaction scheme.