Light-induced fast transient absorbance changes were detected by time-resolved spectroscopy in 38 of 51 haloalkaliphilic isolates from alkaline salt lakes in Kenya and the Wadi Natrun in Egypt. They indicate the presence of two retinal pigments, P and P, which undergo cyclic photoreactions with half-times of 2 ms and 500 ms respectively. P absorbs maximally near 580 nm and P near 500 nm. The pigments differ in their sensitivity to hydroxylamine and detergent bleaching and the photoreactions of P are strongly dependent on chloride concentration. Of the 38 pigment-containing strains, 29 possess both P and P, 9 possess only P.

Inhibition of retinal synthesis with nicotine blocks pigment formation and addition of retinal restores it. Hydroxylamine-bleached pigments can be reconstituted with retinal or retinal analogues. Their similarity to the retinal pigments of strongly suggests that they are also rhodopsin-like retinyledene proteins. P in all properties tested is almost identical to halorhodopsin, the light-driven chloride pump of , and may serve the same function in the haloalkaliphiles. P has photocycle kinetics similar to sensory rhodopsin and a far-blue-shifted long-lived photocycle intermediate, but its ground state absorption maximum is near 500 nm instead of 587 nm. We have not found a bacteriorhodopsin-like pigment in the haloalkaliphiles.


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