Temperature dependence of the photoresponse ofHalobacterium halobium

The temperature dependence of the phototactic behavior of bacteriorhodopsin-pigmentedHalobacterium halobium was measured separately toward attractive yellow-green and repellent violet light. A switchoff phenomenon was found for the former below ca. 27°C, while the response to the latter was constant down to about 7°C. This agreement between the transition temperature for the attractive photosystem with that observed in vitro for several properties of purple membrane, of which bacteriorhodopsin is the sole protein pigment, supports the idea that the attractive photosystem is associated with bacteriorhodopsin. Conversely, the absence of such a transition for the photosystem controlling photorepulsion confirms the separate identity of the two photosystems.