Light-regulated retinal-dependent reversible phosphorylation of Halobacterium proteins.

By (32P)orthophosphate pulse-labeling in vivo, we have identified a class of phosphoproteins in Halobacterium halobium. Exposing labeled whole cells to light results in a rapid dephosphorylation of two of the proteins, which are rapidly rephosphorylated upon darkening of the cells. The light sensitivity of these two proteins is abolished by the retinal synthesis inhibitor nicotine and is restored upon addition of exogenous retinal to the cells. This indicates that the dephosphorylation depends on the rhodopsin-like (retinal-containing) photoreceptors of this organism. Retinal pigments, one of which is the retinyldene protein bacteriorhodopsin, are responsible for both photoenergy and photosensory transduction by Halobacterium. The light-regulated phosphoproteins are of molecular weights of approximately 80,000 and 100,000. Acid and hydroxylamine resistance of the phosphate bonds in both proteins suggests phosphoserine or phosphothreonine linkages.