P588, A SECOND RETINAL-CONTAINING PIGMENT IN HALOBACTERIUM HALOBIUM

Abstract— Halobacterium mutant strains with defects in the biosynthesis of various pigments have been isolated. One of these strains, mutant ET-15, is incapable of producing the light-driven proton pump bacteriorhodopsin and the carotenoid bacterioruberin. However, ET-15 synthesizes another photochemically active, retinal-containing pigment, P₅₈₈, which mediates light-induced proton uptake enhanced by uncouplers. P₅₈₈ and bacteriorhodopsin are simultaneously present in wild-type cells grown under normal conditions; however, they can be distinguished by the following criteria.

• 1 They can be separated by independent mutational events.
• 2 Proton ionophores such as FCCP diminish bacteriorhodopsin-driven proton translocation but enhance P₅₈₈-mediated proton flows. We define here proton translocations which can be diminished by the addition of uncouplers (e.g. FCCP) as ‘active’, others which can be enhanced by FCCP as ‘passive’.
• 3 The kinetic and spectral properties of the photocycle intermediates of bacteriorhodopsin and P₅₈₈ are different.
• 4 The action spectrum for photocycling of P₅₈₈ is red-shifted with respect to that for bacteriorhodopsin.

A comparison of action spectra for proton translocations with that for photocycling links pigment P₅₈₈ and light-driven, passive proton uptake by ET-15 envelope vesicles. When chemically bleached P₅₈₈ was regenerated with all-trans-retinal, both photochemical activity and light-induced proton inflow were restored to equal extents. This identifies P₅₈₈ with the energizer for the passive proton flows.