TRANSFORMATION OF A BOP‐HOP‐SOP‐I‐SOP‐II‐Halobacterium halobiumMUTANT TO BOP+: EFFECTS OF BACTERIORHODOPSIN PHOTOACTIVATION ON CELLULAR PROTON FLUXES AND SWIMMING BEHAVIOR

We have transformed Pho 81, aHalobacterium halobiurnmutant strain which does not contain any of the four retinylidene proteins known in this species, with thebopgene cluster to create Pho81BR, a BR'HR‐SR‐I‐SR‐11– strain. The absorption spectrum, pigment reconstitution process, light‐dark adaptation and photochemical reaction cycle of the expressed protein are indistinguishable from those of native bacteriorhodopsin (BR) in purple membrane of wild type strains. Strain Pho81BR permits for the first time characterization of effects of BR photoactivation alone on cell swimming behavior and energetics in the absence of the spectrally similar phototaxis receptor sensory rhodopsin I (SR‐I) and electrogenic chloride pump halorhodopsin (HR). A non‐adaptive upward shift in spontaneous swimming reversal frequency occurs following 3 s of continuous illumination of Pho81BR cells with green light (550 ± 20 nm). This effect is abolished by low concentrations of the proton ionophore carbonylcyanidem‐chlorophenylhydrazone. Although BR does not mediate phototaxis responses in energized Pho81BR cells under our culture conditions, proton pumping by BR in Pho81BR cells partially deenergized by inhibitors of respiration and adenosine triphosphate synthesis results in a small attractant response. Based on our measurements, we attribute the observed effects of BR photoactivation on swimming behavior to secondary consequences of electrogenic proton pumping on metabolic or signal transduction pathways, rather than to primary sensory signaling such as that mediated by SR‐I. Proton extrusion by BR activates gated proton influx ports resulting in net proton uptake in wild‐type cells. We observe a gated secondary proton influx also in Pho81BR which is deactivated byN, N'‐dicyclohexyl carbodiimide (DCCD) and heating, but is not affected by pre‐illumination. Based on previous studies, these properties exclude the known proton uptake from HR chloride pumping and the gated Na+/H+antiporter, and are consistent with H+‐ATPase activation. We do not observe a DCCD‐insensitive early proton inflow after illumination of Pho81BR cells, confirming that this previously reported proton inflow results from active chloride pumping by HR rather than from a change in the preexisting membrane potential.