Action of phenazine methyl sulfate, inhibitors, and uncouplers on the light-induced proton transport by cells ofRhodospirillum rubrum

Abstract

Suspensions of log phase cells ofRhodospirillum rubrum at pH 5.5 show a light-induced decrease in the pH of the medium which is reversed during the subsequent dark period. The velocity and magnitude of the pH change were the same whether the cells were bubbled with air, CO2-free air or N2 during experimentation. The pH response is temperature dependent. Phenazine methyl sulfate (PMS) at concentrations above 0.05MM stimulates the light-induced pH change. PMS at 1MM gives a 2-fold increase in the initial rate upon illumination and a 1.5-fold increase in the total change in pH after 2 min of illumination. The inhibition of the proton transport by 10 μg/ml antimycin A or 20 μM 2-n-heptyl-4-hydroxyquinoline-N-oxide can be partially relieved by PMS. However, inhibition of the light-induced proton transport with 0.5MM 2,4-dinitrophenol or 3 μM carbonylcyanide-m-chlorophenylhydrazone (CCCP) cannot be overcome by addition of PMS. Valinomycin, at a concentration of 3 μM, caused a slight stimulation of the light-induced proton transport in the presence of 200MM KCl. The inhibition of proton transport by 3 μM CCCP was partially relieved with 3 μM valinomycin in the presence of 200MM KCl, but the antibiotic was without effect when the cells were suspended in 200MM NaCl. The results are discussed in terms of current theories of the action of PMS, antimycin A, valinomycin, and uncouplers on the light-induced electron flow and photophosphorylation inR. rubrum.