EFFECTS OF 2,5-DIBROMO-3-METHYL-6-ISOPROPYL BENZOQUINONE (DBMIB) ON PHOTOCHEMICAL EVENTS IN RHODOSPIRILLUM RUBRUM*

Abstract. The quinone antagonist dibromothymoquinone (2,5-dibromo-3-methyl-6-isopropyl benzoquinone, DBMIB)? was used to inhibit early photochemical changes in chromatophores of the photo-synthetic bacterium, Rhodospirillum rubrum. With continuous illumination with near infrared light, we observed an approximately threefold decrease in efficiency of P865⁺ formation at 100–200 μM DBMIB as measured by absorbance changes at 605 or 430 nm. However, with continuous illumination of several seconds duration, maximal absorbance changes were observed. At low concentrations (1–10 μM) of DBMIB, a decrease in the decay rate of the ΔA₄₃₀ or ΔA₆₀₅ was observed. Using a short (2–10 μs), low intensity light pulse for excitation, we observed that DBMIB inhibits P865⁺ formation. The concentration dependency of this inhibition corresponds to that seen with continuous illumination. We tested the possibility that DBMIB was displacing the first stable electron acceptor, which has been shown to be a tightly bound ubiquinone (UQ) in R. rubrum. Chromatophores prepared from cells grown with ¹⁴C-p-hydroxybenzoic acid in their media provided a sample in which the benzoquinones were labelled specifically with ¹⁴C. These labelled chromatophores were first extracted with petroleum ether to remove the loosely bound ubiquinone pool and then were treated with DBMIB. DBMIB displaced about 1/2 of the remaining tightly-bound UQ present. The DBMIB inhibition was tested for reversibility by adding an excess of UQ to DBMIB-treated samples. In these cases, restoration to 80% could be achieved from samples which were only 20% active relative to the untreated samples. A similar inhibitory effect by DBMIB was also demonstrated in whole cells of Rhodospirillum rubrum, the G-9 mutant of Rhodospirillum rubrum, Rhodopseudomoms sphaeroides, Rhodopseudomonas capsulata, and Chromatium vinosum. The mechanism for inhibition is most consistent with displacement of, or interference with the first stable electron acceptor (ubiquinone) rather than by random quenching of excited states at the level of antenna bacteriochlorophyll.