DISSIMILATORY IRON(III) REDUCTION BY RHODOBACTER-CAPSULATUS

The photosynthetic proteobacterium Rhodobacter capsulatus was shown to be capable of dissimilatory Fe(III) reduction. Activity was expressed during anaerobic phototrophic and microaerobic growth with malate as the carbon source, but not during equivalent aerobic growth. A variety of Fe(III) complexes were demonstrated to act as substrates for intac t cells and membrane fractions of strain N22DNAR(+) using a ferrozine assay for Fe(II) formation. Rates of reduction appeared to be influenc ed by the reduction potentials of the Fe(III) complexes. However, Fe(I II) complexed by citrate, which is readily reduced by Shewanella putre faciens, was a poor substrate for dissimilation by R. capsulatus. The Fe(III)-reducing activity of R. capsulatus was located solely in the m embrane fraction. The reduction of Fe(III) complexes by intact cells w as inhibited by 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO), suggesting the involvement of ubiquinol: cytochrome c oxidoreductases in the ele ctron transport chain. Lack of sensitivity to myxothiazol plus data fr om mutant strains implies that the cytochrome be, complex and cytochro me c, are not obligatory for dissimilation of Fe(III)(maltol)(3). Alte rnative pathways of electron transfer to Fe(III) must hence operate in R. capsulatus. Using strain N22DNAR(+), the reduction rate of Fe(III) complexed by nitrilotriacetic acid (NTA) was elevated compared to tha t of Fe(III)(maltol)(3), and moreover was sensitive to myxothiazol. Ho wever, these differences were not observed in the absence of the elect ron donor malate. The governing factor for the reduction rate of Fe(II I)(maltol)(3) thus appears to be the limited Fe(III)-reducing activity , whilst the reduction rate of Fe(III) complexed by NTA is controlled by the flux of electrons through the respiratory chain. The use of mut ant strains confirmed that the role of the cytochrome be, complex in F e(III) reduction becomes apparent only with the superior substrate. Th e energy-conserving nature of Fe(III) reduction by R. capsulatus was d emonstrated by electrochromic measurements, with the endogenous carote noid pigments being employed as indicators of membrane potential gener ation in intact cells. Using Fe(III)EDTA as electron acceptor, periods of membrane potential generation were directly proportional to the qu antity of complex added, and were extended in the presence of HQNO. Fe (III)-dependent carotenoid bandshifts were abolished by addition of th e protonophoric uncoupler carbonyl cyanide p-trifluoromethoxyphenylhyd razone.