Purification and characterization of a novel 3-chlorobenzoate-reductive dehalogenase from the cytoplasmic membrane of Desulfomonile tiedjei DCB-1

Although reductive dehalogenation by anaerobic microorganisms offers great potential for the degradation of halocarbons, little is known about the biochemical mechanisms involved. It has previously been demonstrated that the dehalogenase activity involved in 3-chlorobenzoate dehalogenation by Desulfomonile tiedjei DCB-1 is present in the membrane fraction of the cell extracts. We report herein the purification of a 3-chlorobenzoate-reductive dehalogenase from the cytoplasmic membrane of D. tiedjei DCB-1. The dehalogenase activity was monitored by the conversion of 3-chlorobenzoate to benzoate with reduced methyl viologen as a reducing agent. The membrane fraction of the cell extracts was obtained by ultracentrifugation, and the membrane proteins were solubilized with either the detergent CHAPS (3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate) or Triton X-100 in the presence of glycerol. The solubilized dehalogenase was purified by ammonium sulfate fractionation and a combination of anion exchange, hydroxyapatite, and hydrophobic interaction chromatographies. This procedure yielded about 7% of the total dehalogenase activity with a 120-fold increase in specific activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the purified dehalogenase consisted of two subunits with molecular weights of 64,000 and 37,000. The enzyme converted 3-chlorobenzoate to benzoate at its highest specific activity in 10 mM potassium phosphate buffer (pH 7.2) at 38 degrees C. The enzyme was yellow and probably a heme protein. The enzyme had an adsorbance peak at 408 nm. The dithionite-reduced enzyme displayed absorbance peaks at 416, 522, and 550 nm. The dithionite-reduced enzyme was able to complex with carbon monoxide. The nature of the heme chromophore is currently unknown.

Potassium extrusion by the moderately halophilic and alkaliphilic methanogen methanolobus taylorii GS-16 and homeostasis of cytosolic pH

Methanolobus taylorii GS-16, a moderately halophilic and alkaliphilic methanogen, grows over a wide pH range, from 6.8 to 9.0. Cells suspended in medium with a pH above 8.2 reversed their transmembrane pH gradient (delta pH), making their cytosol more acidic than the medium. The decreased energy in the proton motive force due to the reversed delta pH was partly compensated by an increased electric membrane potential (delta psi). The cytosolic acidification by M. taylorii at alkaline pH values was accompanied by K+ extrusion. The cytosolic K+ concentration was 110 mM in cells suspended at pH 8.7, but it was 320 mM in cells suspended at neutral pH values. High external K+ concentrations (210 mM or higher) inhibited the growth of M. taylorii at alkaline pH values, perhaps by preventing K+ extrusion. Cells suspended at pH 8.5 and 300 mM external K+ failed to acidify their cytosol. The key observation indicative of the involvement of K+ transport in cytosolic acidification was that valinomycin (0.8 microM), a K+ uniporter, inhibited the growth of M. taylorii only at alkaline pH values. Experiments with resting cells indicated that at alkaline pH values valinomycin uncoupled catabolic reactions from ATP synthesis. Thus, K+/H+ antiport activity was proposed to account for the K+ extrusion and the uncoupling effect of valinomycin at alkaline pH values. Such antiport activity was demonstrated by the sharp drop in pH of the bulk medium of the cell suspension upon the addition of 0.1 M KCl. The antiporter appeared to be active only at alkaline pH values, which was in accordance with a possible role in pH homeostasis by M. taylorii growing at alkaline pH values.

Transfer of Methanolobus siciliae to the genus Methanosarcina, naming it Methanosarcina siciliae, and emendation of the genus Methanosarcina

A sequence analysis of the 16S rRNA of Methanolobus siciliae T4/M(T) (T = type strain) showed that this strain is closely related to members of the genus Methanosarcina, especially Methanosarcina acetivorans C2A(T). Methanolobus siciliae T4/M(T) and HI350 were morphologically more similar to members of the genus Methanosarcina than to members of the genus Methanolobus in that they both formed massive cell aggregates with pseudosarcinae. Thus, we propose that Methanolobus siciliae should be transferred to the genus Methanosarcina as Methanosarcina siciliae.