Strains were grown in TYEP medium with 0.8% (w/v) glucose, pH 6.5. Equivalent amounts of Triton

X-100-treated crude extract (50 μg of protein) were applied to each lane. The activity bands corresponding to Hyd-1 and Hyd-2 are indicated, as is the slowly migrating activity band (designated by an arrow) that corresponds to a hydrogenase-independent H2:BV oxidoreductase enzyme activity. Formate dehydrogenases N and O catalyze hydrogen:BV oxidoreduction In order to identify the enzyme(s) responsible for this new hydrogen: BV oxidoreductase activity, the hypF deletion mutant was grown anaerobically and the membrane fraction was prepared (see Methods). The hydrogen: BV oxidoreductase activity could be released from the membrane in soluble form by treatment with the detergent Triton X-100. Enrichment of the activity was achieved by separation from contaminating membrane proteins using Q-sepharose anion exchange, phenyl sepharose hydrophobic C188-9 molecular weight PARP signaling interaction chromatography and finally gel filtration on a Superdex-200 size exclusion column (see Methods for details). Fractions with enzyme activity were monitored during the enrichment procedure using activity-staining after

non-denaturing PAGE. A representative elution profile from the Superdex-200 chromatography step, together with the corresponding activity gel identifying the active enzyme, are shown in Figure 2. Two distinct peaks that absorbed at 280 nm could be separated (Figure 2A) and the hydrogen: BV oxidoreductase activity was found to be exclusively associated with the higher molecular mass symmetric peak labelled P1 (Figure 2B). This peak eluted after 47 ml (Vo = 45 ml) and was

estimated to have a mass of between 500-550 kDa (data not shown). Figure 2 Chromatographic separation of the H 2 : BV oxidoreductase activity on a Superdex-S200 column. A. A representative elution profile of the enriched H2: BV oxidoreductase enzyme activity after size exclusion chromatography on Superdex-S200 is shown. The absorbance at 280 nm was monitored not and the two main elution peaks were labelled P1 and P2. B. Samples of the fractions across the elution peaks P1 and P2 were separated by non-denaturing PAGE and subsequently stained for hydrogenase enzyme activity. Lane 1, crude cell extract (50 μg protein); lane 2, membrane fraction (50 μg protein); lane 3, solubilised membrane fraction (50 μg protein); lane 4, aliquot of the 400 mM fraction from the Q-sepharose column. The arrow identifies the H2: BV oxidoreductase enzyme activity. The band showing hydrogen: BV oxidoreductase activity in Figure 2B was carefully excised and the polypeptides within the fraction were analyzed by mass spectrometry. Both Fdh-O and Fdh-N enzymes were unambiguously identified: the polypeptides FdoG, FdoH, FdoI, FdnG, and FdnH were identified. The catalytic subunits of Fdh-O and Fdh-N share 74% amino acid identity and both enzymes are synthesized at low levels during fermentative growth.