4 g dry biomass mol−1 fructose (20.4 g dry biomass mol−1 substrate carbon). Genes encoding the enzymes required for both pathways of glycolysis are present in the genome of S. stellata. The Ymax observed during fructose-limited growth (64.2 g dry biomass mol−1 fructose) is closer to that predicted for dissimilation via the Enter–Doudoroff pathway, suggesting that it is probably in use in this case; however, it must be noted that many
bacteria use multiple pathways of glycolysis simultaneously during growth on hexoses (Wood & Kelly, 1977). The Ymax in the presence of DMS (73.2 g dry biomass mol−1 fructose, a 14% increase in Ymax from growth on fructose alone) is closer to the theoretical Ymax, indicative of a tighter coupling Roscovitine order of fructose oxidation to growth in the presence of DMS, with less dissimilation to carbon dioxide to meet the energy requirements of growth and maintenance. The oxidation of DMS to DMSO is catalyzed by DMS dehydrogenase in R. sulfidophilum (McDevitt et al., 2002): The subunits of DMS dehydrogenase have been shown to be encoded by the operon ddhABDC (McDevitt et al., 2002). Searching the S. stellata genome using the blastp algorithm reveals predicted proteins with >55% identity to DdhABC, clustered together
and annotated as components of a nitrate reductase NarYZV (EBA07058–EBA07060). It is worth noting that González et al. (1997) did not observe nitrate reduction during heterotrophic growth of S. stellata under anoxic conditions. Additionally, genes annotated as a DMSO reductase-like molybdopterin-containing dehydrogenase are also present
in the genome selleck chemicals of S. stellata (EBA06368–EBA06370); a tblastx search against the GenBank™ database confirms the annotation. The oxidation of DMS to DMSO could potentially be catalyzed by this enzyme performing its reverse reaction (Adams et al., 1999). Enzyme assays were many conducted for DMS dehydrogenase and DMSO reductase on cell-free extracts prepared from cells obtained from succinate-limited chemostats (D=0.03 h−1) grown with DMS (Table 2). It can be seen that DMS dehydrogenase (DCPIP or ferricyanide linked) activity was absent, although it could be assayed in the control organism R. sulfidophilum; DMSO reductase activity was also absent, but could be assayed in the positive control (H. sulfonivorans). It is, of course, possible that a DMS dehydrogenase is present in S. stellata grown under these conditions, but is either too unstable in cell-free extracts to assay or does not couple to DCPIP or ferricyanide in vitro. Additional assays were conducted in the presence of 1 mM NAD+ and NADP+, but no activity was observed. The ATP content of whole cells obtained from a succinate-limited chemostat (D=0.03 h−1) grown in the presence of DMS was monitored over time after the addition of DMS to 1 mM and the results are shown in Fig. 1. It can be seen that ATP is produced in the presence of DMS by cells of S.