, 1995; Honda et al., 1998) on cell growth and desulfurizing
activity. In a study on desulfurization by R. erythropolis IGTS8 in an acetate-based medium, Honda et al. (1998) observed that sulfate promoted higher cell growth than DBT. To study this phenotype, we performed flux balances for two scenarios (Table 2) with unlimited acetate uptake. In run 1, we fixed the DBT (sulfate) uptake at 20 (0.0) mg g−1 dcw h−1. In run 2, we fixed sulfate (DBT) at 20 (0.0) mg g−1 dcw h−1. Our model gave a higher cell growth rate (1.29 vs. 0.84 h−1) for sulfate (run 2) than DBT (run 1). Then, we fixed the acetate uptake at 20 mg g−1 dcw h−1 RAD001 ic50 and studied two more scenarios (Table 2). In run 3, we allowed unlimited (zero) sulfate (DBT) uptake, and did the reverse in run 4. Again, we obtained a higher growth (1.4 vs. 1.06 h−1) for sulfate (run 3) than DBT (run 4). After studying sulfate and DBT separately, we also studied them together (run 5 in Table 2) for a fixed acetate uptake of 20 mg g−1 dcw h−1. We fixed the sulfate uptake at 2.16 mg g−1 dcw h−1 and allowed unlimited DBT. This sulfate
uptake is 10% of its maximum (21.6 mg g−1 dcw h−1) observed in run 4. The model showed a higher growth rate of 1.12 h−1 compared with 1.06 h−1 obtained previously for run 3 (unlimited DBT, zero sulfate). The DBT uptake was also lower (22.08 vs. 25.76 mg g−1 dcw h−1). This suggests that the CHIR-99021 cost organism may grow faster when it fulfills a part of its sulfur needs via sulfate rather than DBT. In other words, the organism may prefer sulfate when both DBT and sulfate are present. Because sulfate yields a higher growth rate than DBT, the organism may use DBT only if sulfate is not present. This clearly confirms the results of Honda et al. (1998). Honda et al. (1998) reasoned that the observed lower cell growth with DBT was due to the toxic effect of HBP (its desulfurized product). Because our model does not include such toxic effects, we cannot deny this
as a probable explanation. However, we have the following alternate explanation from our study. Rhodococcus erythropolis needs sulfate and sulfide to synthesize its sulfur-containing biomass precursors. If Amino acid it uses DBT as the sulfur source, then it must use the 4S pathway. 4S converts DBT to sulfite, which is converted to sulfate and sulfide by the sulfur metabolism and then incorporated into the biomass precursors. However, the organism needs 4 mol NADH mol−1 DBT to use DBT in the above manner. In contrast, the organism does not need this extra NADH for metabolizing sulfate. Thus, the organism prefers the energetically less expensive sulfate over DBT for its growth. Although our reduced model does not include all the reactions involving NADH, it is known that NADH is an essential component for growth. When the organism is forced to use DBT, NADH available for other growth-critical activities inside the cell reduces, and thus cell growth reduces.