490) for pyruvate formate-lyase activating enzyme. The upregulated genes included pgk (SMU.361) for phosphoglycerate click here kinase, adhAB (SMU.127/8) for acetoin dehydrogenase, pdhAB (SMU.1422/3)
for pyruvate dehydrogenase, adhE (SMU.148) for alcohol-acetaldehyde dehydrogenase and frdC (SMU.1410) for fumarate reductase. Malolactic enzyme MleS catalyzes decarboxylation of malic acid, yielding lactate. It was recently shown that malolactic fermentation is a major system for alkali production and that deficiency of MleS as well as MleP in S. mutans resulted in loss of protection against acid killing (Sheng et al., 2010). In addition, the malolactic fermentation system was also found to be protective against oxidative stress and starvation. Glutathione reductase, GshR, is known to play a significant role in defense against oxidative stress in both eukaryotes and Gram-negative bacteria, and similar results were also reported in S. mutans (Yamamoto et al., 1999). Downregulation of mleSP and gshR will certainly have an impact on the ability of the deficient mutants to survive oxidative stress, which could at least in part attribute to the observed defects in tolerance against MV and H2O2, and consequently to the decreased ability to form biofilms by TW239. Pyruvate
formate lyase-activating enzyme (PflC or Act) is shown to be the sole enzyme able to activate pyruvate formate lyase (Yamamoto et al., 2000), which is known to be highly sensitive to oxygen and play a critical role in sugar fermentation, ATP synthesis and NAD+ and/or NADH recycling under anaerobic conditions ABT 199 (Yamada et al., 1985). Acetoin dehydrogenase (AdhAB), pyruvate dehydrogenase (PdhAB), alcohol-acetaldehyde dehydrogenase (AdhE) and fumarate reductase (FrdC) are all key enzymes in heterofermentation, ATP synthesis and
NAD+ and/or NADH regeneration. Unlike S. aureus, but similar to B. subtilis (Larsson et al., 2005; Pagels et al., 2010), the lactate dehydrogenase gene ldh was not among the genes aberrantly expressed in TW239. Coupled with the increased expression Dichloromethane dehalogenase of adhAB, pdhAB, adhE and frdC and the downregulation of pflC in response to Rex-deficiency, the data presented here also support an important role for Rex in the regulation of glycolysis and acid production by S. mutans in the plaque. Recently, it has been shown that exposure of S. mutans to aeration causes a substantial alteration in the expression of genes involved in oxidative stress (e.g. nox for NADH oxidase), energy metabolism and fermentation (e.g. pdhAB and adhE) and biofilm formation (e.g. gftB) (Ahn et al., 2007). Cross-referencing of these two transcriptional profiles (aeration vs. rex mutation) revealed that of the genes identified in TW239, 11 (10 upregulated and one downregulated, respectively) were also found to be consistently altered in S. mutans stressed by aeration (Table 2 and Table S1), indicating that Rex-mediated regulation could be part of the pathway that S.