The Streptococcus mutans aminotransferase encoded by ilvE is regulated by CodY and CcpA.

The aminotransferase IlvE was implicated in the acid tolerance response of Streptococcus mutans when a mutation in its gene resulted in an acid-sensitive phenotype (B. Santiago, M. MacGilvray, R. C. Faustoferri, and R. G. Quivey, Jr., J. Bacteriol. 194:2010-2019, 2012). The phenotype suggested that amino acid metabolism is important for acid adaptation, as turnover of branched-chain amino acids (bcAAs) could provide important signals to modulate expression of genes involved in the adaptive process. Previous studies have demonstrated that ilvE is regulated in response to the external pH, though the mechanism is not yet established. CodY and CcpA have been shown to regulate expression of branched-chain amino acid biosynthetic genes, suggesting that the ability to sense carbon flow and the nutritional state of the cell also plays a role in the regulation of ilvE. Electrophoretic mobility shift assays using the ilvE promoter and a purified recombinant CodY protein provided evidence of the physical interaction between CodY and ilvE. In order to elucidate the signals that contribute to ilvE regulation, cat reporter fusions were utilized. Transcriptional assays demonstrated that bcAAs are signaling molecules involved in the repression of ilvE through regulation of CodY. In a codY deletion background, ilvE transcription was elevated, indicating that CodY acts a repressor of ilvE transcription. Conversely, in a ccpA deletion background, ilvE transcription was reduced, showing that CcpA activated ilvE transcription. The effects of both regulators were directly relevant for transcription of ilvE under conditions of acid stress, demonstrating that both regulators play a role in acid adaptation.

The branched-chain amino acid aminotransferase encoded by ilvE is involved in acid tolerance in Streptococcus mutans.

The ability of Streptococcus mutans to produce and tolerate organic acids from carbohydrate metabolism represents a major virulence factor responsible for the formation of carious lesions. Pyruvate is a key metabolic intermediate that, when rerouted to other metabolic pathways such as amino acid biosynthesis, results in the alleviation of acid stress by reducing acid end products and aiding in maintenance of intracellular pH. Amino acid biosynthetic genes such as ilvC and ilvE were identified as being upregulated in a proteome analysis of Streptococcus mutans under acid stress conditions (A. C. Len, D. W. Harty, and N. A. Jacques, Microbiology 150:1353-1366, 2004). In Lactococcus lactis and Staphylococcus carnosus, the ilvE gene product is involved with biosynthesis and degradation of branched-chain amino acids, as well as in the production of branched-chain fatty acids (B. Ganesan and B. C. Weimer, Appl. Environ. Microbiol. 70:638-641, 2004; S. M. Madsen et al., Appl. Environ. Microbiol. 68:4007-4014, 2002; and M. Yvon, S. Thirouin, L. Rijnen, D. Fromentier, and J. C. Gripon, Appl. Environ. Microbiol. 63:414-419, 1997). Here we constructed and characterized an ilvE deletion mutant of S. mutans UA159. Growth experiments revealed that the ilvE mutant strain has a lag in growth when nutritionally limited for branched-chain amino acids. We further demonstrated that the loss of ilvE causes a decrease in acid tolerance. The ilvE strain exhibits a defect in F(1)-F(o) ATPase activity and has reduced catabolic activity for isoleucine and valine. Results from transcriptional studies showed that the ilvE promoter is upregulated during growth at low pH. Collectively, the results of this investigation show that amino acid metabolism is a component of the acid-adaptive repertoire of S. mutans.