D-101. Identification of a Gene Cluster Regulating Acid Tolerance and a (p)ppGpp Synthase in Streptococcus mutans

K. C. Seaton, S-J. Ahn, R. A. Burne;
Univ. of Florida, Gainesville, FL.

Streptococcus mutans is the major etiological agent of dental caries in humans and is well adapted to the continually varying conditions in the oral cavity. A conserved strategy of of bacteria to cope with fluctuations in nutrient availability and other stresses is the accumulation of (p)ppGpp in response to nutrient limitation. S. mutans expresses a bi-functional RelA enzyme that governs (p)ppGpp levels, but also produces two additional (p)ppGpp synthases, RelP and RelQ. Importantly, relP is co-transcribed with, and under partial control of, a two component system encoded by relRS. A recent microarray analysis we conducted provided evidence that there could be regulatory or functional overlap of RelP with a gene cluster (Smu835-839 ) linked to relPRS that encodes a potential regulatory protein, a transporter, a thiolperoxidase and a protein of unknown function. To test this hypothesis, a series of polar and non polar deletion:insertion mutants were constructed in the genes. The growth rate of strains carrying polar insertions in Smu835 or Smu836 was significantly slower at both pH 7 and pH 5.5 than the parental strains or strains with non-polar insertions in the same genes. Quantitative Real-Time PCR revealed that there was significant down regulation of relP and relRS expression in the strains lacking Smu835 and Smu836, but no significant differences in expression of relA or relQ. Notably, Smu835 and Smu836 were also down-regulated in relP and relRS mutant strains, but not in relA deletion strain. The data reveal a regulatory connection between the RelPRS system and the genetically linked-transporter that is critical for maintaining homeostasis and low pH tolerance, possibly through modulation of the cellular levels of (p)ppGpp by RelP to properly balance growth and survival modes.