Calcium and Stress Regulate cia Operon Expression in Streptococcus mutans

Background: Streptococcus mutans is a primary pathogen causing dental caries in humans. The cia operon of S. mutans encodes a two-component signal transduction system (TCS), CiaRH. Previous studies have shown that the cia operon plays an important role in the cell's stress responses including mutacin production, acid tolerance, competence development, and biofilm formation. Objective: the objective of this study was to determine the environmental signals that regulate the cia operon expression, as well as the genes that are regulated by the CiaRH TCS. Methods: S. mutans strain UA140, site-directed mutagenesis, Microarray, and real-time RT-PCR were used for this study. Results: In chemically defined medium (CDM) the cia operon was regulated by calcium, and the operon expression was turned on before the cells entered into log phase. The first gene of the operon encodes a small peptide, CiaX, which appeared to act as a calcium sensing peptide for the regulation of operon expression. In complex medium such as BHI, the operon expression followed the growth curve. The operon is self-regulatory under both conditions. Transcriptome analysis of cells gown on BHI plates revealed 14 genes down-regulated ³2-fold and12 genes up-regulated ³2 fold in the ciaH mutant. Conclusion: 1) In chemically defined medium, the cia operon appears to play an important role in the cell's exit from stationary phase or enter into log phase; in complex medium, the cia operon is more involved in the cell's stress response. 2) Except for genes in the cia operon, most down- or up-regulated genes in the ciaH mutant encode hypothetical proteins or transposases, supporting the notion that the CiaRH TCS is a stress response regulator in complex medium. Support: R01-DE014757, COBRE P20.