Characterization of a VicK-regulated operon in Streptococcus mutans

Background: Signal transduction systems serve an important role in sensing and responding to transient environments, and are important for bacterial survival and virulence. The VicK/R system in Streptococcus mutans, a primary causative agent of dental caries, comprises a membrane-located sensor kinase and its cognate responder protein, respectively. In S. mutans VicKR is involved in stress tolerance, biofilm formation and genetic transformation; however, the signals recognized by VicK and its downstream regulatory pathways remain unknown. Objective: To identify the VicK regulon and investigate the involvement of a novel operon (HY11) in the stress tolerance of S. mutans.  Methods: DNA microarray analysis of S. mutans UA159 wild-type and its VicK-deficient mutant (SmuVicK) was performed to identify VicK-regulated genes.  To characterize HY11, a 10-gene deletion mutant spanning Smu1753 to Smu1764 (designated SmuHY11) was constructed using PCR ligation mutagenesis.  Growth of SmuHY11 versus the wild-type strain under various stressors (e.g. NaCl, pH, SDS and H₂O₂) was evaluated.  Results: Microarray analysis revealed 56 genes that were differentially expressed in SmuvicK, with 22 up-regulated and 34 down-regulated genes. A high percentage of affected genes had roles in cell wall biosynthesis, nutrient uptake and stress tolerance. Also notably, genes in HY11, an operon with no significant homology as revealed by BLAST search, were down-regulated in SmuvicK, suggesting a VicK-mediated positive regulatory role in their transcription. Preliminary growth kinetics using SmuHY11 did not reveal drastically altered growth rates under various stressors compared with wild type. Conclusions: This report gives insight into the VicK regulon of S. mutans. The HY11 operon was not however responsible for any of the previously observed stress-related phenotypes associated with the SmuVicK knockout. The function of the HY11 operon and the characterization of VicK regulon genes potentially involved in stress response continues.  Acknowledgements: NIH Grant R01DE0132320 and CIHR Grant MT-15431