Expression of an extracellular protein (SMU.63) is regulated by SprV in Streptococcus mutans

ABSTRACT

In Streptococcus mutans, SprV (SMU.2137) is a pleiotropic regulator that differentially regulates genes related to competence, mutacin production, biofilm formation, stress tolerance response along with some other pathways. In this study, we established a link between SprV and a ∼67KDa protein in culture supernatant of UA159 strain that was later confirmed as SMU.63 by MALDI ToF/MS analysis. We discovered that SprV downregulates the transcription and translation of SMU.63. We found that the seven amino acids from the C-terminal region of SprV were also crucial for the expression of SMU.63. Deletion of SMU.63 leads to increased sucrose-independent biofilm formation and competence. The sprV deletion also increased biofilm formation although this could be partially attributed to the down-regulation of SMU. 63. In sprV-SMU.63 double mutant, a synergistic effect observed in biofilm formation in contrary to competence development. We found that low or excess magnesium ion represses sprV transcription that in turn affected the expression of SMU.63. As expected, a magnesium ion dependent effect of competence and biofilm formation was observed in the UA159 strain. We also replicated the results of SMU.63 expression and competence in S. mutans GS-5 that encodes both SprV and SMU.63 homologs and found that the GS-5 strain behaves similarly to the UA159 strain, indicating that SprV's effect is strain independent.

IMPORTANCE We previously identified a pleiotropic regulator, SprV, in Streptococcus mutans. This regulator appears to be highly conserved among streptococci. Here we showed that SprV regulates the expression of a secreted protein encoded by SMU.63 in S. mutans. SMU.63 has been known to impact biofilm formation and genetic competence, two important characteristics that help in colonization of the organism. SMU.63 is also unique since it is known to form amyloid fiber. We found that SprV regulates the expression of SMU.63 both at the transcription and translation level. We also found that the expression of SprV is regulated by magnesium ion concentration. Interestingly, both low and high magnesium ion concentration affected biofilm formation and genetic competence. Since SMU.63 is also highly conserved among streptococci, we hypothesized that SprV will have similar effect on its expression.