Control of natural transformation in salivarius streptococci through specific degradation of σX by the MecA-ClpCP protease complex

Abstract

Competence for natural DNA transformation is a tightly controlled developmental process in streptococci. In mutans and salivarius species, the abundance of the central competence regulator σX is regulated at two levels: transcriptional, by the ComRS signaling system via the σX/ComX/SigX-inducing peptide (XIP), and posttranscriptional, by the adaptor protein MecA and its associated Clp ATPase, ClpC. In this study, we further investigated the mechanism and function of the MecA-ClpC control system in the salivarius species Streptococcus thermophilus. Using in vitro approaches, we showed that MecA specifically interacts with both σX and ClpC, suggesting the formation of a ternary σX-MecA-ClpC complex. Moreover, we demonstrated that MecA ultimately targets σX for its degradation by the ClpCP protease in an ATP-dependent manner. We also identify a short sequence (18 amino acids) in the N-terminal domain of σX as essential for the interaction with MecA and subsequent σX degradation. Finally, increased transformability of a MecA-deficient strain in the presence of subinducing XIP concentrations suggests that the MecA-ClpCP proteolytic complex acts as an additional locking device to prevent competence under inappropriate conditions. A model of the interplay between ComRS and MecA-ClpCP in the control of σX activity is proposed. © 2014, American Society for Microbiology.