c-di-AMP-Regulated K+ Importer KtrAB Affects Biofilm Formation, Stress Response, and SpeB Expression in Streptococcus pyogenes

The second messenger cyclic di-AMP (c-di-AMP) controls biofilm formation, stress response, and virulence in Streptococcus pyogenes. The deletion of the cdi-AMP synthase gene, dacA, results in pleiotropic effects including reduced expression of the secreted protease SpeB. Here, we report a role for K+ transport in c-diAMP-mediated SpeB expression. The deletion of ktrB in the Delta dacA mutant restores SpeB expression. KtrB is a subunit of the K+ transport system KtrAB that forms a putative high-affinity K+ importer. KtrB forms a membrane K+ channel, and KtrA acts as a cytosolic gating protein that controls the transport capacity of the system by binding ligands including c-di-AMP. SpeB induction in the Delta dacA mutant by K+ specific ionophore treatment also supports the importance of cellular K+ balance in SpeB production. The Delta dacA Delta ktrB double deletion mutant not only produces wildtype levels of SpeB but also partially or fully reverts the defective Delta dacA phenotypes of biofilm formation and stress responses, suggesting that many Delta dacA phenotypes are due to cellular K+ imbalance. However, the null pathogenicity of the Delta dacA mutant in a murine subcutaneous infection model is not restored by ktrB deletion, suggesting that c-di-AMP controls not only cellular K+ balance but also other metabolic and/or virulence pathways. The deletion of other putative K+ importer genes, kup and kimA, does not phenocopy the deletion of ktrB regarding SpeB induction in the Delta dacA mutant, suggesting that KtrAB is the primary K+ importer that is responsible for controlling cellular K+ levels under laboratory growth conditions.

Automated summary

The study reveals that in Streptococcus pyogenes, the c-di-AMP pathway regulates the expression of the specific protein SpeB through K+ transport. Cellular K+ balance plays a crucial role in SpeB production, with KtrAB identified as the primary K+ importer responsible for controlling cellular K+ levels.