Cyclic Di‐<scp>AMP</scp> Affects Cell Membrane Integrity of <i>Streptococcus pneumoniae</i>

Competence is an important bioprocess for Streptococcus pneumoniae. Previously, we demonstrated that the bacterial second messenger cyclic di-adenosine monophosphate (c-di-AMP) modulates pneumococcal competence. Surprisingly, cdaA*, a strain producing less c-di-AMP due to a point mutation in the diadenylate cyclase CdaA, is susceptible to competence-stimulating peptide (CSP). In this study, we screened cdaA* suppressor mutants resistant to CSP to explore the underlying mechanism. Of 14 clones sequenced, nine clones possessed mutations in the c-di-AMP phosphodiesterase Pde1, indicating that the susceptibility to CSP of the cdaA* strain is correlated to c-di-AMP levels. Another two clones exhibited a mutation in FabT, a transcription factor controlling cell membrane fatty acid biosynthesis. We further showed that deletion of fabT, disruption of the FabT-binding site within the P_fabK promoter, deletion of a fabT activator BriC, or disruption of K⁺ uptake in the cdaA* mutant all rescued the growth defect of the cdaA* strain in media supplemented with CSP. Finally, we found that a c-di-AMP phosphodiesterase-null mutant with high levels of c-di-AMP is highly sensitive to treatment with either ethanol or Triton X-100, which could be corrected by reducing c-di-AMP levels through introducing point mutations in CdaA. Together, these findings indicate that c-di-AMP affects cell membrane integrity.