Essentiality of c-di-AMP in Bacillus subtilis: Bypassing mutations converge in potassium and glutamate homeostasis

PLoS Genet. 2021 Jan 22;17(1):e1009092. doi: 10.1371/journal.pgen.1009092. eCollection 2021 Jan.


In order to adjust to changing environmental conditions, bacteria use nucleotide second messengers to transduce external signals and translate them into a specific cellular response. Cyclic di-adenosine monophosphate (c-di-AMP) is the only known essential nucleotide second messenger. In addition to the well-established role of this second messenger in the control of potassium homeostasis, we observed that glutamate is as toxic as potassium for a c-di-AMP-free strain of the Gram-positive model bacterium Bacillus subtilis. In this work, we isolated suppressor mutants that allow growth of a c-di-AMP-free strain under these toxic conditions. Characterization of glutamate resistant suppressors revealed that they contain pairs of mutations, in most cases affecting glutamate and potassium homeostasis. Among these mutations, several independent mutations affected a novel glutamate transporter, AimA (Amino acid importer A, formerly YbeC). This protein is the major transporter for glutamate and serine in B. subtilis. Unexpectedly, some of the isolated suppressor mutants could suppress glutamate toxicity by a combination of mutations that affect phospholipid biosynthesis and a specific gain-of-function mutation of a mechanosensitive channel of small conductance (YfkC) resulting in the acquisition of a device for glutamate export. Cultivation of the c-di-AMP-free strain on complex medium was an even greater challenge because the amounts of potassium, glutamate, and other osmolytes are substantially higher than in minimal medium. Suppressor mutants viable on complex medium could only be isolated under anaerobic conditions if one of the two c-di-AMP receptor proteins, DarA or DarB, was absent. Also on complex medium, potassium and osmolyte toxicity are the major bottlenecks for the growth of B. subtilis in the absence of c-di-AMP. Our results indicate that the essentiality of c-di-AMP in B. subtilis is caused by the global impact of the second messenger nucleotide on different aspects of cellular physiology.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacillus subtilis / genetics
  • Bacillus subtilis / growth & development
  • Bacillus subtilis / metabolism*
  • Bacterial Proteins / genetics
  • Cyclic AMP / genetics
  • Cyclic AMP / metabolism
  • Dinucleoside Phosphates / genetics
  • Dinucleoside Phosphates / metabolism*
  • Gene Expression Regulation, Bacterial / genetics
  • Glutamic Acid / genetics
  • Glutamic Acid / metabolism*
  • Homeostasis / genetics
  • Ion Transport / genetics
  • Mutation / genetics
  • Potassium / metabolism*
  • Second Messenger Systems / genetics


  • Bacterial Proteins
  • Dinucleoside Phosphates
  • cyclic diadenosine phosphate
  • Glutamic Acid
  • Cyclic AMP
  • Potassium

Grants and funding

This work was supported by a grant from the Deutsche Forschungsgemeinschaft (STU214/16-2) via SPP 1879. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.