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In bacteria, the so-called stringent response is responsible for adaptation to changing environmental conditions. This response is mediated by guanosine derivatives [(p)ppGpp], synthesized by either large mono-functional RelA or bi-functional SpoT (synthesis and hydrolysis) enzymes in β -  and γ - proteobacteria, such as  Escherichia coli . In Firmicutes and α - , δ - , and  - proteobacteria, large bifunctional Rel-SpoT-homologs (RSH), often accompanied by small (p)ppGpp synthetases and/or hydrolases devoid of regulatory domains, are responsible for (p)ppGpp turnover. Here, we report on surprising  in vitro  and  in vivo  properties of an RSH enzyme from  Methylobacterium extorquens  (RSH Mex ). We find that this enzyme possesses some unique features, e.g., it requires cobalt cations for the most efficient (p)ppGpp synthesis, in contrast to all other known specific (p)ppGpp synthetases that require Mg 2+ . In addition, it can synthesize pppApp, which has not been demonstrated  in vitro  for any Rel/SpoT/RSH enzyme so far.  In vivo , our studies also show that RSH Mex  is active in  Escherichia coli  cells, as it can complement  E. coli  ppGpp 0  growth defects and affects  rrnB  P1- lacZ  fusion activity in a way expected for an RSH enzyme. These studies also led us to discover pppApp synthesis in wild type  E. coli  cells (not carrying the RSH Mex  enzyme), which to our knowledge has not been demonstrated ever before. In the light of our recent discovery that pppApp directly regulates  E. coli  RNAP transcription  in vitro  in a manner opposite to (p)ppGpp, this leads to a possibility that pppApp is a new member of the nucleotide second-messenger family that is widely present in bacterial species.

frontiers in microbiology

Methylobacterium extorquens RSH Enzyme Synthesizes (p)ppGpp and pppApp in vitro and in vivo, and Leads to Discovery of pppApp Synthesis in Escherichia coli