title: A Synonymous Mutation Upstream of the Gene Encoding a Weak-Link Enzyme Causes an Ultrasensitive Response in Growth Rate.
abstract: When microbes are faced with an environmental challenge or opportunity, preexisting enzymes with promiscuous secondary activities can be recruited to provide newly important functions. Mutations that increase the efficiency of a new activity often compromise the original activity, resulting in an inefficient bifunctional enzyme. We have investigated the mechanisms by which growth of Escherichia coli can be improved when fitness is limited by such an enzyme, E383A ProA (ProA*). ProA* can serve the functions of both ProA (required for synthesis of proline) and ArgC (required for synthesis of arginine), albeit poorly. We identified four genetic changes that improve the growth rate by up to 6.2-fold. Two point mutations in the promoter of the proBA* operon increase expression of the entire operon. Massive amplification of a genomic segment around the proBA* operon also increases expression of the entire operon. Finally, a synonymous point mutation in the coding region of proB creates a new promoter for proA* This synonymous mutation increases the level of ProA* by 2-fold but increases the growth rate by 5-fold, an ultrasensitive response likely arising from competition between two substrates for the active site of the inefficient bifunctional ProA*.
  
  
  
  The high-impact synonymous mutation we discovered in proB is remarkable for two reasons. First, most polar effects documented in the literature are detrimental. This finding demonstrates that polar effect mutations can have strongly beneficial effects, especially when an organism is facing a difficult environmental challenge for which it is poorly adapted. Furthermore, the consequence of the synonymous mutation in proB is a 2-fold increase in the level of ProA* but a disproportionately large 5.1-fold increase in growth rate. While ultrasensitive responses are often found in signaling networks and genetic circuits, an ultrasensitive response to an adaptive mutation has not been previously reported.
authors: Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, Boulder, Colorado, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Boulder, Colorado, USA.
authors: Jamie P
authors: JP
authors: Kershner
authors: Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, Boulder, Colorado, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Boulder, Colorado, USA.
authors: Sean
authors: S
authors: Yu McLoughlin
authors: Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, Boulder, Colorado, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Boulder, Colorado, USA.
authors: Juhan
authors: J
authors: Kim
authors: Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, Boulder, Colorado, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Boulder, Colorado, USA.
authors: Andrew
authors: A
authors: Morgenthaler
authors: Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, Boulder, Colorado, USA.
authors: Christopher C
authors: CC
authors: Ebmeier
authors: Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, Boulder, Colorado, USA.
authors: William M
authors: WM
authors: Old
authors: Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, Boulder, Colorado, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Boulder, Colorado, USA shelley.copley@colorado.edu.
authors: Shelley D
authors: SD
authors: Copley
mesh_terms: D001483:Base Sequence; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000235:genetics* / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D050698:Glutamate-5-Semialdehyde Dehydrogenase / Q000235:genetics* / Q000378:metabolism; D007700:Kinetics; D008969:Molecular Sequence Data; D020125:Mutation, Missense; D009876:Operon; D017851:Phosphotransferases (Carboxyl Group Acceptor) / Q000235:genetics* / Q000378:metabolism; D017354:Point Mutation; D011401:Promoter Regions, Genetic
chemical_list: D029968:Escherichia coli Proteins; D050698:Glutamate-5-Semialdehyde Dehydrogenase; C000620909:proA protein, E coli; D017851:Phosphotransferases (Carboxyl Group Acceptor); C020650:glutamate 5-kinase