title: Phosphate sink containing two-component signaling systems as tunable threshold devices.
abstract: Synthetic biology aims to design de novo biological systems and reengineer existing ones. These efforts have mostly focused on transcriptional circuits, with reengineering of signaling circuits hampered by limited understanding of their systems dynamics and experimental challenges. Bacterial two-component signaling systems offer a rich diversity of sensory systems that are built around a core phosphotransfer reaction between histidine kinases and their output response regulator proteins, and thus are a good target for reengineering through synthetic biology. Here, we explore the signal-response relationship arising from a specific motif found in two-component signaling. In this motif, a single histidine kinase (HK) phosphotransfers reversibly to two separate output response regulator (RR) proteins. We show that, under the experimentally observed parameters from bacteria and yeast, this motif not only allows rapid signal termination, whereby one of the RRs acts as a phosphate sink towards the other RR (i.e. the output RR), but also implements a sigmoidal signal-response relationship. We identify two mathematical conditions on system parameters that are necessary for sigmoidal signal-response relationships and define key parameters that control threshold levels and sensitivity of the signal-response curve. We confirm these findings experimentally, by in vitro reconstitution of the one HK-two RR motif found in the Sinorhizobium meliloti chemotaxis pathway and measuring the resulting signal-response curve. We find that the level of sigmoidality in this system can be experimentally controlled by the presence of the sink RR, and also through an auxiliary protein that is shown to bind to the HK (yielding Hill coefficients of above 7). These findings show that the one HK-two RR motif allows bacteria and yeast to implement tunable switch-like signal processing and provides an ideal basis for developing threshold devices for synthetic biology applications.
authors.affiliation: Biosciences, College of Life and Environmental Sciences, University of Exeter, Exeter, United Kingdom; Systems Biology Program, College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, United Kingdom.
authors.forename: Munia
authors.initials: M
authors.lastname: Amin
authors.affiliation: Systems Biology Program, College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, United Kingdom.
authors.forename: Varun B
authors.initials: VB
authors.lastname: Kothamachu
authors.affiliation: Department of Mathematical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Elisenda
authors.initials: E
authors.lastname: Feliu
authors.affiliation: Department of Biological Sciences, Virginia Tech, Blacksburg, Virginia, United States of America.
authors.forename: Birgit E
authors.initials: BE
authors.lastname: Scharf
authors.affiliation: Biosciences, College of Life and Environmental Sciences, University of Exeter, Exeter, United Kingdom.
authors.forename: Steven L
authors.initials: SL
authors.lastname: Porter
authors.affiliation: School of Life Sciences, University of Warwick, Coventry, United Kingdom.
authors.forename: Orkun S
authors.initials: OS
authors.lastname: Soyer
mesh_terms: D002633:Chemotaxis / Q000502:physiology; D008954:Models, Biological; D010710:Phosphates / Q000378:metabolism*; D015203:Reproducibility of Results; D015398:Signal Transduction / Q000502:physiology*; D016962:Sinorhizobium meliloti / Q000378:metabolism; D058615:Synthetic Biology / Q000379:methods*
chemical_list: D010710:Phosphates