title: Modular organization of the Phd repressor/antitoxin protein.
abstract: The P1 plasmid addiction operon is a compact genetic structure consisting of promoter, operator, antitoxin gene (phd), and toxin gene (doc). The 73-amino-acid antitoxin protein, Phd, has two distinct functions: it represses transcription (by binding to its operator) and it prevents host death (by binding and neutralizing the toxin). Here, we show that the N terminus of Phd is required for repressor but not antitoxin activity. Conversely, the C terminus is required for antitoxin but not repressor activity. Only a quarter of the protein, the resolution limit of this analysis, was required for both activities. We suggest that the plasmid addiction operon is a composite of two evolutionarily separable modules, an operator-repressor module and an antitoxin-toxin module. Consideration of similar antitoxin proteins and their surroundings indicates that modular exchange may contribute to antitoxin and operon diversity.
authors.affiliation: Department of Biological Sciences, University of Alabama in Huntsville, Huntsville, AL 35899, USA.
authors.forename: Jeremy Allen
authors.initials: JA
authors.lastname: Smith
authors.forename: Roy David
authors.initials: RD
authors.lastname: Magnuson
mesh_terms: D000595:Amino Acid Sequence; D000992:Antitoxins / Q000737:chemistry / Q000235:genetics*; D005075:Biological Evolution; D008969:Molecular Sequence Data; D009876:Operon; D017433:Protein Structure, Secondary; D012097:Repressor Proteins / Q000737:chemistry / Q000235:genetics*
chemical_list: D000992:Antitoxins; D012097:Repressor Proteins