Modular organization of the Phd repressor/antitoxin protein.
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.
Smith
Jeremy Allen
JA
Department of Biological Sciences, University of Alabama in Huntsville, Huntsville, AL 35899, USA.
Magnuson
Roy David
RD
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*
D000992:Antitoxins; D012097:Repressor Proteins