Modular Organization of the Phd Repressor/Antitoxin Protein
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FIG. 1.(A) Schematic structure of the P1 addiction operon. (B) Deletion analysis of Phd: predicted protein products and summary of their activities. The predicted protein products of phd and various deletion mutants of phd are shown. The repressor (REP) and antitoxin (ANT) activities of each construct are indicated. Repressor activity was indicated by the ability to repress transcription of a lacZ reporter fused to the P1 addiction promoter (Table 3). Antitoxin activity was indicated by the ability of the cells to grow in the presence of an otherwise lethal level of the Doc toxin (Table 4). (C) Secondary structure predictions for Phd and a Klebsiella homolog. The aligned sequences of Phd and a Phd homolog from K. pneumoniae (Kp) are given along with their predicted secondary structures, as determined by the Chou-Fasman algorithm (CF) (8) and by the Garnier-Osguthorpe-Robson algorithm (GOR) (16).
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FIG. 2.(A) Evidence of recombinant antitoxins. Inspection of selected antitoxin alignments provided evidence of recombination events within the antitoxin sequences. The N termini of Phd and Axe are marginally similar, but their C termini, and their corresponding toxins, are dissimilar. The apparent recombination junction revealed by the alignment of Axe and Phd falls within the repressor-antitoxin transition region (indicated by bold type) as defined by deletion mapping of Phd (Table 1). Conversely, the N termini of RelE and StbD are dissimilar, but their C termini, and their corresponding toxins, are very similar, as previously noted by Hayes (25). The complementary RelE/StbD recombination junction noted by Hayes occurs in a comparable position. Protein sequences were globally aligned by using Clustal X (28) with default parameters. Alignments of these proteins have been previously reported (20, 25). (B) Modular model for antitoxin-toxin systems. We propose that toxin-antitoxin systems are a composite of two evolutionarily separable modules: an operator-repressor module and an antitoxin-toxin module. Recombination between modules may contribute to operon and antitoxin diversity. A schematic of two analogous parental genetic structures (the first in plain text, the second in bold text) and a recombinant structure are depicted. Such a recombinant is likely to be functional even if the parental structures are distantly related or heterologous.
- American Society for Microbiology
