Doc of Prophage P1 Is Inhibited by Its Antitoxin Partner Phd through Fold Complementation

Abstract

Prokaryotic toxin-antitoxin modules are involved in major physiological events set in motion under stress conditions. The toxin Doc (death on curing) from the phd/doc module on phage P1 hosts the C-terminal domain of its antitoxin partner Phd (prevents host death) through fold complementation. This Phd domain is intrinsically disordered in solution and folds into an ␣-helix upon binding to Doc. The details of the interactions reveal the molecular basis for the inhibitory action of the antitoxin. The complex resembles the Fic (filamentation induced by cAMP) proteins and suggests a possible evolutionary origin for the phd/doc operon. Doc induces growth arrest of Escherichia coli cells in a reversible manner, by targeting the protein synthesis machinery. Moreover, Doc activates the endogenous E. coli RelE mRNA interferase but does not require this or any other known chromosomal toxin-antitoxin locus for its action in vivo.Small operons encoding a toxin and its antitoxin are common in the genomes of bacteria and archaea and are also found on certain plasmids and bacteriophages. These so-called toxinantitoxin (TA) 2 modules have been proposed to regulate the pace of metabolism and may induce a state of dormancy in case of nutritional stress (1-3). TA modules are highly abundant in opportunistic pathogens such as Mycobacterium tuberculosis (4), and their presence has been linked to persistence (5).On plasmids, TA modules act as addiction systems, aiding plasmid maintenance in the bacterial population by post-segregational killing (6), filling in a function related to apoptosis and programmed cell death in eukaryotes (7). Related effects have been observed for chromosome-located TA systems as some of them have been shown to diminish large scale genome reductions in the absence of selection (8). In the presence of the plasmid, both toxin and antitoxin are expressed, leading to a steady state equilibrium where the antitoxin counteracts the effect of the toxin. In its free state, the antitoxin, usually a modular protein that contains a functional intrinsically disordered region (9 -11), is under constant proteolytic attack. The toxinantitoxin complex acts as an autorepressor for the TA operon, ensuring that only small amounts of the proteins are present in the cell. Upon plasmid loss, the antitoxin is degraded by a specific intracellular protease, releasing the toxin. Without the possibility of replenishing the antitoxin population, the toxin action becomes irreversible, resulting in cell death.The phd/doc operon encodes a TA module aiding the maintenance of the plasmid-prophage P1 in Escherichia coli (12). Doc is an inhibitor of translation elongation through its association with the 30 S ribosomal subunit in a way similar to the antibiotic hygromycin (13). The action of Doc is suppressed by the antitoxin Phd, which consists of two domains. Its C-terminal domain (residues 52-73) harbors the interaction site with Doc and on its own prevents Doc-mediated growth arrest (14,15). The N-terminal region (residues 1-51) of Phd i...

The C-Terminal Portion of an Archaeal Toxin, aRelE, Plays a Crucial Role in Protein Synthesis Inhibition

“…12) However, consistent with the present results is the observation by Garcia-Pino et al that RelE induced cleavage of mRNA at several codons, especially just downstream of the start codon. 16) The present results further indicate that aRelE was able to bind mRNAs to the same extent irrespective of tRNA fMet (Fig. 3A, c and d), inconsistently with the results obtained in preliminary experiment, described above.…”

Functional equivalence and evolutionary convergence in complex communities of microbial sponge symbionts

“…R-M and toxin-antitoxin (T-A) systems also are often considered selfish elements that are involved in MGE competition and an "arms-race" between the chromosomes and the MGEs (87,88). The evolutionary accumulation of these systems in sponge symbionts further supports the postulated high rate of HGT inside symbiont communities of sponges (see above and SI Results and Discussion).…”

HicA of Escherichia coli Defines a Novel Family of Translation-Independent mRNA Interferases in Bacteria and Archaea

“…4). Although many TA loci encode mRNA-cleaving enzymes, there are examples of such translational inhibitors that mediate indirect mRNA cleavage (13), and the final test to determine whether HicA is an RNase awaits biochemical experiments.…”