Plasmid Addiction Genes of Bacteriophage P1: doc, which Causes Cell Death on Curing of Prophage, and phd, which Prevents Host Death when Prophage is Retained

Abstract

P1 lysogens of Escherichia coli carry the prophage as a stable low copy number plasmid. The frequency with which viable cells cured of prophage are produced is about 10(-5) per cell per generation. Here we show that a significant part of this remarkable stability can be attributed to a plasmid-encoded mechanism that causes death of cells that have lost P1. In other words, the lysogenic cells appear to be addicted to the presence of the prophage. The plasmid withdrawal response depends on a gene named doc (death on curing), encoding a 126 amino acid protein. Expression of doc is not SOS-inducing and killing by Doc is recA-independent. In cells that retain P1 the killing is prevented by the product of a gene named phd (prevent host death), encoding a 73 amino acid protein. The genes phd and doc have been cloned and expressed from a 0.7 kb segment of P1 DNA. The two genes constitute an operon and the synthesis of Doc appears to be translationally coupled to that of Phd. Homologs of the P1 addiction genes are found elsewhere, but phd and doc are unrelated to previously described genes of other plasmids that also cause an apparent increase in plasmid stability by post-segregational killing.

Complete genome sequence of the probiotic lactic acid bacteriumLactobacillus acidophilusNCFM

“…A second protein, Phd, coexpressed with Doc, counteracts the killer protein. Absence of Phd causes cell death, usually induced upon phage or plasmid curing (22). A second ORF, La30, separated from La31 by only one codon, was also predicted, revealing striking similarities to the systems described for P1 and phage F. Both ORFs are very small (213 and 399 bp, respectively), likely organized in an operon, and both exhibit low sequence similarities to other known proteins.…”

The YefM Antitoxin Defines a Family of Natively Unfolded Proteins

“…According to data analysis, a k a of 3.06 ϫ 10 3 (M Ϫ1 s Ϫ1 ) and a k d of 1.22 ϫ 10 3 (M Ϫ1 s Ϫ1 ) were calculated (arithmetic mean). Accordingly, an equilibrium con- stant (K D ) of 0.4 M was determined for the YoeB-YefM [11][12][13][14][15][16][17][18][19][20][21][22][23] complex. This dissociation constant is consistent with a specific binding between the toxin and the peptide fragment.…”

“…One case in which a natively unfolded state of a protein appears to have physiological significance is that of the Phd protein of the phage P1 (11). This protein is a part of a bimolecular complex that acts as the "plasmid addiction" module of the phage (12). The addiction module mechanism assures an efficient inheritance of the extrachromosomal phage and is based on the differential physiological stability of its two components, the stable toxin Doc and the labile antitoxin Phd.…”

The ParE2–PaaA2 toxin–antitoxin complex fromEscherichia coliO157 forms a heterodocecamer in solution and in the crystal

“…Among the arsenal of macromolecules that are available to function in the context of bacterial stress response are the so-called 'toxin-antitoxin (TA)' modules. These are pairs of genes, usually forming a separate small operon, that were originally discovered to be involved in the maintenance of low-copy-number plasmids (Ogura & Hiraga, 1983;Gerdes et al, 1986;Bravo et al, 1987;Lehnherr et al, 1993). They are now known to be abundant on bacterial chromosomes, with some organisms such as Mycobacterium tuberculosis possessing over 60 TA modules (Pandey & Gerdes, 2005).…”