Postsegregational killing does not increase plasmid stability but acts to mediate the exclusion of competing plasmids

Postsegregational killing does not increase plasmid stability but acts to mediate the exclusion of competing plasmids

Tim F. Cooper* and
Jack A. Heinemann†

Department of Plant and Microbial Sciences, Private Bag 4800, University of Canterbury, Christchurch, New Zealand

Edited by Joshua Lederberg, The Rockefeller University, New York, NY, and approved September 8, 2000 (received for review February 23, 2000)

Abstract

Postsegregational killing (PSK) systems consist of a tightly linked toxin–antitoxin pair. Antitoxin must be continually produced to prevent the longer lived toxin from killing the cell. PSK systems on plasmids are widely believed to benefit the plasmid by ensuring its stable vertical inheritance. However, experimental tests of this “stability” hypothesis were not consistent with its predictions. We suggest an alternative hypothesis to explain the evolution of PSK: that PSK systems have been selected through benefiting host plasmids in environments where plasmids must compete during horizontal reproduction. In this “competition” hypothesis, success of PSK systems is a consequence of plasmid–plasmid competition, rather than from an adaptive plasmid–host relationship. In support of this hypothesis, a plasmid-encoded parDE PSK system mediated the exclusion of an isogenic ΔparDE plasmid. An understanding of how PSK systems influence plasmid success may provide insight into the evolution of other determinants (e.g., antibiotic resistance and virulence) also rendering a cell potentially dependent on an otherwise dispensable plasmid.

Footnotes

↵ * Present address: Center for Microbial Ecology, Plant and Soil Science Building, Michigan State University, East Lansing, MI 48824.
↵ † To whom reprint requests should be addressed. E-mail: j.heinemann{at}pams.canterbury.ac.nz.
This paper was submitted directly (Track II) to the PNAS office.
Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073/pnas.220077897.
Article and publication date are at www.pnas.org/cgi/doi/10.1073/pnas.220077897
Abbreviations:

PSK,

postsegregational killing;

rm,

restriction–modification;

Gmr,

resistance to gentamicin;

HME,

horizontally mobile element;

LBH,

Luria–Bertani–Herskowitz medium