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Journal of Bacteriology, May 2000, p. 2416-2421, Vol. 182, No. 9
Gene Regulation and Chromosome Biology
Laboratory, ABL Basic Research Program, NCI-Frederick Cancer
Research and Development Center, Frederick, Maryland 21702-1201
Received 5 November 1999/Accepted 10 February 2000
The large virulence plasmid pMYSH6000 of Shigella
flexneri contains a determinant that is highly effective in
stabilizing otherwise unstable plasmids in Escherichia
coli. Expression of two small contiguous genes, mvpA
and mvpT (formerly termed STBORF1 and STBORF2), was shown
to be sufficient for stability. Mutations in mvpT abolished
plasmid stability, and plasmids expressing only mvpT killed
the cells unless mvpA was supplied from a separate plasmid
or from the host chromosome. When replication of a plasmid carrying the
minimal mvp region was blocked, growth of the culture stopped after a short lag and virtually all of the surviving cells retained the plasmid. Thus, the mvp system stabilizes by a
highly efficient postsegregational killing (PSK) mechanism, with
mvpT encoding a cell toxin and mvpA encoding an
antidote. The regions that surround the mvp genes in their
original context have an inhibitory effect that attenuates plasmid
stabilization and PSK. The region encompassing the mvp
genes also appears to contain an additional element that can aid
propagation of a pSC101-based plasmid under conditions where
replication initiation is marginal. However, this appears to be a
relatively nonspecific effect of DNA insertion into the plasmid vector.
0021-9193/00/$04.00+0
The Stability Region of the Large Virulence Plasmid of
Shigella flexneri Encodes an Efficient Postsegregational
Killing System
and
*
Corresponding author. Mailing address: Gene Regulation
and Chromosome Biology Laboratory, ABL Basic Research Program,
NCI-Frederick Cancer Research and Development Center, Frederick, MD
21702-1201. Phone: (301) 846-1266. Fax: (301) 846-6988. E-mail:
austin{at}ncifcrf.gov.
Present address: Biology and Biotechnology Research Program,
Lawrence Livermore National Laboratory, Livermore, CA 94551.
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