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J. Bacteriol., 06 1997, 3670-3675, Vol 179, No. 11
L Radnedge, MA Davis, B Youngren and SJ Austin
The large virulence plasmid pMYSH6000 of Shigella flexneri contains a
replicon and a plasmid maintenance stability determinant (Stb) on adjacent
SalI fragments. The presence of a RepFIIA replicon on the SalI C fragment
was confirmed, and the complete sequence of the adjacent SalI O fragment
was determined. It shows homology to part of the transfer (tra) operon of
the F plasmid. Stb stabilizes a partition- defective P1 miniplasmid in
Escherichia coli. A 1.1-kb region containing a homolog of the F trbH gene
was sufficient to confer stability. However, the trbH open reading frame
could be interrupted without impairing stability. Deletion analysis
implicated the involvement of two small open reading frames, STBORF1 and
STBORF2, that fully overlap trbH in the opposite direction. These open
reading frames are closely related to the vagC and vagD genes of the
Salmonella dublin virulence plasmid and to open reading frame pairs in the
F trbH region and in the chromosomes of Dichelobacter nodosus and
Haemophilus influenzae. Stb appears to promote better-than-random
distribution of plasmid copies and is a plasmid incompatibility
determinant. The F homolog does not itself confer stability but exerts
incompatibility against the activity of the Stb system. Stb is likely to
encode either an active partition system or a postsegregational killing
system. It shows little similarity to previously studied plasmid stability
loci, but the genetic organization of STBORF1 and STBORF2 resembles that of
postsegregational killing mechanisms.
Copyright © 1997, American Society for Microbiology
Plasmid maintenance functions of the large virulence plasmid of Shigella flexneri
Gene Regulation and Chromosome Biology Laboratory, ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, Maryland 21702-1201, USA.
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