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transcription units suggests that the genes are co-regulated and encode factors
that function together in a multicomponent virulence (invasion) system. The
mxi
and
spa
operons encode subunits of a type III secretion system (T3SS; see
Chapter 14) and the
ipa
operon encodes proteins (effectors; see Chapter 15)
secreted by this system. T3SS are widely used by animal and plant pathogens
(enterohemorrhagic and enteropathogenic
E. coli
,
Salmonella
spp.
, Yersinia
spp.
, Pseudomonas
spp.
, Erwinia caratovara,
and
Xanthomonas campestris,
to name a few) for delivery of virulence factors to the surface or interior of
host cells (
Galan and Wolf-Watz, 2006
). These secreted effectors of virulence
allow the pathogens to reprogram host cells to serve the pathogen's particular
needs. The T3SS delivery machines are conserved structurally, functionally, and
genetically and share several core characteristics.
T3SS genes
The products of the
ipa
genes are actively secreted into the extracellular medium
even though they contain no signal sequence for recognition by the usual gen-
eral secretory pathway of Gram-negative bacteria. Ipa secretion is mediated by
a T3SS composed of gene products from the
mxi/spa
loci (
Schroeder and Hilbi,
2008
). A characteristic feature of T3SS is a contact-dependent secretion mecha-
nism. Delivery of effectors to their targets (surface and cytoplasmic) occurs
only after outer membrane elements of the T3SS complex contact cholesterol
in the host cell membrane (
Hayward et al., 2005
). This feature is observed in
the
Shigella
T3SS as the Ipa proteins are deposited on the host cell surface after
contact with host cells (
Menard et al., 1994a
;
Watarai et al., 1995
).
The structural elements of the
Shigella
T3SS are encoded in two adjacent
operons on the virulence plasmid. The
mxi
(
m
embrane e
x
pression of
i
(invasion
plasmid antigens) genes encode several lipoproteins (MxiJ and MxiM), a trans-
membrane protein (MxiA), and proteins containing signal sequences (MxiD,
MxiJ, and MxiM) (
Schroeder and Hilbi, 2008
). MxiH is the major subunit of
the secretion needle (
Blocker et al., 2001
). The
spa
(
s
urface
p
resentation of
Ipa
a
ntigens) genes encode additional components of the
Shigella
T3SS. The
cytoplasmic protein Spa32 controls the switch for T3SS substrate specificity
and thus regulates needle length (
Magdalena et al., 2002
;
Tamano et al., 2002
).
Assembly of the T3SS needle and subsequent effector secretion requires Spa47
which is associated with the cytoplasmic face of the secretion apparatus and
has sequence similarities with ATPases of the flagellar assembly machinery of
other bacteria (
Vankatesan et al., 1992
;
Tamano et al., 2000
). Spa47 likely is the
energy-generating component of the secretion apparatus.
Invasion plasmid antigens
The genes comprising the
ipaBCDA
(
i
(invasion
p
lasmid
a
ntigens) cluster encode
the immunodominant antigens of
Shigella
and EIEC that are detected with sera
from convalescent patients and experimentally challenged monkeys (
Hale et al.,
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