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pathogenicity component of Salmonella typhimurium . This assembly, referred
to as the T3SS injectisome, was shown to be necessary for the translocation
of effectors. Subsequently, the direct visualization of T3SS-encoded injecti-
somes have also been reported for Yersinia pestis ( Journet et al., 2003 ), Shigella
flexneri ( Tamano et al., 2000 ) and EPEC ( Sekiya et al., 2001 ). The similar-
ity between the injectisome and the flagellum is not only morphological: con-
firming a phylogenetic link between the two systems, many flagellar proteins,
specifically those localized at the inner membrane, are homologous to T3SS
injectisome components, suggesting a common evolutionary origin ( Ginocchio
et al., 1994 ).
In this chapter, we will describe the various T3SSs present in E. coli and
related Shigella strains ( Chaudhuri and Henderson, 2012 ) (see also Chapter 7),
and their role in pathogenicity. We will then review the current knowledge of
the organization of the T3SS injectisome with special attention to the struc-
tural detail as well as current insights into the possible molecular mechanisms
employed by these systems for efficient secretion of effector proteins (see also
Chapter 15 for a review of T3SS effector function). Finally, an overview of the
complex regulation implemented for T3SS assembly and secretion will be sum-
marized. Our review will focus on the T3SS injectisome, but various proteins of
orthologous function in the flagellar T3SS will also be discussed.
TYPE 3 SECRETION SYSTEMS IN E. COLI
LEE-encoded T3SS
The locus of enterocyte effacement (LEE) is a pathogenicity island originally
identified in the EPEC strain O127:H6 ( McDaniel et al., 1995 ), and later found
in most EPEC, EHEC, and atypical enteropathogenic E. coli (aEPEC) strains.
It is responsible for the distinctive formation of attaching and effacing (A/E)
lesions by LEE-encoding strains, requiring first adhesion of the bacteria to
the host intestinal epithelium followed by rapid recruitment of various host
cytoskeletal elements that ultimately create the characteristic pedestal (lesion)
beneath the adhered bacterium ( Donnenberg and Kaper, 1992 ).
In most strains, the LEE is found inserted within tRNA-encoding sites (typi-
cally selC , pheU , or pheV ) ( Wieler et al., 1997 ; Muller et al., 2009 ). Sequence
analysis shows a lower GC content within the LEE (38.4%) compared to the
average for the overall E. coli genome (50.8%) as well as homology to trans-
posons ( Donnenberg et al., 1997 ) that suggests a potential mode of horizontal
transfer between E. coli strains ( Reid et al., 2000 ; Sandner et al., 2001 ).
The LEE is ∼35 kb long, and contains 41 reading frames organized into
five distinct operons ( Figure 14.1 A) ( Elliott et al., 1998 ), which include genes
encoding for several T3SS effectors and their chaperones, a T3SS injectisome
and export apparatus, T3SS regulators, and an adhesin ( Jarvis et al., 1995 ).
Secretion of various T3SS effectors in EPEC-infected epithelial cells has been
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