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et al., 1990 ). Sequencing of plasmid pO157 from EHEC identified a T2S system
conserved among all EHEC O157 and some non-O157 strains ( Schmidt et al.,
1997 ). A silenced T2S operon was also found in an E. coli K-12 strain ( Blattner
et al., 1997 ). This system can secrete endochitinase ChiA when the silencing
protein is inactivated ( Francetic et al., 2000 ). In ETEC a T2S system highly
similar to that responsible for cholera toxin secretion in Vibrio cholerae was
discovered to secrete LT ( Tauschek et al., 2002 ). This gene cluster is absent
from E. coli K12, however the K12 genome appears to have once contained this
sequence.
Although discovered years earlier in other bacteria, T4P where first iden-
tified in E. coli with the discovery of the EPEC bundle-forming pilus (BFP)
( Girón et al., 1991 ). Subsequent identification of the gene encoding the major
structural subunit confirmed its similarity to other T4P pilin genes and sug-
gested the presence of a large operon encoding genes required for pilus pro-
cessing and biogenesis ( Donnenberg et al., 1992 ). Additional E. coli T4P were
discovered subsequently.
Distribution
Both T2S and T4P are produced by a diverse number of Gram-negative bac-
teria including many plant, animal, and human pathogens. Within E. coli , one
T2S system and the HCP are ubiquitous, while additional T2S and T4P systems
are found on specific subsets of pathotypes ( Girón et al., 1993, 1994 ; Schmidt
et al., 1997 ; Tauschek et al., 2002 ).
The components of T4P systems share significant sequence similarity and
structural homology with components of T2S systems, DNA uptake systems
( Averhoff and Friedrich, 2003 ; Peabody et al., 2003 ), and filamentous phage
assembly systems ( Linderoth et al., 1996 ; Russel et al., 1997 ). T4P assembly
components also are orthologous to proteins involved in archaeal flagellum
assembly ( Peabody et al., 2003 ). The sequence and structural similarities across
such a wide range of organisms strongly suggest an ancient and shared evolu-
tionary history ( Peabody et al., 2003 ; Pelicic, 2008 ). T4Ps have also been found
in Gram-positive ( Varga et al., 2006 ; Rodgers et al., 2011 ) and archaeal species
( Herdendorf et al., 2002 ; Bardy and Jarrell, 2003 ).
Only a few T4bP systems have been identified in E. coli , where some are
virulence factors used for adherence to eukaryotic cells, an essential first step
for colonization. This is evident in the decreased virulence of EPEC strains
deficient for functional pili ( Bieber et al., 1998 ). The best-characterized system
is the BFP of EPEC. Through the remainder of this chapter we will refer to the
BFP system as the model of T4bP biogenesis, referring to other systems pri-
marily to highlight differences when they occur. In addition to BFP, two related
T4Ps have been identified in ETEC, the colonization factor antigen III (CFA/III)
and Longus systems. Another system called the R64 pilus is encoded on a plas-
mid, and a T4aP called the hemorrhagic coli pilus or HCP has been described in
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