Page 5 - Volume 5, Issue 2, July 2012
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© Benaki Phytopathological Institute
Phytobacterial type III secretion systems in the era of biotechnology
33
ical applications in the field of genetic engi-
neering of crops on a commercial scale – the
subject has been extensively reviewed else-
where (Gelvin, 2010; Van Montagu, 2011).
T3SS function as vehicles of protein traf-
ficking
T3SS are molecular nanomachines found
in many Gram-negative proteobacteria (α,
β, γ and δ subdivisions, also found in some
non-proteobacteria such as
Chlamydiae)
that establish intimate relationships (patho-
genic, symbiotic or commensal) with plant,
human, insect, nematode or animal hosts
and are evolutionarily related to bacteri-
al flagella (Dale and Moran, 2006; Tampak-
aki
et al.
, 2004; Troisfontaines and Cornelis,
2005; Charova
et al.
, 2012). They function as
“injectisomes”, translocating diverse reper-
toires of proteins to extracellular locations
and into eukaryotic cells. Many T3SS-secret-
ed proteins (T3SS effectors) modulate the
function of host regulatory networks, which
determine the outcome of recognition and
biological accommodation with the eukary-
otic host. T3SS have evolved into seven phy-
logenetic families (Troisfontaines and Cor-
nelis, 2005). Some bacteria may harbor more
than one T3SS, which usually belong to dif-
ferent phylogenetic families (non-ortholo-
gous), an indication of relatively recent hori-
zontal gene transfer.
In plant pathogenic bacteria, the T3SS
forms a long pilus that equals or exceeds
in length that of the bacterial cell and has a
central channel through which the secreted
proteins are transported from the prokary-
otic cytoplasm across the inner and outer
bacterial membrane as well as the plant cell
wall, into the host cytosol. The proteins that
build and regulate the assembly and func-
tion of this secretion apparatus are coded
by three classes of genes (reviewed in Tam-
pakaki
et al.
, 2010). Firstly, the
hrc
genes (
hrp
c
onserved), which also have homologs in
the bacterial flagellum and whose protein
products are conserved at the sequence lev-
el among animal and plant pathogenic bac-
teria. They express proteins known as the
T3SS core components, including HrcJ, HrcU,
HrcV, HrcN, HrcR, HrcT, HrcS and HrcC, which
form the basal body of the apparatus that
is embedded in the inner and outer bacte-
rial membrane. HrcC forms a ring-like struc-
ture in the outer membrane. HrcN and its
homologs catalyze ATP hydrolysis provid-
ing energy to the system and are involved
in protein unfolding and transport (Lorenz
and Büttner, 2009). Secondly, transcriptional
regulators of the T3SS operons and effector
genes, such as a) the alternative sigma factor
(σ
L
) HrpL, the master regulator of the secre-
tion system in
P. syringae
pathovars
, Pantoea,
Pectobacterium
and
Erwinia/Dickeya
species,
and b) the AraC-type regulators HrpB and
HrpX in
Ralstonia
and
Xanthomonas
species,
respectively (Büttner and Bonas U., 2009).
Importantly,
avr/
effector genes and
hrp/
hrc
operons are co-regulated, a finding that
provided an early clue about their function-
al linkage. The third class of genes codes for
the extracellular T3SS components (the pilus
subunits, HrpA in
P. syringae
and
Erwinia am-
ylovora
, HrpY in
R. solanacearum
, HrpE in
X.
campestris
), proteins serving as chaperones
assisting in injectisome assembly, secreted
proteins with extracellular enzymatic func-
tion, proteins forming pores in the plant cell
wall or assisting translocation activity (HrpK,
HrpF; reviewed in Büttner and He, 2009) and
proteins such as harpins (HrpZ, HrpW, PopA;
He
et al.
, 1993) that are able to elicit the HR in
purified form.
The
hrp/hrc
genes are generally located
in pathogenicity islands (PAIs) and are pri-
marily found on the bacterial chromosome
rather than on plasmids. Typical
hrp
clus-
ters, such as those of
P. syringae
pathovars,
consist of core regions of 6 operons and
27 open reading frames (ORFs), flanked by
two effector rich loci, the conserved effec-
tor locus (CEL) and the exchangeable effec-
tor locus (EEL; Alfano
et al.
, 2000). New
in sil-
ico
analysis revealed multiple T3SS clusters
in the same strain, found not to be non-or-
thologous and functionally non-redundant
but serving bacterial virulence in different
ways. For example, in case of
E. amylovo-
ra,
one T3SS mediates interaction with the
plant host, while the second might be serv-
