Biology Reference
In-Depth Information
The conserved organization of group 1 capsules-related clusters appears
to be reminiscent of the modular structure of the genetic loci required for
the production of the ABC-dependent group 2 capsules in
E. coli
(
Fig. 7.3
),
which comprise three distinct regions (
Rahn et al., 1999
). Region 1
encodes the KpsF, E, D, U, C, and S, which are necessary for translocation
of the polysaccharide, thus playing similar functions as those played by
Wza, Wzb, and Wzc in the group 1 capsule-related clusters. The central
region 2 is serotype specific and is responsible for the biosynthesis and
polymerization of the polymer. This region is equivalent to the 3′ region
of group 1 capsule-related clusters, but lacks the
wzy
and
wzx
. Finally,
region 3 comprises genes encoding components that are not required in
the Wzy-dependent machinery, namely
kpsM
and
kpsT
, which encode the
ABC transporter that characterizes the pathway. The gene products of the
regions 1 and 3 are conserved, functioning independently of the struc-
ture of the capsule polysaccharide (
Whitfield, 2006
;
Whitfield & Roberts,
1999
). Similar organization is observed for the loci encoding the proteins
Figure 7.3
Genetic organization of the loci encoding the proteins involved in the pro-
duction of
E. coli
group 1 capsules (following the wzy-dependent pathway) and
E. coli
group 2 capsules (following the ABC-dependent pathway). In the first case, the 5
′
region
of the loci usually contains the conserved genes
wzi
(coding a protein involved in surface
attachment of the capsule),
wza
,
wzb
, and
wzc
, whereas the 3
′
region is serotype specific
and encodes the enzymes involved in the biosynthesis of the nucleotide sugars, glyco-
syltransferases and the Wzx and Wzy proteins (
Rahn et al., 1999
;
Whitfield, 2006
). The
genetic loci required for the production of group 2 capsules comprises three regions.
Region 1 encodes the KpsF, E, D, U, C and S (similar functions as those played by Wza,
Wzb and Wzc). Region 2 is equivalent to the 3
′
region of group 1 capsule- related clusters,
but lacks the
wzy
and
wzx
. Region 3 comprises the
kpsM
and
kpsT
encoding the ABC
transporter that characterizes this pathway. The gene products of the regions 1 and 3 are
conserved, functioning independently of the monosaccharidic composition/structure of
the capsule polysaccharide (
Rahn et al., 1999
;
Whitfield, 2006
;
Whitfield & Roberts, 1999
).
