Biology Reference
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implementation. Therefore, it is crucial to elucidate the biosynthesis of the
EPS in these organisms.
2. CONSERVED MECHANISMS OF BACTERIAL EPS
PRODUCTION
In the past years, several studies have focused on the genetics of EPS
biosynthesis in bacteria. However, cyanobacteria have not been thoroughly
examined and, consequently, the information available is very limited. Stud-
ies performed with other bacteria point out that, regardless of the vari-
ety of surface polysaccharides produced, their biosynthetic pathways are
relatively conserved, beginning with the activation of monosaccharides and
its conversion into nucleotide sugars in the cytoplasm, and finishing with
an assembled complex polymer outside the cell wall (
Whitfield & Larue,
2008
). This process requires the participation of three groups of proteins,
including (1) enzymes involved in the biosynthesis of the nucleotide sug-
ars, (2) glycosyltransferases, which catalyse the transfer of the nucleotide
sugars from activated donor molecules to specific acceptors in the plasma
membrane, and (3) proteins involved in EPS assembly and export (
Reeves
et al., 1996
). While the sugar activation/modification enzymes and the gly-
cosyltransferases are strain specific, the proteins involved in the polymeriza-
tion, assembly and export of the polymer seem to be well conserved, with
most bacterial EPS being assembled and exported by one of the following
pathways: (1) the synthase-dependent, (2) the Wzy-dependent, and (3) the
ABC transporter-dependent (also referred to as Wzy-independent) (
Cuth-
bertson, Kos, & Whitfield, 2010
;
Geremia & Rinaudo, 2005
;
Whitfield &
Larue, 2008
;
Yother, 2011
). Unquestionably, the Wzy-dependent pathway is
the most widely distributed mechanism, being present in large number of
bacteria (
Whitfield, 2010
;
Whitfield & Larue, 2008
). These pathways are not
confined to the assembly and export of the EPS, but may also be involved
in the formation of other surface polysaccharides, such as the O-antigen
of LPS (
Cuthbertson et al., 2010
;
Mozzi, Savoy de Giori, & Font de Valdez,
2003
). Regarding the synthase-dependent pathway, although it is used for
the production of important biological polysaccharides, including bacterial
cellulose, hyaluronan and alginate, the information available is still quite
limited (
Cuthbertson et al., 2010
;
Franklin, Nivens, Weadge, & Howell,
2011
;
Raetz & Whitfield, 2002
). It is known that the assembly and export
of these polymers rely in the activity of a processive glycosyltransferase -
the synthase - that serves as both a polymerase and an exporter. However,
