Biology Reference
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andhighnumberofdifferentmonosaccharides(upto13),makethemverypromising
forbiotechnologicalapplications.Despitetheincreasinginterestinthesepolymers,the
informationabouttheirbiosyntheticpathwaysisstilllimited.Studiesperformedinother
bacteria revealed that themechanismsofEPSassemblyandexportare relativelycon-
served,generallyfollowingtheWzy-dependentortheABC-dependentpathways,which
require the involvement of polysaccharide copolymerase (PCP) and outermembrane
polysaccharideexport(OPX)proteins.Ourpreviousstudiesrevealedthat incyanobac-
teria,thegenesencodingtheseproteinsoccurinmultiplecopies,scatteredthroughout
thegenome,eitherisolatedorinsmallclusters.However,itisnecessarytoidentifyother
genesthatmayberelatedtothisprocess,understandtheirgenomicdistribution,and
reconstructtheirevolutionaryhistory.Thedatagatheredhereprovideafirstinsighton
thephylogenetichistoryoftheEPS-relatedgenes,andconstitutearobustbasisforsub-
sequentstudiesaimingtooptimizeEPSproductionincyanobacteria.
1. INTRODUCTION
Cyanobacteria possess an unusual prokaryotic envelope that combines
the presence of an outer membrane, as in Gram-negative bacteria, with a
thick and highly cross-linked peptidoglycan layer similar to that of Gram-
positive organisms ( Hoiczyk & Hansel, 2000 ; Stewart, Schluter, & Shaw,
2006 ). Many cyanobacterial strains also produce and export extracellular
polymeric substances (EPS), mainly composed of polysaccharides, that can
remain attached to the cell surface or be released into the surrounding envi-
ronment (released polysaccharides (RPS)). The cyanobacterial EPS attached
to the cell wall can be referred to as sheaths, capsules and slimes, according
to their thickness, consistency and appearance ( Pereira et al., 2009 ). The
sheath is usually a thin and dense layer that surrounds cells or cell groups
and is visible in light microscopy without staining. The capsule generally
consists of a thick and slimy layer intimately associated with the cell sur-
face, characterized by its sharp outlines and for being structurally coherent
to exclude particles (e.g. India ink). The slime refers to the mucilaginous
material dispersed around the cells, usually not reflecting their shape
( De Philippis & Vincenzini, 1998 ; Pereira et al., 2009 ). Certain amounts of
these external layers can be released into the surrounding medium, consti-
tuting the RPS. Differences observed in the monosaccharidic composition
of sheaths and RPS of the same strain suggest that RPS are not merely
due to the fragmentation of the external layers, but may also result from a
divergent biosynthetic process ( Li, Harding, & Liu, 2001 ; Micheletti et al.,
2008 ; Ortega-Calvo & Stal, 1994 ; Tease et al., 1991 ). The production of the
EPS boundary between the cell and its immediate environment requires
a large amount of energy consequently, it is reasonable to assume that it
 
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