Biology Reference
In-Depth Information
three copies of their circular chromosome per cell (
Griese, Lange, & Soppa,
2011
). The figures fluctuate between 3 and 16 for
Synechococcus
PCC 7942
(formerly designated as
Anacystis nidulans
) (
Griese et al., 2011
;
Mann &
Carr, 1974
) and 12 and 218 for
Synechocystis
PCC 6803 (
Griese et al., 2011
;
Labarre, Chauvat, & Thuriaux, 1989
). By contrast, little is known concerning
the ploidy of cyanobacterial plasmids, and whether it is possibly influenced
by their size, or the nature and growth conditions of their cyanobacterial
hosts (
Beria & Pakrasi, 2012
;
Ma, Paulsen, & Palenik, 2012
). Recent genom-
ics studies suggest that horizontal gene transfer events might be frequent in
cyanobacterial (
Hess, 2011
). In fresh-water species, two major modes of gene
transfer have been identified: natural transformation (
Chauvat, Astier, Vedel,
& Joset-Espardellier, 1983
;
Grigorieva & Shestakov, 1982
), and conjugation
with either cyanobacterial (
Wolk, Vonshak, Kehoe, & Elhai, 1984
) or non-
cyanobacterial plasmids (
Mermet-Bouvier, Cassier-Chauvat, Marraccini, &
Chauvat, 1993
). In addition, insertion sequences and cyanophages could con-
tribute to gene transfers among cyanobacteria. Indeed, several cyanobacterial
insertion sequences are truly mobile (
Cassier-Chauvat, Poncelet, & Chauvat,
1997
), and some cyanophages possess genes presumably involved in photo-
synthesis (
Clokie & Mann, 2006
) or anti-oxidant defences (glutaredoxins, see
below section 5) that could be involved in or resulting from gene transfer.
1.3. Redox Stress in Cyanobacteria
Because of their lifestyle, cyanobacteria are continuously challenged with
toxic
r
eactive
o
xygen
s
pecies (ROS) present in our oxygenic atmosphere
(ozone, O
3
), or generated by photosynthesis (
Kirilovsky, 2010
), respiration
and cell metabolism (
Latifi, Ruiz, & Zhang, 2009
). These oxidative agents
are, namely, singlet oxygen (
1
O
2
), the superoxide anion (O
2
.
−
), hydrogen per-
oxide (H
2
O
2
), and the hydroxyl radical (OH
%
) (
Imlay, 2008
). Among other
ROS-generated damages, cysteines can be oxidized to form thiyl (sulfenyl)
radical (-S°) by one-electron transition; sulfenic acid (-SOH) and disul-
fide (-S-S-) by a two-electrons transition; sulfinic acid (-SO
2
H) by a four-
electrons transition; and eventually sulfonic acid (-SO
3
H) by a six-electrons
transition (
Forman, Maiorino, & Ursini, 2010
). Two types of disulfide can
be distinguished considering whether they link two cysteinyl residues, from
the same or different proteins (intra- or intermolecular disulfide bridges)
or from a protein and a molecule of the anti-oxidant tripeptide glutathione
(glutathione-protein mix disulfide, also termed glutathionylation). These
sulphur switches can provide an important and flexible means of reversibly
controlling protein function. Glutathionylation is regarded as a transient
