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subjected to chilling injury revealed fracture faces containing particle-free and particle-containing
regions (phase-separation state). The cells from normal growth temperatures revealed the existence
of particles distributed at random (liquid-crystalline state). Moreover,
Anabaena variabilis
, a species
that is not susceptible for chilling, revealed intramembrane particles distributed at random in cells
derived from growth temperatures as well as chilling treatments. Further, the studies of Murata
et al
.
(1979) and Sato
et al
. (1979) have confi rmed that when growth temperatures are lowered a decrease
in saturated fatty acids takes place with a concomitant increase in the unsaturated fatty acids. Ono
and Murata (1982) observed that due to variation in growth temperature the fatty acid composition
also varies and when double bonds in the fatty acids of lipids are introduced it leads to a decrease
in the thermotrophic phase transition of the membranes. So the role of fatty acid desaturases in
introducing double bonds in the fatty acids to produce more unsaturated fatty acids and their role
in low temperature tolerance has received attention. A number of studies have correlated changes
in fatty acid composition of the membrane lipids to the ambient growth temperature (Murata
et
al
., 1979, 1992a,b; Sato and Murata, 1980, 1981; Wada and Murata, 1990) and this has been regarded
as an adaptive response (Murata and Nishida, 1987; Murata, 1989). Cossins (1994) termed this as
homeoviscous adaptation.
ii) Role of fatty acid desaturases
:
Desaturases introduce double bonds into fatty acids and thus
convert saturated fatty acids into unsaturated ones. Three types of desaturases, i.e. acyl-CoA
desaturases, acyl-ACP (acyl-carrier protein) desaturases and acyl-lipid desaturases are known.
The fi rst type is present bound to the endoplasmic reticulum of animal, yeast and fungal cells.
These introduce double bonds into fatty acids bound to coenzyme A. The second type, acyl-ACP
desaturases, is present in the stroma of plant plastids and these can introduce double bonds into
fatty acids that are bound to ACP. The third type acyl-lipid desaturases introduce double bonds
into fatty acids of glycerolipds. They are bound to the endoplasmic reticulum and the chloroplast
membranes of plant cells as well as the thylakoid membranes of cyanobacteria. It is the acyl-lipid
desaturases that respond to a shift-down of growth temperature and convert saturated fatty acids
into unsaturated fatty acids there by increasing the fl uidity of the membranes. Murata
et al
. (1992)
classifi ed cyanobacterial strains into four groups based on the patterns of fatty acid desaturation.
S. elongatus
PCC 7942 (
A
.
nidulans
R2),
Synechococcus vulcanus
(a thermophilic cyanobacterium) and
Mastigocladus laminosus
belong to group 1 that can only introduce a double bond at Δ
9
position of
fatty acids either at the
sn-
1 or
sn-
2
position during a 10-14 h temperature shift-down. In group 2,
Synechococcus
sp. strain PCC 7002,
A
.
variabilis
M3,
Plectonema boryanum
and
N
.
muscorum
are included.
These strains can introduce double bonds at the Δ
9
, Δ
12
and Δ
15
(ω
3
) positions of the C18 acids at the
sn-
1 position and at the Δ
9
, Δ
12
positions of C16 acids at the
sn-
1
position.
Synechococcus
sp. strain
PCC 7002 has three acyl-lipid desaturase genes (
desA
, Δ
12
desaturase;
desB
, ω
3
or Δ
15
desaturase and
desC
, Δ
9
desaturase). In case of
A
.
variabilis
M3, conversion of 16:0 to 16:1Δ
9
occurred in light as well
as in dark due to cold shock and polyunsaturated fatty acids 16:2Δ
9,12
and 18:3Δ
9,12,15
also increased
(Sato and Murata, 1980). Group 3 consists of
Synechocystis
sp. strain PCC 6714 and
S. platensis
that can
introduce three double bonds at Δ
6,9,12
positions of C18 acids at
sn-
1 position.
Synechocystis
sp. strain
PCC 6803 and
Tolypothrix tenuis
belong to group 4 and these strains can introduce double bonds at the
Δ
6,9,12,15
positions of the C18 acids at the
sn
-1 position.
Synechocystis
sp. strain PCC 6803 contains four
genes for acyl-lipid desaturases that have been cloned and their expression and activities noted in
E
.
coli
. These are
desA
(Wada
et al
., 1993),
desB
(Sakamoto
et al
., 1997a,b),
desC
(Sakamoto
et al
., 1994)
and
desD
(Reddy
et al
., 1993) encoding acyl-lipid desaturases that introduce double bonds specifi cally
at Δ
12
, ω
3
, Δ
9
and Δ
6
positions of C18 fatty acids, respectively. A temperature shift-down (38°C to 22°C)
