STRESS RESPONSES - Cyanobacteria - page 647

Δ 9 desaturase gene ( desC ) also showed a temperature-dependent expression in the wild-type and
desA + transformants of S. elongatus PCC 7942. Since the wild-type was able to synthesize only
monounsaturated fatty acids, the desC gene expression occurred at 30°C. However, desA + transformant
cells synthesized diunsaturated fatty acids at the expense of monounsaturated fatty acids and in
these the induction of desC gene took place at 28°C. Due to the presence of diunsaturated fatty acids,
an increase in the fl uidity of membrane shifted the cold-inducible expression of desC gene towards
higher temperatures by 2°C. The enhanced level of expression of desaturase genes and consequently
elevated levels of unsaturated fatty acids at the low temperature has been explained to be due to
negative temperature coeffi cients of the desaturases. That is why desaturases can be more active at
lower temperatures (Murata and Wada, 1995).
The regulatory role of light has been emphasized in the induction of acyl-lipid desaturases in
Synechocystis sp. strain PCC 6803 (Kis et al ., 1998). Cells grown photoautotrophically at 25°C in normal
(70 µmol m -2 s -1 ), moderately strong (500 µmol m -2 s -1 ) and strong (2000 µmol m -2 s -1 ) light did not
show any difference in the level of fatty acid unsaturation. Light-activated heterotrophic growth (with
5 mM glucose in dark at 25°C and illuminated once daily for 10 min with a light intensity of 40 µmol
m -2 s -1 ) supported a 2-fold increase in monounsaturated fatty acids at the expense of polyunsaturated
fatty acids. When compared to photoautotrophically grown cultures where the level of 18:1 fatty acid
was 7%, in dark grown cultures these fatty acids increased to 17%. More signifi cantly, in dark grown
cultures, the level of α 18:3 and 18:4 fatty acid levels were reduced to traces. Temperature or light
did not infl uence the level of desC gene transcripts for Δ 9 desaturase as it seems to be constitutively
expressed at 25°C with or without illumination. The expression of desB gene in dark-grown cells
was negligible but in light the mRNA of this gene increased by 10-fold reaching a maximum level
within 15 min. The level of mRNAs for desA and desD were higher than the transcripts of desB but
the expression of the former two in light increased by 10-fold. When photoaututrophically grown
cultures at 35°C were transferred to 25°C in dark, the induction of des genes did not occur. It means
the low-temperature-induction was blocked in the dark.
Studies of Yin et al . (2007) showed that the challenge from chill plus light stress could be overcome
in Synechocystis sp. strain PCC 6803 by the presence of a gene sll1242 named as ccr - 1 ( c yanobacterial
cold resistant) gene. Synechocystis sp. strain PCC 6803 can survive for >2 months at 5°C in the
absence of light but in presence of light (100 µmol photons m -2 s -1 ) at 5°C the cells completely lose
their viability within 10 days. The tolerance of a strain to 5°C in presence (100 µmol photons m -2
s -1 ) or absence of light is determined on the basis of its ability to reinitiate growth (ARG) after an
exposure for 5 days. Photoautotrophic and mixotrophic (in presence of glucose) growth of such
strains was assayed at a photosynthetic photon fl ux of 30 µmol photons m -2 s -1 light intensity at
30°C. From a random insertion mutant library (6000 clones) of Synechocystis sp. strain PCC 6803, 14
mutants have been isolated that exhibited increased chill-light sensitivity with an ARG of less than
20% of the wild-type. Inverse PCR and sequencing confi rmed the presence of an interrupted gene
sequences in six mutants, out of which mutant sll1242 was selected for a further study. This mutant
showed increased sensitivity to chill plus light and was unable to grow photoautotrophically and
mixotrophically at 15°C whereas the wild-type could do so at different temperatures (30, 20 and 15°C).
The photosynthetic effi ciency of the mutant sll1242 greatly decreased. Complementation of the wild-
type gene sequence in the mutant sll1242 confi rmed the role of this gene in maintaining chill plus light
tolerance. Another mutant with interrupted gene sequence for desD marker designated as desD :C.K
2 also showed increased sensitivity to chill-light stress. After a chill-light stress for 5 days, the ARG
value of this mutant decreased to 5.6% of that of wild-type. Complementation of the mutant by desD
gene increased the viability and the ARG reached up to 113.6%. This probably refl ects on the role of