STRESS RESPONSES - Cyanobacteria - page 646

that the level of desD mRNA increased 10-fold within 15 min of a temperature shift-down and
the level of the Δ 6 desaturase enzyme doubled within 4 h of the start of cold treatment. The desB
gene, that encodes ω 3 desaturase, responded more quickly to the temperature shift-down as
the transcripts started appearing within 10 min and the mRNA levels increased 15-fold higher.
This has been ascribed to be due to enhanced transcription as well as to the stabilization of
the transcript. The desB mRNA levels gradually declined when the cells were shifted to high
temperature (Los and Murata, 1994; Los et al ., 1997). Sakamoto and Bryant (1997) demonstrated cold-
inducibility of the desC gene in Synechococcus sp. strain 7002. When the cultures of this cyanobacterium
were shifted from 38°C to 22°C, the level of desC mRNA increased within 5 min. The desC gene, that
encodes Δ 9 desaturase, in S. elongatus PCC 7942 has been shown to be induced within 30 min after
a temperature shift from 36°C to 24°C (Ishizaki-Nishizawa et al ., 1996). Other important features of
des gene expression in Synechocystis sp. strain PCC 6803 are: (i) transcription start sites for the desA ,
desB and desD genes are located at -114,-35, and -347 bp positions at both 34°C and 22°C; (ii) RNA
polymerase utilizes the same promoters at both temperatures and (iii) the existence of a consensus
sequence of 5'-GTTTGTTTT-3' downstream the start sites for the cold-inducible desaturases.
The expression of three desaturase genes ( desA , Δ 9 desaturase; desB , ω 3 or Δ 15 desaturase and
desC , Δ 9 desaturase) and the fatty acid composition were compared at two growth temperatures
(38°C and 22°C) in two wild-type strains of the marine cyanobacterium Synechococcus sp. strain
PCC 7002 (NIBB and PR6000). The levels of mRNA of the three genes increased within 15 min after
a temperature shift-down to 22°C but these soon disappeared after a shift-up to 38°C. The overall
fatty acid composition of the cells gown at 38°C was identical at low (50 µE m -2 s -1 ) and high light
(250 µE m -2 s -1 ) intensities. However, ω 3 desaturation increased in cells grown at both light intensities
at only 22°C. This signifi es that fatty acid desaturation at the ω 3 position is regulated by the low
temperature-dependent expression of desB gene. Based on the above results, Sakamoto et al . (1997)
concluded that the rate-limiting step for increasing the desaturation of membrane lipids at the low
temperature is post-transcriptional.
From the above discussion it is clear that a lowering of growth temperature increases the fl uidity
of the cytoplasmic membrane due to the synthesis of polyunsaturated fatty acids. What exactly is
the threshold limit for perception of a decrease in temperature? A comparative study of Synechocystis
sp. strain PCC 6803 grown at two different temperatures (36°C and 32°C) for fatty acid content and
expression of desA gene revealed that the cells grown at 32°C contained more unsaturated fatty
acids than the cells grown at 36°C. This suggests that the fl uidity of membranes has increased at
32°C. When cells grown at 36°C were shifted to 28°C, the expression of desA gene was not noted
whereas in the cells grown at 32°C, the expression of desA gene occurred at 26°C. It means the cells of
Synechocystis sp. strain PCC 6803 were able to sense a change in temperature only when it exceeded
6°C (Los et al ., 1993). Vigh et al . (1993) examined the effects of hydrogenation of cells of Synechocystis
sp. strain PCC 6803 grown at 36°C for 4 min. This treatment converted 5% of the unsaturated fatty
acids to saturated fatty acids in the cell membrane but not in the thylakoid membranes. As a result,
the membrane fl uidity decreased and the desA gene expression took place at 30°C instead of 28°C.
It means hydrogenation of lipids in plasma membrane increased the threshold temperature for the
expression of desA gene by 2°C. Likewise, a temperature-dependent expression of the desB gene
was compared between the wild-type and desA - /desD - mutant of Synechocystis sp. strain PCC 6803.
Wild-type cells grown at 36°C showed expression of desB gene at 28°C whereas desB gene was
expressed in mutant desA - /desD - cells at 32°C. Since the mutant contained only monounsaturated
fatty acids, the fl uidity of the cytoplasmic membrane decreased and the cold-induced expression of
desB gene was shifted toward a lower temperature by 5°C (Los and Murata, 1999). The cold-inducible