STRESS RESPONSES - Cyanobacteria - page 644

resulted in the desaturation at ω 3 position due to which the accumulation of 18:3Δ 9,12,15 and 18:4Δ 6,9,12,15
acids was noted (Wada and Murata, 1990).
Wada and Murata (1989) fi rst isolated mutants of Synechocystis sp. strain PCC 6803 defective in
fatty acid desaturases (FAD) at the 6th and 12th positions and designated them as Fad6 and Fad12,
respectively. These mutants exhibited growth to the same extent as that of the wild-type at 34°C but
they differed in their growth rate at 22°C. The growth rate of Fad12 was much lower than wild-type
whereas Fad6 grew to the same rate as that of the wild-type. Fad6 mutant lacked polyunsaturated
fatty acids 18:3Δ 6,9,12 whereas Fad12 lacked 18:2Δ 9,12 and 18:3Δ 6,9,12 but contained low levels of 18:2Δ 6,9
fatty acids. Since Fad6 mutant grew to the same extent as that of the wild-type at 22°C, the absence
of 18:3 fatty acid did not make any difference but at the same time due to low levels of synthesis of
18:2 fatty acid in Fad12 the growth was lower than wild-type. So the importance of 18:2 for growth
at low temperature has been recognized. The desA gene from Synechocystis sp. strain PCC 6803 has
been cloned and used to transform S. elongatus PCC 7942 (which possesses only Δ 9 desaturase). The
transformants acquired the ability to desaturate at Δ 12 position also and synthesized 16:2Δ 9,12 and
18:2Δ 9,12 fatty acids and exhibited more cold tolerance (Wada et al ., 1990, 1994). The transformant
desA + cells also appeared to be more tolerant to photoinhibition (Gombos et al ., 1997). The fact that
Synechocystis sp. strain PCC 6803 can synthesize mono-, di- and triunsaturated fatty acids in its
lipids (Wada and Murata, 1989) has been further exploited by the isolation of mutants defi cient
in desaturases (see Table 4, Chapter 2 for details of number and types of genes of desaturases in
various cyanobacteria). Mutant Fad6, defective in desaturation of fatty acids at the Δ 6 position,
synthesized only mono- and di-unsaturated fatty acids (Wada and Murata, 1989). This mutant was
transformed by an interrupted gene sequence of desA with kanamycin resistance marker ( Km r ) and
the transformant was designated as Fad6/ desA : Km r (Wada et al ., 1992). The transformants defective
in desaturation at the Δ 6 and Δ 12 positions produced only monounsaturated fatty acids in its lipids.
A comparison of the fatty acid profi les of wild-type, Fad6 and Fad6/ desA : Km r revealed that the
content of saturated fatty acids (16:0) were 58, 59 and 57 mol% , respectively. While Fad6 showed 11
mol % of monounsaturated fatty acid 18:1Δ 9 and 25 mol% of diunsaturated fatty acid 18:2Δ 9,12 , the
wild-type showed 7 mol% and 12 mol% of the respective mono- and disaturated fatty acids. The
transformant Fad6/ desA : Km r produced higher levels of monounsaturated fatty acid up to 41 mol%
and di- and polyunsaturated fatty acids were completely absent. At low temperature, Fad6 and
transformant Fad6/ desA : Km r experienced severe impairment in growth due to a phenomenon termed
as low temperature photoinhibition (Gombos et al ., 1992). Likewise, Wada et al . (1994) transformed
A . nidulans R-2 SPc with desA gene from Synechocystis sp. strain PCC 6803 and demonstrated that
the transformants of A . nidulans acquired the capability to introduce a second double bond into
palmitoleic (16:1Δ 9 ) and oleic (18:1Δ 9 ) acids at the Δ 12 position and at the same time showed tolerance
to chilling stress.
Tasaka et al . (1996) isolated a series of mutants of Synechocystis sp. strain PCC 6803 defective in
acyl-lipid desaturases and presented evidences in support for the role of polyunsaturated membrane
lipids in growth, respiration and photosynthesis. Mutants defi cient in Δ 12 desaturase ( desA gene) and
Δ 6 desaturase ( desD gene) showed increased levels of monounsaturated oleic acid at the expense of
polyunsaturated fatty acids such as di-, tri- and tetraunsaturated fatty acids. The growth of these
mutants was similar to that of the wild-type at 35°C but they did not grow well at 25°C. The desA - /
desD - mutants were unable to grow at 15°C probably due to their non-recovery from photo-induced
damage to the PSII complex. Their inability to process precursor of D1 protein might be responsible
for a short supply of mature D1 protein which is an essential component of the reaction center of
the PSII complex. The transformants of S. elongatus PCC 7942 with desA gene from Synechocystis