STRESS RESPONSES - Cyanobacteria - page 648

18:3 fatty acids in chill-light tolerance. The essentiality of α-tocopherol in tolerating chill-light stress
has ben identifi ed by subjecting the mutants ( slr1736 , slr1737 , slr0089 , slr0090) unable to synthesize
α-tocopherol to chill-light stress. Acquired chill-light tolerance (ACLT) is a property that is exhibited
by the wild-type when the cells are pre-conditioned at a temperature of 15°C. This pre-conditioning
enhances considerably the cellular levels of α-tocopherol. The wild-type and the mutants exhibited
almost the same growth pattern when subjected to chill in dark but when subjected to chill in light
the mutants completely lost the ACLT. Complementation of the slr0089 mutant with the wild-type
gene regained the ACLT thus showing the essential nature of α-tocopherol in conferring chill-light
tolerance to Synechocystis sp. strain PCC 6803 (Yang et al ., 2008).
iii) Microarray studies : Suzuki et al . (2000 a,b) carried out a systematic disruption of putative genes
for Hiks and identifi ed Hik33 ( hik33 ) as a regulator of the cold-inducible expression of desB gene
in a strain that carried a reporter gene for bacterial luciferase. Genome-wide microarrays revealed
that close to 50 genes are strongly induced in Synechocystis sp. strain PCC 6803 under cold stress. In
addition to these, several other genes that are induced at low temperature belong to genes of known
function, those responsible for oxidative stress and several others of unkown function. The important
genes of known function that are induced at low temperature are genes for: (i) RNA polymerase ( rpo );
(ii) sigma factor D ( sigD ); (iii) elongation factor EF-G ( fus ); (iv) high-light-inducible proteins ( hliA ,
hliB , hliC ); (iv) subunit 4 of NADH dehydrogenase and (v) alternative form of cytochrome c ( cytM )
(Suzuki et al ., 2001). The number of genes under the control of Hik33 were found to be 23 out of 38
highly cold-inducible genes. It means the rest 15 of the 38 cold-inducible genes were not regulated
by Hik33. So it was suggested that Synechocystis sp. strain PCC 6803 might possess another sensor
or pathway for transduction of the low-temperature signal (Suzuki et al. , 2001). At this juncture, it
is important to note that the Hik33 contains two transmembrane domains and several conserved
domains, i.e. a HAMP-linker, a leucine zipper, a PAS domain and a histidine kinase domain (Williams
and Stewart, 1999; Taylor and Zuhlin, 1999). The HAMP-linker possesses two helical regions that are
responsible for transducing the stress signals via intramolecular structural changes between the two
helical regions and leads to the intermolecular dimerization of the protein (Williams and Stewart,
1999; Arvind and Ponting, 1999; Arvind et al ., 2003). Inaba et al . (2003) conducted fourier transform
infrared (FTIR) spectrometry of the cytoplasmic membranes of the double mutant desA - /desD - of
Synechocystis sp. strain PCC 6803 (isolated by Tasaka et al .,1996) and showed that due to double
mutation the cytoplasmic membrane acquired more rigidity than those from the wild-type cells
at 22°C. Microarray analysis of cold-inducible genes in wild-type and the double mutant enabled
them to recognize three groups of genes. The fi rst group consisted of those genes that were induced
by cold in the wild-type but were strongly cold-inducible in the desA - /desD - cells. Included in this
group are certain heat shock genes ( hspA , dnaK2 , and clpB2 ), genes of the sulphate transport system
( sbpA , cysA , cysT and cysW ), RNA polymerase sigma factor ( sigB ), a histidine kinase ( hik34 ) and a
gene for a putative penicillin-binding protein ( psp ). The second group consisted of genes whose
cold-inducibility is moderately enhanced by the double mutation and these are genes for high-light-
inducible proteins ( hli genes) and some genes of unknown function. The cold-inducibility of the third
group of genes was unaffected by the double mutation and included here are cold shock genes for
RNA-binding protein (such as rbp1 ) and cyanobacterial RNA helicase ( crhL ). Maximum induction
of hspA and dnaK2 was noted in desA - /desD - mutant cells at 24°C but wild-type cells did not show
such induction. The cold-inducibility of rbp1 (25°C) and crhL (~20°C) was unaffected in the double
mutant. The role of hik33 as a membrane sensor to detect a cold stress signal by sensing the rigidity of
cytoplasmic membrane has been evaluated by the generation of another mutant by the inactivation