STRESS RESPONSES - Cyanobacteria - page 640

small, heat-stable oxidoreductases that function as alternative hydrogen donor in the reduction
of intramolecular disulfi de in ribonucleotide reductase, the essential enzyme for DNA synthesis.
First discovered in E . coli (Grx1 and Grx3) and in yeast (Grx1 and Grx2), these three Grxs are small
proteins with a molecular weight of 10 kDa and contain an active site of CPYC. A second group of
Grxs comprising of Grx3, Grx4 and Grx5 with an active site of GFS corresponds to yeast. The Grxs
found in Synechocystis share a high identity to the Grxs from other organisms. The mRNA levels of
Grx2 of Synechocystis increased under oxidative stress conditions caused by high salinity, chilling
or the addition of H 2 O 2 , methylviolgen or t-butylhydroperoxide. The overexpression of the gene
( ssr2061 ) of Synechocystis in E . coli JM109 produced Grx2 that protected the cells of the latter to salt
stess greater than 700 mM.
II. TEMPERATURE STRESS
The optimum growth temperature of mesophilic prokaryotes ranges between 25°C to 40°C. Beyond
this the mesophilic prokaryotes are unable to grow and so this temperature seems to be demarcation
point for mesophiles and those that are equipped to grow at temperatures above 40°C. These are
known as thermophiles which have an optimum temperature range of growth between 50°C
and 70°C while at the same time some are able to grow albeit slowly at 40°C as well. Examples of
thermophiles are found among eubacteria, actinomycetes, fungi, protozoa and algae. Few other
eubacteria and archaebacteria that are able to grow at still higher temperatures (between 80°C and
110°C) are known as hyperthermophiles. The hyperthermophiles are unable to survive below 60°C.
Due to a “shift-down” or “shift-up” of the microbial cells from their optimum growth temperature,
they experience a cold shock or heat shock, respectively.
A) Cold shock: A sudden shift-down in the growth temperature results in a cold shock leading to a
transient cessation of growth. During this period, the general protein synthesis is severely inhibited
and is accompanied by the expression of a group of well defi ned proteins. Among these, cold shock
family of proteins (Csps), low temperature protein chaperones such as GroEL, GroES, caseinolytic
proteases (Clps), RNA binding proteins (Rbps) and RNA helicases are important in bacteria (Weber
and Marahiel, 2003; Gualerzi et al. , 2003; Phadtare and Severinov, 2010). Eventually, the synthesis
of these proeins also decreases, followed by a phase of acclimatization to the low temperature and
growth resumes. In contrast to heat shock, cold shock response appears to be predominantly post-
transcriptional as no cold-specifi c σ factor has so far been identifi ed. In E . coli many of the proteins
that are up-regulated during cold shock are associated with the translational apparatus (Phadtare,
2004; Nierhaus and Wilson, 2004). The perceptible effects of cold shock are felt at different levels in
the cells starting with a decrease in membrane fl uidity (thus affecting active transport and protein
secretion), stabilization of secondary structures of RNA and DNA (with a consequential decrease in
translation and transcription) to slow or ineffi cient folding of proteins and fi nally to the cold-adapted
functionality of the structure of ribosomes.
Studies on the responses of cyanobacteria to cold stress (Murata and Wada, 1995; Murata and Los,
1997; Los and Murata, 1998, 1999, 2000, 2004; Prakash et al ., 2010; Los et al ., 2010) have contributed to
our understanding of the following six areas. These are as follows: (i) fatty acid desaturases that are
responsible for adjustments in membrane fl uidity; (ii) Rbps that probably function as RNA chaperones
(similar to the Csps of E . coli and Bacillus subtilis ); (iii) RNA helicases that help in the removal of
secondary structures of mRNAs and improve effi ciency in translation at the low temperature; (iv)
adjustments in the structure of ribosomal proteins so that the translation effi ciency is improved at