STRESS RESPONSES - Cyanobacteria - page 712

long term desiccation (17 days) led to extensive proteolysis and the dissociation of phycobilisomes
took place in presence of light or in its absence (Potts, 1985). Nitrogen fi xation appeared to be quite
sensitive even for short periods of drying (Potts et al ., 1984) but the enzyme nitrogenase reductase
( nifH product; Fe-protein of nitrogenase) remained intact in immobilized, dehydrated and rehydrated
cells of N . commune as revealed by immunoblotting (Peat et al ., 1988). During rehydration N . muscorum
showed an instantaneous increase in the size of intracellular ATP pool which was inhibited by sodium
azide (10 mmol/l) and carbonyl cyanide m-chlorophenylhydrazone (2 µmol/l) (Potts and Morrison,
1986). Although rapid cessation of nitrogen fi xation occurred upon dehydration of N . commune cells
(Potts et al ., 1984; Potts and Bowman, 1985), the fact that the presence of Fe-protein component of
nitrogenase in a 10 year fi eld-desiccated material of N . commune (Peat et al ., 1988) signifi es that the
enzyme components of nitrogen fi xation appear to be intact during extended periods of desiccation.
The resumption of the processes of photosythesis, respiration and nitrogen fi xation was noted
after specifi c lag periods during rehydration (Scherer et al ., 1984). The polysome turnover in the
immobilized and desiccated cells (drying at -99.5 MPa for 2 h) of N . commune continued to persist
but declined thereafter during long periods of desiccation (Angeloni and Potts, 1986). Thus the
basic enzyme machinery inside the cells remains intact as evidenced by the quick regaining of vital
metabolic processes upon rehydration. The stability of certain proteins even during extended periods
of desiccation has been confi rmed in the desiccated fi eld materials of N . commune as well as in its
clonal isolate N . commune UTEX 584 (Scherer and Potts, 1989; Hill et al ., 1994). The lipid biosynthesis in
the rehydrated cells of N . commune was instantaneous with the appearance of steady-state radiolabel
in PG and SQDG within the fi rst few min whereas the abundant glycolipids MGDG and DGDG
required 2 h time. The lag period was associated with the incorporation of 35 SO 3 and the functional
nature of the proteins required for uptake and phosphorylation of glycerol, uptake of sulphate and
phosphate (Taranto et al ., 1993). The rpoC1C2 gene transcripts governing the synthesis of the two
subunits of RNA polymerase (γ and β') readily disappeared in dehydrated cells of N . commune UTEX
584 but their levels attained to control levels within 60 min of rehydration. This was mainly due to
the presence of RNA polymerase enzyme in a stable form during long-term desiccation as shown
by immunoblotting analyses of protein extracts (Xie et al ., 1995). All the above studies point out that
the proteins and metabolic potential of the cyanobacterial cells is retained during desiccation and
the desiccated cells represent 'simply a collection of dried components' (Potts, 1994).
The damages caused to DNA during desiccation may be through chemical modifi cations
(alkylation or oxidation), cross-linking or base removal such as depurination. Based on the
corollary evidences of variations in the pH of internal cellular compartments and the rate of in vitro
depurination of DNA, it was estimated that Nostoc DNA would achieve 1% depurination of the
genome after storage at 37ºC for 10 years (Potts, 1994). But in actual fi eld conditions the organism
faces extremely high temperatures and it is expected that the depurination rates must be higher
than those mentioned. The presence of signifi cant amounts of 6-methyladenine in the genome of N .
commune has been reported but these residues are lost more readily than other purines (Jägger and
Potts, 1988). The DNA of laboratory grown N . commune UTEX 584 showed light-dependent nicking
for short periods when the cells were dried that lacked the normal capsular polysaccharide (Stulp
and Potts, 1987). However, Shirkey et al. (2003) pointed out that the DNA from 13 year desiccated
N . commune cells is devoid of the oxidatively modifi ed DNA biomarkers such as 8-hydroxyguanine,
8-hydroxyadenine and 5-hydroxyuracil. Fifteen samples of form species of N . commune (obtained
from different Herbaria in a desiccated state for varying periods, i.e. 26-149 years) were included
in this study and fi ve form species, i.e. WH002 (64 years dry), WH0014 (55 years dry), ALD 8122 (29
years dry), TOP 1993 (9 years dry) and ENG 1996 (6 years dry) showed viability. The genomic DNA