STRESS RESPONSES - Cyanobacteria - page 716

RPS in cultures exhibited variations as for example N . fl agelliforme possessed mannose, galactose,
glucose and glucoronic acid but no arabonise whereas the RPS of N . sphaeroides and N . commune
contained glucose (the major component), rhamnose, fucose, xylose, mannose and galactose. The
linkage analysis of the EPS of the fi eld samples showed that xylose, glucose and galactose are
1-4 linked, mannose, galactose and xylose are present as terminal residues and the branch points
occurred in glucose (1-3,4 and 1-3,6 linkages) and xylose (1-3,4 linkage). On the other hand, Helm et
al . (2000) reported the presence of glucose, galactose, xylose and nosturonic acid in the ratio of 2:1:1:1,
respectively in the EPS of fi eld sample of N . commune DRH1. Although the separation into individual
monosaccharides was not possible, they suggested the presence of a pentamer as a repeating unit
with a disaccharide (of glucose and xylose) and trisaccharide (β-D-nosturonic acid, β-D-Glucose and
D-Galactose) present in 1:1 molar ratio. The glycan matrix is unique in possessing abundant water
stress proteins (Wsp), a UV-A/B absorbing pigment, oxidized and reduced forms of the pigment
scytonemin and two glycoproteins (75 kDa and 110 kDa). A carbohydrate-modifying enzyme
(68 kDa) is the second abundant protein after Wsp (Hill et al ., 1994a,b). However, the glycan sheath of
N . commune with its unique properties responsible for conferring tolerance to environmental stresses
is also liable to bacterial degradation. A new species of Paenibacillus ( P . glycanilyticus ) which utilizes
rhamnose and L-fucose has the capability to degrade the glycan of N . commune (Dasman et al ., 2002a).
The purifi ed enzyme (128.5 kDa) has been found to be unique as its N-terminal amino acid sequence
did not bear any resemblance to hitherto known polysaccharide degrading enzymes. Complete
degradation of N . commune polysaccharide into tetra and hexasaccharides took place at an optimum
pH and temperature of 5.5 and 35ºC, respectively (Dasman et al ., 2002b).
An unusual water fl ux in the EPS of the desiccated colonies of N . commune (collected from
Topasail Island, North Carolina, U.S.A.) was noted. Desiccated colonies were rehydrated (in light
of 150 µmol of photons m -2 s -1 at 25ºC) for different intervals of time and the increase in weight
was monitored till the time there was no further increase (equilibrium weight). During the fi rst
rehydration cycle it took 30 hr for attaining equilibrium weight. The same colonies were allowed to
desiccate (at 24.5ºC and 25% relative humidity) to attain an internal water cotent of 0.06 g H 2 O g -1
dry weight of the colonies which closely corresponded with the initial dry weight of the colonies
when they were collected. In the second and third rehydration cycles it required 12 h and 3 h for
attaining the equilibrium weight, respectively. The increase in water content contributed to 20-40
fold increase in their weight as they attained equilibrium weight (Shaw et al ., 2003).
The accumulation of a novel Wsp inside the desiccated cells of N . commune (Scherer and Potts,
1989), its secretion into the EPS with UV-A/B absorbing pigments (Hill et al ., 1994) and the release
of Fe-SOD upon rehydration of N . commune CHEN/1986 (Shirkey et al ., 2000) have been reported
to play a role in desiccation tolerance. The existence of three of the Wsps (33, 37 and 39 kDa) with
similar consensus N-terminal sequences (Ala-Leu-Tyr-Gly-Tyr-Thr-Ile-Gly-Glu) indicated that the
variation in their molecular masses resulted due to proteolysis of the 39-kDa Wsp. The presence
of the Wsp in the desiccated cells of the fi eld materials of N . commune and the synthesis of Wsp
(39 kDa) in the laboratory grown colonies subjected to drying go in favour of its suggested role in
desiccation. The fi eld materials, collected from China, Europe, North America, Aldabra Atoll (Indian
Ocean) and Antarctica, desiccated (for years) revealed a similar pattern of Wsp on Western blots
(Scherer and Potts, 1989). The presence of the N-terminal consensus sequence and another internal
sequence (Glu-Ala-Arg-Val-Thr-Gly-Pro-Thr-Thr-Pro-Ile-Asp) in the Wsps bring them closer to
carbohydrate-modifying enzymes. The presence of 1, 4-β-D-xylan-xylanohydrolase activity in the
purifi ed Wsps and the ionic interactions between the purifi ed Wsp and and a xylan-containing UV-
A/B absorbing pigment suggest that Wsp plays a role in the synthesis of the latter (Hill et al ., 1994).