STRESS RESPONSES - Cyanobacteria - page 718

among the hydrocarbons. As a result of this, the temperature required to melt these chains (T m )
increases. For example, phosphatidylcholine of egg has a transition temperature of -7ºC when
hydrated and the T m increases to 70ºC when it is fully dehydrated. It means the phosphatidylcholine
of egg is in gel phase at room temperature when it is dry but it will pass through a phase transition
when rehydrated. When the lipid bilayers pass through such phase transitions during rehydration
the membranes tend to become leaky. This is the real challenge faced by the cyanobacterial cells
when desiccated cells are rehydrated. However, when phosphatidylcholine of egg is dehydrated
in presence of disaccharides such as sucrose or trehalose, the T m is considerably lowered to -20ºC
but at the same time when rehydrated it will not pass through a phase transition as it is already in
a liquid crystalline phase.
Studies conducted on liposomes in vitro have been extended to intact cells (Crowe et al ., 1989;
Hoekstra et al ., 1992; Leslie et al ., 1994, 1995; Crowe et al ., 2002). A number of studies conducted
on sucrose, trehalose, glucose and the polymer hydroxyethyl starch (HES) established that these
substances often form glasses (i.e. vitrify) at low water contents and their ability to hold the liposomes
intact and protect them from leakage and fusion very much depended on the combination of two
of these substances rather than each of them individually. Temperatures at which glass formation
takes place, i.e. T g is found to be directly proportional with the molecular weight and decreasing
water content. For example, HES (molecular weight 500,000) though is a good glass former (T g is
more than 150ºC) could not prevent leakage from unilamellar vesicles during drying or rehydration
(because of its inability to interact with the polar head groups) but HES when combined with glucose
which can directly interact with lipid head groups (in 1:1 ratio) effectively formed a stable glass
that inhibited fusion and leakage (Crowe et al ., 1997). When we try to apply these to intact cells,
the formation of stable carbohydrate glass by sucrose and trehalose may be expected as these are
present in signifi cantly high amounts in desiccated cells. As T g in case of trehalose is higher than
sucrose and also due to conversion of certain amounts of trehalose to its crystalline dihydrate form
during absorption of moisture renders the glassy sugar to have exceptionally high T g (Crowe et al .,
2002). Crowe et al . (2002) summarized evidences that go in favour of stabilizing effects of sucrose
and trehalose in desiccated cells and that 'these sugars in the cytoplasm may be both necessary and
suffi cient for anhydrobiosis'. They highlighted the utility of these sugars in maintaing the mammalian
cells in a dry state that has profound use in human clinical medicine. Increased levels of trehalose
production during desiccation, heat shock, cold shock and oxidative stress and the ability of trehalose
to protect proteins and cell membranes from the damages caused by the aforesaid stresses opened
up new vistas to understand the multifunctional role of trehalose (Elbein et al ., 2003). Lipid soluble
extracts of desiccated cells of N . commune revealed the presence of trehalose and sucrose and these
were not detectable after rehydration. The presence of trehalose inside the cells was confi rmed by
immunoblotting (Hill et al ., 1994). Moreover, Hill et al . (1999) demonstrated that a low concentration
of sucrose and trehalose mixture (equivalent to the concentrations present inside the cells of N .
commune DRH1) with EPS in small amounts prevented the leakage of carboxyfl uoroscein from
membrane vesicles.
v) Trehalose synthesis: Avonce et al . (2006) outlined fi ve main pathways for trehalose biosynthesis
(see Fig. 5). The pathway of trehalose synthesis reported in thermophilic archaebacterium Sulfolobus ,
known as Tre Y/Tre Z pathway, involves two steps. In the fi rst step, the terminal glucose subunit at
the reduced end of starch or glycogen is transglycosylated from a α 1-α 4 to α 1-α 1 bond resulting in
the formation of maltooligosyl trehalose mediated by the enzyme maltooligosyl trehalose synthase
(Tre Y or Mts). In the second step, maltooligosyl trehalose with its terminal trehalose is hydrolysed