Term Paper: Desiccation Tolerance in Prokaryotes

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[. . .] It is situated in the extra cellular polysaccharide sheath with a molecular mass of 544 Da and a structure based on indolic and phenolic subunits and is an optically inactive dimeric pigment. [Role of Lipids and fatty acids in stress tolerance]

Scytonemin has been projected to serve as an ultraviolet sunscreen and has an absorption maximum at 386nm. Scytonemin remains highly constant and carries out its screening activity without further metabolic investment from the cells, once it is fused. By its long perseverance in terrestrial cyanobacterial crusts or dried mats, it is obvious that rapid photo degradation of scytonemin does not take place. This approach may be priceless to several scytonemin-containing cyanobacteria which must live on long periods of metabolic inactivity and occupying harsh habitats and subject to regular cycles of desiccation and rewitting. Thus the means of desiccation tolerance by lipids and fatty acids in cyanobacteria seems that it cannot be easily resolved through genetic analysis and is of complex interactions. [Role of Lipids and fatty acids in stress tolerance]

As in this system existence of dehydration is not easily predicted and direct effects of a disaccharide on physical properties of dry tissue cannot be noticed readily, an effort to develop the water replacement hypotheses approach in plants was challenged. The issue was made even more complex by the presence of trehalases in plants. To address these problems, a study was made using desiccation-sensitive Escherichia coli as a model. Desiccation and freeze-drying are acute stresses; they have an effect on gene expression and gene regulation markedly, and multiple targets are damaged, are the results of this study. There are different features that affect the inactivation of dried cells and these factors include the growth phase, cell concentration, the drying method and the storage conditions.

The rates of endurance for their untransformed counterparts are higher when compared to the rates of survival for recombinant strains of Escherichis coli following desiccation. Our main aim was, in order to test the water replacement hypothesis fuse sucrose in desiccation sensitive cells. It has been verified that in vivo sucrose synthesis and a marked protective effect in Eshcerichis coli that is sensitive to drying in air. It has also been seen that under conditions any intrinsic ability of the cells to offer some resistance to drying was reduced. Sucrose and trehalose guard membranes by maintaining the dry membranes in a physical state in the same way to that of fully hydrated membranes and by depressing the Tm of the phase transition when water is removed from the phospholipids bilayer.

From the FTIR date it is evident that the membrane phospholipids are protected by sucrose. The irregularity of the melting curve indicates insufficiency of sucrose to override the Tm of all the phospholipids. About 5.6 x 107 molecules of trehalose per cell are required to saturate the interphospholipid spaces especially in the experiments where extra cellular trehalose was used to protect Escherichia Coli from the effects of freeze drying. This parameter is almost equal to the value calculated for intracellular content of sucrose. Trehalose yet is supposed to be more efficient in protecting membranes from desiccation and freeze drying. The sucrose comes in contact only with one of the two layers. It has been observed that the P = O stretch value is increasing even though it depressed after heating. [Engineering desiccation tolerance in Escherichia coli]

The desiccation effect thus is more prominent in case of the cells under light than upon the cells in dark due to the reactions of photooxide catalysed by residual water. In proteins or in DNA the level of enzyme activity is noticed to be damaged. The residual water go on competing with sucrose for sites in the phospholipids membrane in drying cells at 200 MPa and said to have an effect on light sensitivity of the cells. Storage of sucrose over phosphorus pent oxide causes total desiccation of the membranes enhancing its protective effect. The differential sensitivity caused by light is also being reduced by the complete desiccation. Cells after desiccation, freezing and rehydration are fully recovered by the non-reducing sugars available in vivo and in vitro along with 100mM trehalose. High density clones like clones in gene libraries can be formed and stored by such immobilization of cells. Other cell lines like eukaryote cell lines cab be stabilized in vivo synthesis of sucrose. [Engineering desiccation tolerance in Escherichia coli]

Two types of tolerance can be said to have occurred judging the response to the level of residual water. 'Drought tolerance" is tolerance to the moderate dehydration up to the extent when no bulk cytoplasm water exists. 'Desiccation tolerance' is tolerance of further dehydration causing gradual loss of shell molecules. Desiccation tolerance also succeeds the cells to again hydrate them. Anhydrobiosis sometime alleviates the adversities. Living matters depends on two processes namely Biosynthesis of the appropriate molecules and their assembly in to organized structures. The hydrophobic effect is very important of the organization of cells. Water therefore an important media for assembly of phospholipids into biological membranes and formation of many proteins. This media for organization cells is totally lost when it completely dissipates from the living matter. It caused the membranes to go on a structural change and denature of proteins. [Mechanisms of plant desiccation tolerance]

However some organisms tolerate the severe desiccation proving the fact that mechanisms has existed in the nature that maintains the cellular structures without water. This mechanism differs with the availability of bulk water. The mechanisms in the cells catering to Drought tolerance are based on structural stabilization by preferential hydration, while desiccation tolerance mechanism that replaces water by molecules forming hydrogen bonds. The anhydrobiotes during the process of dehydration has to undergo stages of hydration ranges necessary for protection against drought. The Desiccation tolerance is widely prevalent in the plant kingdom including ferns, mosses and their spores; pollen and seeds of higher plants of course not in gymnosperm, plants. Prokaryotes, protests, fungi and animals like tardigrades, nematodes and crustaceans also exhibit this feature. [Mechanisms of plant desiccation tolerance]

The Drought and Desiccation tolerance are similar in the sense that significant amount of non-reducing di -- and oligosaccharides, compatible solutes and specific proteins such as the late embryogenesis abundant proteins and heat shock proteins are available in both the cases. The process of desiccation tolerance can be started by dehydration and the plant hormone abscisic acid. In the plant tissues of mosses, ferns and angiosperm resurrection plants, sometimes reduction of water level to a slightest extent activates gene expression associated with desiccation tolerance. The gene expression is arises during development as a part of maturation programme in anhydrobiotic seeds. This makes seed embryos capable of desiccation tolerant much before maturation drying.

Since the cellular water potential remains constant up to maturation drying the decrease in water level during the process of seed maturation is not taken as dehydration. Due to consequential accumulation of the dry matter only the water level decreases at this moment. This is an indication of beginning of desiccation tolerance in developing seeds through abscisic acid. The abscisic acid also guards against premature germination. The desiccation tolerance in seeds can also be pursued by premature slow drying, which results in dwarf anhydrobiotic embryos. The cells in the pro-embryo attain the capability of desiccation tolerance though the embryos do not grow the germination-competent stage. Cellular volume reduction due to dehydration results in increasing concentration of cytoplasmic elements and contents of the cell becomes increasingly viscous. This is more conducive for molecular interactions, protein denaturation and membrane fusion. [Mechanisms of plant desiccation tolerance] broad range of compounds like praline, glutamate, glycine-betaine carnitine, mannitol, sorbitol, fructans, polyols, trehalose, sucrose and oligosaccharides are identified for prevention of such adverse molecular interactions. Even though they are chemically heterogeneous they are excluded from the surface of proteins resulting in protein dehydration. Due to the adverse thermo dynamical effect of preferential exclusion surface area of a protein will be minimal and folded conformation are frequent. The denatured state will be most frequent in the presence of preferentially bound co-solvents. It results in summation of the effects due to existence of both types of solutes. What ever may be the reasons of dehydration-drought, freezing or osmotic shock, compatible solutes are accumulated by the plants and microorganisms as a result of cellular dehydration.

Since they do not affect cellular structure and functions the solutes are said to be compatible. The molecules in them are similar to that of the compounds said earlier which stabilizes proteins by preferential exclusion. A causal relationship between increasing concentrations of these compatible solutes and improved stress tolerance is evident from enrichment through external addition or through molecular genetic methods. To increase the water absorption capability of cells their absolute concentration is sometimes found inadequate. For safeguarding macro molecules in organisms against moderate water loss preferential exclusion, therefore, seems to be the prominent mechanism. It is essential… [END OF PREVIEW]

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