and translation of mRNAs and thus play a critical role in regulation of cold genes
(Sledjeski et al. 1996; Majdalani et al. 2005).
6.4.1.3 Sensing Low Temperature via Alteration in Protein
Conformation
At low temperature, changes in protein conformation as a means for temperature
sensing have observed in many bacteria. TlpA, a protein (371 amino acids) in
Salmonella typhimurium has the ability to sense temperature and to regulate gene
expression (Gulig et al. 1993; Hurme et al. 1997). As temperature increases, the
proportion of DNA-binding oligomers decreases, leading to repression of the target
gene. The sensory capacity of TlpA is dependent on the coiled-coil structure of
TlpA, which illustrates sensing of temperature through changes in protein
conformation.
Previous studies had suggested that membrane proteins that undergo temperaturedependent phosphorylation–dephosphorylation in bacteria might act as sensors.
Changes in membrane proteins due to temperature-depended phosphorylation–
dephosphorylation in bacteria act as thermometer (Ray et al. 1994a). In psychrophilic Pseudomonas syringae phosphorylation and dephosphorylation of a set of
membrane proteins in response to upshift and downshift of temperature were
observed. Response to low temperature in P. syringae Lz4w, phosphorylation of a
cytosolic 66 kDa protein, and differential phosphorylation of lipopolysaccharide
were observed (Ray et al. 1994a, c).
6.5
Exopolysaccharide Production
Cold-adapted microorganisms are accustomed to get frozen within their habitats.
Such organisms are also expected to have evolved adaptations to survive repeated
freezing and thawing, as these processes tend to damage living cells and attenuate
cell viability (Mazur 1966). Several diatoms, cyanobacteria and bacteria generate
abundant quantities of exopolysaccharide (Palmisano and Sullivan 1985; Cooksey
and Cooksey 1995; Costerton et al. 1995; Stoderegger and Herndl 1998), which is
stored as a thick gel surrounding the cells. The primary ecological significant
characteristic of exopolysaccharide is that it can form and maintain protective
microhabitats around microorganisms in aquatic and cold environments (Decho
1990). The physical, rheological, and chemical properties of exopolysaccharide are
affected by the length of the polymer chain, which is the principal determinant of the
molecular weight (Christensen 1999). As the length of the polymer increases, a
greater opportunity for complex entanglement of polymer chains and intramolecular
associations occurs and these interactions contribute to the tertiary structure and
physical behavior of the polymer (Sutherland 1994).
The role of extracellular polysaccharides (EPS) in the desiccation and freeze
tolerance was studied in cyanobacterium Nostoc commune (Tamaru et al. 2005). The
cells embedded in EPS were highly desiccation tolerant, freeze–thaw cycles and the
O 2 evolution was not damaged by air drying. When the cells were completely
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 193
(Sledjeski et al. 1996; Majdalani et al. 2005).
6.4.1.3 Sensing Low Temperature via Alteration in Protein
Conformation
At low temperature, changes in protein conformation as a means for temperature
sensing have observed in many bacteria. TlpA, a protein (371 amino acids) in
Salmonella typhimurium has the ability to sense temperature and to regulate gene
expression (Gulig et al. 1993; Hurme et al. 1997). As temperature increases, the
proportion of DNA-binding oligomers decreases, leading to repression of the target
gene. The sensory capacity of TlpA is dependent on the coiled-coil structure of
TlpA, which illustrates sensing of temperature through changes in protein
conformation.
Previous studies had suggested that membrane proteins that undergo temperaturedependent phosphorylation–dephosphorylation in bacteria might act as sensors.
Changes in membrane proteins due to temperature-depended phosphorylation–
dephosphorylation in bacteria act as thermometer (Ray et al. 1994a). In psychrophilic Pseudomonas syringae phosphorylation and dephosphorylation of a set of
membrane proteins in response to upshift and downshift of temperature were
observed. Response to low temperature in P. syringae Lz4w, phosphorylation of a
cytosolic 66 kDa protein, and differential phosphorylation of lipopolysaccharide
were observed (Ray et al. 1994a, c).
6.5
Exopolysaccharide Production
Cold-adapted microorganisms are accustomed to get frozen within their habitats.
Such organisms are also expected to have evolved adaptations to survive repeated
freezing and thawing, as these processes tend to damage living cells and attenuate
cell viability (Mazur 1966). Several diatoms, cyanobacteria and bacteria generate
abundant quantities of exopolysaccharide (Palmisano and Sullivan 1985; Cooksey
and Cooksey 1995; Costerton et al. 1995; Stoderegger and Herndl 1998), which is
stored as a thick gel surrounding the cells. The primary ecological significant
characteristic of exopolysaccharide is that it can form and maintain protective
microhabitats around microorganisms in aquatic and cold environments (Decho
1990). The physical, rheological, and chemical properties of exopolysaccharide are
affected by the length of the polymer chain, which is the principal determinant of the
molecular weight (Christensen 1999). As the length of the polymer increases, a
greater opportunity for complex entanglement of polymer chains and intramolecular
associations occurs and these interactions contribute to the tertiary structure and
physical behavior of the polymer (Sutherland 1994).
The role of extracellular polysaccharides (EPS) in the desiccation and freeze
tolerance was studied in cyanobacterium Nostoc commune (Tamaru et al. 2005). The
cells embedded in EPS were highly desiccation tolerant, freeze–thaw cycles and the
O 2 evolution was not damaged by air drying. When the cells were completely
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 193