A set of proteins which are distinct from CSPs, and are specifically synthesized during continuous growth at low temperatures, are termed CAPs (Roberts and Inniss 1992; Whyte and Inniss 1992; Berger et al. 1996; Colucci and Inniss 1996). Recently, CAPs distinct from CSPs have been identified in the mesophilic bacteria Enteroccoccus faecalis during continuous growth at 8°C and Listeria monocytogenes at 10°C (Panoff et al. 1997; Liu et al. 2002).
From peptide analysis of the whole-cell lysates of E. sibiricum 255-15, 39 proteins with Mr ranging from 7 to 95 kDa were identified to be present at an increased level at the lower temperature and were considered to be CAPs, 16 of which were not detected at 25°C (Qiu et al. 2006). Some of these CAPs, such as trigger factor (TF) and pyruvate dehydrogenase, were characterized as CSPs in E. coli (Kandror and Goldberg 1997; Jones et al. 2006). TF in E. coli is a molecular chaperone with prolyl-isomerase activity, and associates with nascent polypeptides on ribosomes, binds to GroEL, enhances GroEL's affinity for unfolded proteins, and promotes degradation of certain polypeptides (Kandror and Goldberg 1997). TF levels increased progressively as growth temperature decreased and even rose in cells stored at 4°C. E. coli cells with reduced TF content die faster, while cells overex-pressing TF showed greater viability. Thus, TF represents an example of an E. coli protein which protects cells against low temperatures. Unlike the TF, the role of pyruvate dehydrogenase has not yet been well-understood. Presumably, it is involved in the intensification of glycolysis and the suppression of the tricarboxylic acid cycle, i.e., in the processes that are observed upon the retardation of cell growth, and the adaptation of cells to stresses (Graumann and Marahiel 1996; Qiu et al. 2006).
The overexpression of heat-shock protein 70 (Hsp70) molecular chaperones was observed in E. sibiricum 255-15 during the cold-adaptation process. The heat-shock proteins may function as molecular chaperones that play an important role in protein folding, and — like DnaK — have functions in refolding of misfolded proteins that are essential under stress. Thus, these so-called "heat-shock proteins" are not simply heat-shock-specific proteins. They should more appropriately be called "temperature-stress proteins" (Qiu et al. 2006). While Hsp70 of P. articus 273-4 was overexpressed only in response to low temperature, chaperonin Hsp60 was found to be induced by low temperature or salt, where it was down-regulated if both of these extremes were present (Zheng et al. 2007).
Chaperone proteins DnaK and GroEL were found to be actively synthesized in response to heat, cold, and chemical stress (Salotra et al. 1995; Phan-Thanh and Gormon 1997). The phage-shock protein A (PspA) of E. sibiricum 255-15 was the highest overexpressed protein at low growth temperatures, whose expression ratio was over 70 (Qiu et al. 2006). Presently, the exact function of PspA remains elusive. High-level synthesis of PspA occurs only under extreme stress conditions including heat shock, cold shock, osmotic shock, and exposure to ethanol (Brissette et al. 1990; Kleerebezem and Tommassen 1993; Model et al. 1997). These stress conditions might all lead to the dissipation of the proton-motive force, and expression of the PspA may help the cells to maintain the proton-motive force under such stress conditions (Kleerebezem et al. 1996).
The penicillin tolerance protein of E. sibiricum 255-15 was also found greatly overexpressed at 4°C. In P. articus 273-4, 18 proteins were up-regulated at 4°C in xh TSB and only four proteins were up-regulated at 4°C in xh TSB supplemented with 5% NaCl (Zheng et al. 2007). These facts suggest that a single stress could induce other stress-induced proteins that are organized in a complex and highly sophisticated adaptation network.
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