CSPs comprise a family of small proteins that are structurally highly conserved, bind to single-stranded nucleic acids and are involved in a variety of cellular processes, such as transcription (Ermolenko and Makhatadze 2002). Bacterial
CSPs are rich in aromatic and basic amino acids, and their expression peak occurs shortly after a rapid temperature downshift to regulate the adaptation to cold stress, but they are also present under normal conditions to regulate other biological functions (Barbaro et al. 2002; Guo and Gong 2002). The cold-shock phenomenon was originally found in Escherichia coli at a temperature downshift from 37°C to 10°C (Jones and Inouye 1994), and was later found to be a cold-shock response common to many bacterial species (Kim et al. 1998b; Lottering and Streips 1995; Obata et al. 1998) some eukaryotes (Somer et al. 2002), and archaea (Cavicchioli et al. 2000). The major cold-shock protein CspA of E. coli has high sequence similarity with eukaryotic Y-box DNA-binding proteins that are known to be involved in regulation of several transcription and translation processes (Lee et al. 1994). A homolog of CspA was found to be upregulated following cold shock in psychrotrophic bacterium Arthrobacter globiformus SI55, but unlike its mesophilic counterparts, it was still expressed during prolonged growth at 4°C. The synthesis of this CspA-like protein was regulated at the translational level, and it was shown that growth resumption following a temperature downshift correlated with CspA expression (Berger et al. 1997). Similarly, psychroactive bacteria from permafrost showed overexpression of the CSPs during continuous low-temperature growth.
The presence of homologous cold-shock protein C (CspC, 7.255 kDa) in Exiguobacterium sibiricum 7-3 and three Csps (with Mr 7.150, 7.414 and 7.444 kDa) in E. sibiricum 255-15 was detected by high-performance liquid chromatography (HPLC) associated with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) (Chong et al. 2000; Qiu et al. 2006). Along with CspC, the overexpression of two other CSPs (CSP CSI4B 1,924.3 kDa, CSP CSI5 1,359.7 kDa) was observed in E. sibiricum 7-3 during low temperature growth (Chong et al. 2000). Three major CSPs from E. sibiricum 255-15 were homologous with 65.15%, 66.67%, and 59.09% sequence overlap to CspA in E. coli, and over 74% when compared to CspB, CspC, and CspD in Bacillus subtilis (Qiu et al. 2006). What is interesting is that unlike in E. coli, B. subtilis, and E. sibiricum 7-3 family of CSPs, those in E. sibiricum 255-15 were found similarly expressed at 25°C and 4°C, and represent about 10% of the total soluble proteins in cells grown at both temperatures. This result suggests that the genes for these proteins are turned on continuously to produce "shock" proteins to protect the cells from damage during abrupt changes in environmental conditions. Such behavior has been observed in other psychroactive bacterium such as Psychrobacter arcticus 273-4, where it has been shown that certain proteins (e.g. ribosomal proteins, ATP-depend-ent helicase, Elongation factor Ts) are always synthesized (Zheng et al. 2007). Apparently these organisms, which survive for long periods of time under extreme conditions, have adapted such a continuous expression as a means of survival. However, a putative CSP (8,111 Da/4.9 pi) was detected in P. arcticus 273-4 at 4°C and at both 22°C and 4°C when grown in medium with 5% NaCl, but was not detected at 22°C in // Tryptic Soy Broth (TSB) (Zheng et al. 2007). Another strain, P. cryohalolentis K5, showed the presence of CSP (CspA, 7.45 kDa) only at temperatures of 4°C and -4°C (Bakermans et al. 2007).
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