Deletion analysis of cspA of Escherichia coli: requirement of the AT-rich UP element for cspA transcription and the downstream box in the coding region for its cold shock induction

A superfamily of proteins that contain the cold-shock domain

Members of a family of cold-shock proteins (CSPs) are found throughout the eubacterial domain and appear to function as RNA-chaperones. They have been implicated in various cellular processes, including adaptation to low temperatures, cellular growth, nutrient stress and stationary phase. The discovery of a domain--the cold-shock domain--that shows strikingly high homology and similar RNA-binding properties to CSPs in a growing number of eukaryotic nucleic-acid-binding proteins suggests that these proteins have an ancient origin.

The CspA family in Escherichia coli: multiple gene duplication for stress adaptation

CspA was originally found as the major cold-shock protein in Escherichia coli, consisting of 70-amino-acid residues. It forms a beta-barrel structure with five anti-parallel beta-strands and functions as an RNA chaperone. Its dramatic but transient induction upon cold shock is regulated at the level of transcription, mRNA stability and translation. Surprisingly, E. coli contains a large CspA family, consisting of nine genes from cspA to cspI. Phylogenetic analysis of these gene products and the cold-shock domain of human YB-1 protein reveals that there are two major branches in the evolution of CspA homologues: one branch for CspF and CspH, and another for all the other known CspA homologues from both prokaryotes and eukaryotes. The locations of these genes on the E. coli chromosome suggest that the large CspA family probably resulted from a number of gene duplications and, after subsequent adaptation, resulted in specific groups of genes that respond to different environmental stresses; for example, cspA, cspB and cspG for cold-shock stress and cspD for nutritional deprivation. The E. coli CspA family will be discussed in terms of their structures and functions, and their gene structures and regulation.

Role of the cold-box region in the 5' untranslated region of the cspA mRNA in its transient expression at low temperature in Escherichia coli

Upon temperature downshift, a group of proteins called cold shock proteins, such as CspA, CspB, and CsdA, are transiently induced in Escherichia coli. However, when the 5' untranslated region (5' UTR) of cspA mRNA is overproduced at low temperature, the expression of cold shock genes is prolonged or derepressed. It has been proposed that this effect is due to highly conserved 11-base sequences designated the "cold box" existing in the 5' UTRs of cspA, cspB, and csdA. Here, we demonstrate that the overproduction of the 5' UTR of not only cspA but also cspB and csdA mRNAs causes derepression of all three genes at the same time. Conversely, when the cold-box region was deleted from the cspA 5' UTR its derepression function was abolished. The amount of mRNA from the chromosomal cspA gene was much higher in cells overproducing the wild-type 5' UTR by means of a plasmid than it was in cells overproducing the cold-box-deleted 5' UTR. The stability of the chromosomal cspA mRNA in cells overproducing the wild-type 5' UTR was almost identical to that in cells overproducing the cold-box-deleted 5' UTR. Therefore, the derepression of cspA caused by overproduction of 5' UTR at the end of the acclimation phase occurs at the level of transcription but not by mRNA stabilization, indicating that the cold-box region plays a negative role in cspA transcription in cold shock-adapted cells. The role of the cold-box region was further confirmed with a cspA mutant strain containing a cold-box-deleted cspA gene integrated into the chromosome, which showed a high level of constitutive production of CspA but not CspB during exponential growth at low temperature.

Cold shock and adaptation

Adaptation to environmental stresses, such as temperature fluctuation, is essential for the survival of all living organisms. Cellular responses in both prokaryotes and eukaryotes to high temperature include the synthesis of a set of highly conserved proteins known as the heat shock proteins. In contrast to the heat shock response, adaptation to low temperatures has not been as extensively studied. However, a family of cold-inducible proteins is evident in prokaryotes. In addition, most organisms have developed adaptive mechanisms that alter both membrane fluidity and the protein translation machinery at low temperature. This review addresses the different adaptive mechanisms used by a variety of organisms with a focus on the molecular mechanisms of cold adaptation that have recently been identified during the cold shock response in Escherichia coli.