Transcription and translation are tightly coupled in bacterial cells. However, the transcription machinery and ribosomes generally occupy different subcellular regions in bacteria such as Escherichia coli and Bacillus subtilis, indicating the need.for (a) mechanism(s) coupling these processes. A prime function of this mechanism(s) would be ensuring the transfer of unfolded mRNA from the nucleoid to ribosomes, which require linear mRNA for the initiation of translation. During conditions of a sudden decrease in temperature (cold shock), secondary structures in mRNA would pose an even greater problem for the initiation process. Two conserved classes of proteins, cold shock proteins (CSPs) and cold induced RNA helicases (CSHs), appear to be major players in the prevention of secondary mRNA structures and in transcription/translation coupling. CSPs are general mRNA-binding proteins, and like CSH-type RNA helicases, the presence of at least one csp gene in the cell is essential for viability. Members of both protein families have recently been shown to interact, suggesting that a two-step process achieves the coupling process, removal of secondary mRNA structures through CSHs and prevention of reformation through CSPs.
Keywords: cold shock, cold shock proteins, transcription, translation, mRNA, ribosomes, nucleoid
Coping with cold!!
Exposure to low temperatures is a frequent event that is encountered by different bacterial species in various situations. A striking aspect in nature is the estimation that the temperature of more than 80% of biosphere is below 5[degrees]C (1). Food-related bacteria are particularly and repeatedly exposed to low temperatures during food handling and storage (2). This exposure to cold temperatures has increased over last decades because of the extended use of refrigeration and freezing in food preservation. While low temperatures inhibit the growth of some food spoilage and pathogenic bacteria, they enrich the numbers of others. Among the latter are serious pathogens such as Listeria monocytogenes that causes spontaneous abortion in pregnant women or the birth of a severely sick baby and may affect the bloodstream and the central nervous system in humans (3). Chilling and freezing weather temperatures in different areas of the world also represent cold stress to bacteria residing in soil and on the skin of animals and plants.
There has been a growing trend in research to study bacterial response to low temperatures over the last few decades. Focus has been particularly placed on the cell's response to the sudden decline in temperature (cold shock). This response is generally considered as a model for the cell's adaptive behavior during cold stress. It is currently appreciated that most bacterial species can respond to cold shock by transient induction of arrays of specific proteins, termed cold-induced proteins (CIPs), and repression of other proteins synthesized during active growth or on exposure to other stressful conditions such as heat shock (4,5). Such a response is presumed to aid cells overcoming physiological stress generated by cold shock.
The negative influence of exposure to cold stress stems mainly from the physical effect of low temperature on the cell's structures and enzymatic reactions. For instance, low temperature lowers the fluidity of the cytoplasmic membrane and induces the formation of nonfunctional mRNA secondary structures (for a comprehensive review see Phadtare et al. (5)). Low temperatures also decrease the rate of enzymatic reactions due to a reduction in molecular dynamics (6). These structural and metabolic changes bring about difficulties in central functions in cells involving membrane transport, transcription and translation. Coping with cold stress thus requires the synthesis of diverse mechanisms that enable cells to keep surviving under this hardship.
A prime cold-adaptive mechanism is the synthesis of membrane fatty acid desaturases. …