Sensing and responding to temperature are essential for bacterial survival
There are always challenges for bacterial survival, especially to keep suitable living status facing fluctuations of chemical and physical parameters. The common chemical perturbations include sudden deprivation of nutrients or key metabolites and changes in surrounding pH，and the most representative physical deviation is temperature shift. But in most cases, the ambient temperature change is more essential for bacterial life cycle than chemical parameters, since the efficiency of all cellular processes is temperature dependent, especially the expression of any given gene[2,3]. Therefore, it is indispensable for bacteria to evolve more sensitive and more effective abilities, which can respond to temperature forwardly before the damages such as unfolded proteins or membrane rigidification occur (figure 1).
Figure 1. To survival in the change of many physical and chemical parameters, bacteria have evolved many kinds of biosensors to respond to its surroundings.
Biological signal transduction systems that sense temperature shifts
When it comes to the biological behavior of temperature sensing, it is crucial to elicit a proper response that could coordinate the expression of temperature-relevant genes. The common detecting systems in bacterial cells are based on three manners as following:
Temperature sensing in this level involves local DNA topological change followed by transcriptional events. For example, the expression of transcription factor σ32 in bacteria E.coli increases rapidly when temperature upshifts, which could mediate the transcription of genes including σ32 -recognizing promoters(figure 2). But this regulatory pathway takes some time to come into effect because the thermosensing proteins require time for upregulation.
Figure 2. The example of transcriptonal thermoregulation (1) Heat-induced up-regulation of transcription factor σ32. (2) σ32 recognizes the specific promoters and recruits the core enzyme of RNA polymerase. (3) RNA polymerase consisting of core enzyme and σ32 promotes the transcriptional initiation and elongation. (4) The heat-induced chaperones or proteases are expressed increasingly.
Temperature sensing also bases on many types of thermoregulatory RNA elements, also known for RNA-based thermosensors (RTs), which are located in the 5’ untranslated region (5’ UTR) of mRNA. The 5’ UTR is a part of bacterial transcript of gene, which would not be translated into peptide. Therefore, RTs are none-coding RNA elements. These types of thermoregulation respond to temperature and modulate the translation of already existing or nascent mRNAs, which could induce altered mRNA stability and ribosome accessibility of ribosome binding site (RBS) thereby controlling the translation efficiency (figure 3).
>See more details about RTs，which are we focus on this year.<
Figure 3. The example of translational thermoregulation (A) At low temperatures, the stem-loop in 5’- UTR of mRNA hides the SD sequence by base-paring with vicinal anti-SD sequence. Ribosome could not bind to RBS and the translation switches off. (B) At high temperatures, the stem-loop melts, resulting the full liberation of the SD sequence and start codon. This conformational change promotes the entire ribosome accessibility of RBS. The translation switches on.
Unfolded or misfolded protein conformations are the most common example of the consequences of temperature-induced damages. When temperature upshifts, chaperone proteins that are overexpressed can promote nascent polypeptide chains to fold into correct conformation, which is known as post-translational thermoregulation[7,8](figure 4). But this process is more like to lock the barn door after the horse was stolen, which means cellular processes have already been altered due to temperature shifts.
Figure 4. The example of post-translational thermoregulation. (1) Heat-induced unfolding or misfolding of protein. (2) Heat-induced up-regulation of chaperones. (3) The increased chaperones bind to peptides. (4) Chaperones help peptides fold into correct conformations.
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