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Heat shock response in prokaryotes
In both eukaryotic and prokaryotic, heat shock response is the efficient response by changing temperature. This can be defined as an efficient production of heat shock proteins (Savada at all., 2017). These heat shock proteins are produced in order to compensate the temperature changes in the environment (Savada at all., 2017). This essay will discuss the heat shock response in prokaryotes as they are capable of adapting to changes such as external pH changes, temperature changes, concentration of nutrients and toxins as well as oxidative stress but this essay will focus on how bacteria will survive during the heat shock (Kumar at all., 2017).
Heat Shock Response
Heat Shock Response in prokaryotes is the capacity of a prokaryotic cell to respond to a dramatic change in temperature by translating Heat Shock Proteins to restore homeostasis (Tulin at all., 2019)(Kumar at all., 2017). According to Kumar at all (2017), protein start to be denatured and they start getting unfolded due to high temperature. This results in non-functional proteins as they lose their second and tertiary structures and their 3D shape conformation (Savada at all., 2017). Heat shock genes encodes for chapestones, protease and other stress related proteins are then controlled by rpoH and its product 32 (Roy at all., 2012). At normal condition, the level of 32 intercellular, however the level rises as temperature rises as well (Roy at all., 2012). So this is a dynamic response system highly controlled at 4 different levels: transcription and translation of rpoH as well as activity and stability of 32 protein (Roy at all., 2012).
Heat Shock Response and Transcription
As a consequence of the unfolding of proteins caused by the increase of temperature, heat shock proteins are translated in order to keep cellular homeostasis. It follows that, to achieve this, the rate at which heat shock proteins are produced, increases because of heat shock genes coding for heat shock proteins. According to Yura at all. (1993) at the transcription level, all bacteria use of one multiunit RNA polymerase used during transcription of genes. Those genes usually have in front of them by one or more promoters that is then sensed by the sigma factor which is part of RNA polymerase. “While all bacterial species code for one housekeeping sigma factor only, they code for a various number of alternative sigma factors depending on the species. So far, three different alternative sigma factors have been described as regulators of HSGs (heat shock proteins), namely, Sig32, SigE, and SigH. discovered”(Kumar at all., 2017).
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RpoH encodes for Sigma 32 which at standard conditions is unstable and it is found at very low concentration in the cell (Yura at all., 1993) (Kumar at all., 2017). In order to function and so being able to adapt to the cells’ needs, it is controlled by three mechanisms at the level of transcription, activity and stability (Kumar at all., 2017). From the level of transcription, the two regions A and B from coding region of rpoH controls the translational level; further those regions, at low temperature, form a structure which prevents translation of the genes (Kumar at all., 2017).
We will now evaluate the activity of sigma 32. It is regulated by DnaK/J/GrpE and GroEL/S chapestones (Kumar at all., 2017). Chaperones are proteins that are involved in keeping the shape of macromolecules by controlling their folding and unfolding. In the event of high temperatures, the chaperones bind to the unfolded proteins while at lower temperature are free in the cells (Kumar at all., 2017). To increase the sigma activity and to start the transcription in prokaryotes, there is a decrease of chaperone system or overexpression of chaperone substrate. On the other hand, overexpression of chaperone system leads to a decrease in the sigma 32 activity (Kumar at all., 2017). As already mentioned, the sigma 32 is unstable and so the inner membrane protease FtsH has the ability to control it.
Sigma E regulates transcription to keep the proteins properly folded as they are in the periplasm and the outer membrane (Kumar at all., 2017). Whenever the prokaryotic cells is not under heat stress, SigmaE is inactive. As a result of this, strong and stable interaction between Sigma E and the C-terminal domain of the anti-sigma factor RseA are formed and its C-terminal domain is located in the cytoplasm while the N-terminal is exposed toward the periplasm(Kumar at all., 2017). The N-termination is then bounded an anti-sigma factor RseB and the rseA gene is deleted when the cell is in normal temperature condition (Kumar at all., 2017). When the temperature rises RseA is broken down into three different products: DegS and RseP which can be found in the inner membrane proteins while the third one is present in the cytoplasm (Kumar at all., 2017).
The alternative Sigma H is responsible to control the oxidative and heat stress. At transcriptional level, it is regulated by autoregulation of sigH promoter while after transcription, it is controlled by anti-sigma factor RshA (Kumar at all., 2017). However, this complex is denatured at high temperatures and during oxidative conditions (Kumar at all., 2017).
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Another way to control the regulation of heat shock protein is by using transcriptional repressor. After an increase in temperature, the RNA polymerase can bind to the promoter region and start transcription because the repressor protein had dissociated from the operon (Kumar at all., 2017). The repressor analysed are HrcA,CtsR, RheA and HspR. HrcA repression is present in both active and inactive form and its activity is controlled by GroEL (Kumar at all., 2017). Due to this interaction, we can deduce that inactive form is present in prokaryotic cells and, whenever the repressor interacts with the GroEL it becomes active and it then binds to CIRCE (Kumar at all, 2017). The CtsR, instead, is made up of 3 operons: tetracitrosmic clpC and tow moree monocistronic clpP and clpE operon (Kumar at all., 2017). It binds on the operon CtsR box and so it represses transcription (Kumar at all., 2017). CtsR is inactivated by being phorylated by ClpC in order to not rembing to its operon after a heat shock (Kumar at all., 2017). Another repressor is RheA. At normal condition, low level of RheA and HSP18 protein are present, while high level of hsp18mRNA (Kumar at all., 2017). However, after a dramatic increase in temperature, the level of hsp18 increases (Kumar at all., 2017). RheA becomes inactive as it starts unfolding since it is active at normal condition (Kumar at all., 2017). On the other hand, HspR is regulated by DnaK chaperone as it represses HspR in binding HAIR operon (Kumar at all., 2017).
RNA thermosensors are sequences of RNA on the 5’-UTR able to sense temperature changes (Kumar at all., 2017). It controls the initiation of translation by forming secondary structure but this new structure can be easily changed by the change in temperature. It liberates RBS and so the initiation complex it formed (Kumar at all., 2017). Increasing the temperature, results in the break down of the secondary structure and ribosome are able to detect the RBS and this is a faster process due to the mRNA directly controls gene expression (Kumar at all., 2017). Because of the rise in temperature, the RNA thermosensor structure start denaturing and translation begins (Kumar at all., 2017). during normal conditions, RpoH mRNA has a secondary structure that prevents SD sequence and the downstream initiation codon.
DNA has a similar mechanism using DNA thermosensors using DNA supercoiling, promoter curvature and DNA-associated proteins as a way to detect changes in temperature (Kumar at all., 2017). A positive supercoil results from the increase of temperature and this has consequences on the efficiencies of the transcription (Kumar at all., 2017). Promoter Curvature are section of curved DNA placed in front of a promoter that have AT-tracts (Kumar at all., 2017). Looking at the nucleoid-associated proteins have the ability to change structurally but also DNA replication, recombination and transcription (Kumar at all., 2017). At physiological temperature, “the H-NS to DNA ratio increases several folds during growth at low temperatures. The temperature-dependent accessibility of promoters kept inactive by binding of H-NS” (Kumar at all, 2017).
Heat Shock Proteins and Survival of the Prokaryotic Cells
Heat shock proteins are the survival tool in bacteria to survive whenever there is a change in temperature (Yura at all., 1993). The majority of these proteins have been synthesised and used under normal conditions as they can be used not only under heat stress conditions (Yura at all., 1993). More in general, heat shock proteins are capable to protect other proteins when they are present at high concentration, at the presence of GroE and DnaK Proteins (Yura at all., 1993).
In conclusion, bacteria have a high capacity to adapt to external changes and, thus, surviving in different environments. In the event of changes in temperatures, the Heat Shock Proteins are synthesised in order to allow the cell to survive and to restore homeostasis. In order to achieve this, three different mechanisms (Alternative Sigma Factors, Transcriptional Repressor, RNA Thermorespressor and DNA Thermorepressor help the survival of prokaryotes.
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