Anti-sigma factors
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Introduction
Anti-sigma factors are small
Anti-sigma factors are simultaneously transcribed with their associated sigma factor. This pairing creates a negative feedback loop, maintaining proper levels of both contrasting factors as there can only be one anti-sigma factor per sigma factor that is transcribed.[5]
Research shows anti-sigma factors have more activities than contouring sigma factors effects. Anti-sigma factors can also activate some cells while inhibiting others, meaning they have an essential role in cell function.[5][6]
Mechanism
There are three main categories for triggering the release of sigmas factors from anti-sigma factors: partner switching, direct signaling, and a mechanism regulated by proteolysis.[1]
The partner-switching mechanism is commonly found in Gram-positive bacteria. It consists of four key players: a sigma factor, an anti-sigma factor, an anti-anti-sigma factor, and an input phosphatase complex. A cell that is not under stress has an anti-sigma factor that is bound to the sigma factor on the gene and keeps it inactive. In times of stress, a phosphatase complex dephosphorylates the anti-sigma factor, allowing the anti-sigma factor to switch partners and bind to the anti-anti-sigma factor. This frees the sigma factors to activate the gene. Environmental stressors, such as heat, often activate this mechanism.[1]
The direct signaling mechanism is as it sounds: the anti-sigma factor binds to a signal, which causes conformation changes in the structure of the anti-sigma factors, resulting in the release of the sigma factors.[1]
The
Anti-sigma factors in Escherichia coli
E. coli has seven main sigma factors, five of which have a specific anti-sigma factor. The anti-sigma factor binding to its sigma factors depends upon environmental cues. This mechanism blocks the transcription of genes that are unnecessary in new conditions. The table below shows five sigma factors, what process it affects, and its corresponding anti-sigma factor. In E. coli, sigma factors transcribe their anti-sigma factors; this creates a negative feedback loop. The sigma factor can be regulated when the anti-sigma factor is transcribed and the anti-sigma factor when the sigmas gene is transcribed. Sigma factors 70 and 54 don't have specific anti-sigma factors; they have other negative feedback loop mechanisms.[4]
| Sigma factor | Sigma Effect | Related anti-sigma factor |
| σ38 | Master regulator of general stress response
|
RssB |
| σ32 | Heat shock response ≥ 37 °C | Dnak |
| σ28 | Active late gene of flagellum assembly | FIgM |
| σ24 | Signals release of factors to fix misfolded proteins | RseA |
| σ19 | One signal in the EC signaling pathway | FecR |
Anti-Anti-Sigma Factors
Anti-anti-sigma factors allow for the dissociation of the matching anti-sigma factor from its sigma factors, thought binding to the anti-sigma factor, forcing its release from the sigma factor. This allows for tighter regulation of the transcription of genes as a response to environmental conditions. Anti-anti-sigma factors can thereby function as negative or positive regulatory elements, depending on the corroding sigma factor and gene involved.[7][8]
In Bacteriophage
AsiA is an anti-sigma factor gene that is required for bacteriophage T4 to be developed). Which means that AsiA is an essential anti-sigma factor in bacteriophage.[6][4][9][8]
Sigma B Factor in Bacillus subtilis
Sigma B was the first anti-sigma factor identified in a bacterium. It is found in Bacillus subtilis and other similar bacteria. Sigma B is a stress response factor that plays a role in survival and against destruction that could be caused by other organisms such as mammals. General stress responses that are controlled by Sigma B are stimulated by things like temperature, salt concentration, energy depletion, etc. Once activated, Sigma B binds to the RNAP and recognizes a promoter, causing inhibition of the stimuli. Because Sigma B orthologs are conserved in various gram-positive bacteria, this anti-sigma factor plays an essential role in the evolution of different bacteria and their ability to respond to stressing factors. Scientist have found that the anti- sigma factor, Sigma B controls more than 150 genes that are influential in stress response.[10][11]
RsbW in Bacillus subtilis
When Bacillus subtilis is not under stress conditions, it is negatively regulated by the anti-sigma factor, Rsbw. RsbW is an anti-sigma factor that regulates another anti-sigma factor, sigma B. RsbW binds to sigma B and prevents it from forming an RNA polymerase holoenzyme. However, in stressed conditions, the unphosphorylated form of the protein, RsbV, competes with Sigma B for binding to RsbW. RsbV binds to RsbW, allowing sigma B to bind to the core RNA polymerase, resulting in the expression of stress response.[12][13]
References
Further reading
- Kang JG, Paget MS, Seok YJ, Hahn MY, Bae JB, Hahn JS, Kleanthous C, Buttner MJ, Roe JH (August 1999). "RsrA, an anti-sigma factor regulated by redox change". The EMBO Journal. 18 (15): 4292–8. PMID 10428967.
- Ohnishi K, Kutsukake K, Suzuki H, Lino T (November 1992). "A novel transcriptional regulation mechanism in the flagellar regulon of Salmonella typhimurium: an antisigma factor inhibits the activity of the flagellum-specific sigma factor, sigma F". Molecular Microbiology. 6 (21): 3149–57. S2CID 41958324.
- Helmann JD (April 1999). "Anti-sigma factors". Current Opinion in Microbiology. 2 (2): 135–41. PMID 10322161.
- Sevcikova B, Rezuchova B, Homerova D, Kormanec J (November 2010). "The anti-anti-sigma factor BldG is involved in activation of the stress response sigma factor σ(H) in Streptomyces coelicolor A3(2)". Journal of Bacteriology. 192 (21): 5674–81. PMID 20817765.