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Introduction

Anti-sigma factors are small
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...
that bind to sigma factors and inhibit transcriptional activity in regulating prokaryote gene expression. Anti-sigma factors have both a sigma-binding domain and a sensory/signaling domain; this allows them to respond to signals inside and outside the cell. Anti-sigma factors have been found in several bacteria, including ''
Escherichia coli ''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
'' and ''
Salmonella ''Salmonella'' is a genus of bacillus (shape), rod-shaped, (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two known species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' ...
'', and viruses such as the
T4 bacteriophage Escherichia virus T4 is a species of bacteriophages that infect ''Escherichia coli'' bacteria. It is a double-stranded DNA virus in the subfamily '' Tevenvirinae'' of the family '' Straboviridae''. T4 is capable of undergoing only a lytic li ...
. Anti-sigma factors have an antagonistic effect on sigma factors. Each sigma factor has an associated anti-sigma factor that regulates it. These anti-sigma factors are divided into
cytoplasmic The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell and ...
-bound anti-sigma factors and inner
membrane-bound A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of the ...
anti-sigma factors. The differences in these sigma factors are where in the cell they are bound. Cytoplasmic-bound anti-sigma factors include FlgM, DnaK, RssB, and HscC. Inner membrane-bound anti-sigma factors, also called extra-cytoplasmic function (ECF) anti-sigma factors, include FecR and RseA. ECF anti-sigma factors tend to be more diverse in genetic sequence than cytoplasmic-bound anti-sigma factors. These factors regulate many cellular processes, such as
flagellum A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
assembly, transport of materials,
cell growth Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...
, and the cell's stress response. Sigma factors are essential proteins that start the transcription by binding with RNAP; anti-sigma factors are proteins that inhibit the activities of sigma factors affected by several mechanisms. These mechanisms include adding up the anti-sigma factor between sigma or twisting the anti-sigma factor around sigma—
gene regulation Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products (protein or RNA). Sophisticated programs of gene expression are wide ...
, especially in bacteria, allows for adaptivity and normal
cell differentiation Cellular differentiation is the process in which a stem cell changes from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellular ...
and development. Gene regulation has many different layers of regulators. Yet, initiating transcription is crucial in controlling which genes are expressed. Anti-sigma factors are simultaneously transcribed with their associated sigma factor. This pairing creates a
negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused ...
loop, maintaining proper levels of both contrasting factors as there can only be one anti-sigma factor per sigma factor that is transcribed. 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.


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 Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Protein degradation is a major regulatory mechanism of gene expression and contributes substantially to shaping mammalian proteomes. Uncatalysed, the hydrolysis o ...
. The partner-switching mechanism is commonly found in
Gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain ...
. It consists of four key players: a sigma factor, an anti-sigma factor, an anti-anti-sigma factor, and an input
phosphatase In biochemistry, a phosphatase is an enzyme that uses water to cleave a phosphoric acid Ester, monoester into a phosphate ion and an Alcohol (chemistry), alcohol. Because a phosphatase enzyme catalysis, catalyzes the hydrolysis of its Substrate ...
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. 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. The regulated intramembrane proteolysis (RIP) mechanism allows signal transduction across membranes. This mechanism is often used to regulate ECF sigma factors. The mechanism involves two sequential cleavages, the first being an external cleavage of membrane-traversing anti-sigma factor and the second cleavage of the anti-sigma factors in the membrane's plane, resulting in a free cytoplasmic domain.


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.


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.


In Bacteriophage

T4 bacteriophage Escherichia virus T4 is a species of bacteriophages that infect ''Escherichia coli'' bacteria. It is a double-stranded DNA virus in the subfamily '' Tevenvirinae'' of the family '' Straboviridae''. T4 is capable of undergoing only a lytic li ...
uses anti-sigma factor to ruin the ''Escherichia coli'' polymerase in order that direct exclusive transcription of its own genes. 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.


Sigma B Factor in ''Bacillus subtilis''

Sigma B was the first anti-sigma factor identified in a bacterium. It is found in ''
Bacillus subtilis ''Bacillus subtilis'' (), known also as the hay bacillus or grass bacillus, is a gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus ''Bacill ...
'' 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.


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.


References


Further reading

* * * * {{Transcription factors Gene expression