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Arc two-component system
The Arc (Anoxic Redox Control) system is a two-component signal transduction system in E.coli under anaerobic conditions. Changes in the redox condition of growth are sensed and signaled in E.coli by this system. The Arc system has been found to be responsible for the anaerobic respiration of genes that code for Citric Acid cycle enzymes, glycoxylate cycle enzymes, fatty acid oxidation enzymes and several dehydrogenases necessary for aerobic growth. This system is also found to control the anerobic induction of the gene coding for Pyruvate-formate Lyase.

The system consists of the protein ArcB as a membrane associated sensor kinase and protein ArcA as its cognate response regulator. In response to low oxygen levels, autophosphorylated ArcB phosphorylates ArcA and the resulting phosphorylated ArcA functions as a transcriptional regulator of the genes that are essential for anaerobic growth.



Structure of the Arc system in E.coli
In prokaryotes, environmental changes are often signaled by a family of two-component systems which result in adaptive gene expressions. Such a system, comprises of
 * membrane-associated sensor kinase
 * cytoplasmic response regulator

When a signal is received by the membrane-associated sensor kinase, ATP-dependent autophosphorylated is stimulated at a conserved Histidine residue in the cytosolic transmitter domain. In the immediate step, a phosphoryl group gets transferred to a conserved aspartate residue in the response regulator in the cytoplasm. After phosphorylation, the response regulator acts as a transcriptional regulator. Upon termination of signaling, both the cognate response regulator and the sensor kinase undergo dephosphorylation and the pathway is stalled.

Structure of protein ArcB
ArcB contains three catalytic domains
 * N-terminal tranmitter domain(H1) with a conserved His-292 residue
 * Central receiver domain(D1) with a conserved Asp-576 residue
 * C-terminal secondary tranmitter domain(H2) with a conserved His-717 residue

Structure of protein ArcA
Phosphorylated ArcA controls the expression of nearly 30 operons(the Arc Modulon) in response to the redox conditions of growth. Arc A contains two domains
 * N-terminal receiver domain containing a conserved Asp-54 residue
 * C-terminal HTH domain

Regulation
External environmental changes cause a stimulus due to which ArcB autophosphorylates. The phosporyl group is then transferred to ArcA by a phosphorelay from His-->Asp-->His-->Asp, sequentially involving His-292 of H1,Asp-576 of D1, His-717 of H2 and Asp-54 of ArcA. Termination of stimulation results in an ArcA-P dephosphorylation occurs by reverse transfer of the phosphoryl group to His-717 of H2 and then to Asp-576 of D1, where release of Pi takes place. In signal decay, domain H1 has no apparent role. This type of signal transmission in the form of a phosphorelay has been observed in several prokaryotic as well as eukaryotic systems including Kin/Spo system of Bacillus subtilis, the S1n1p/Ypd1p/Ssk1p of Saccharomyces cerevisiae, the BvgS/A of Bordatella pertussis and the TorS/R system of E.coli.



Influence of effectors on phosphorylation of ArcB
The phosphorylating and dephosphorylating pathways of the Arc system have been characterized well by using various combinations of ArcB modules and thus a systematic study of influence of effectors has been done.



Effect of D-Lactate, Acetate and Pyruvate
It has been shown that the initial rate of the H1-D1-H2 phosphorylation and the overall level of the phosphorylated protein increased nearly 3-fold due to the presence of 1mM D-Lactate. Acetate and Pyruvate exerted similar effects but to lesser degrees. Since D-lactate was most effective in raising the rate of net phosphorylation of ArcB, the effect of this metabolite on the phosphorylation of ArcA after autophosphorylation of ArcB was studied. The rate of transphosphorylation of ArcA by H1-D1-H2 increased 3-fold due to the presence of the effector. This is consistent with the earlier observation.

Influence of effectors on the phosphatase activity of ArcB
It is necessary to observe the influence of effectors on the dephosphorylation rate of ArcA-P by H1-D1-H2 as the net phosphorylation of both H1-D1-H2 and ArcA reflects the balance of rates of phosphorylation and dephosphorylation which gives no clear understanding of the reaction pathway on which D-Lactate exerts an effect. Results of experiments conducted to observe the dephosphorylation of ArcA show that there is no effect in the dephosphorylation rate implying the phosphatase activity of ArcB is not influenced by the effectors.

Homologous system in S. Cerevisiae
An osmosensing mechanism consisting of a phosphorelay signal transduction pathway homologous to bacterial two-component signal transducers is present in the eukaryotic system,S. cerevisiae. The mechanism involves a two-component signal transducer consisting of Sln1p, Ypd1p and Ssk1p and a MAP kinase cascade. The transmembrane protein Sln1p contains an extracellular sensor domain and cytoplasmic histidine kinase and receiver domains, whereas the cytoplasmic protein Ssk1p contains a receiver domain. Ypd1p binds to both Ssk1p and Sln1p and mediates the phosphorelay. The phosphorelay is initiated by the autophosphorylation of Sln1p at His-576. This phosphate is then transferred to to Sln1p-Asp-1144 and then to Ypdp1-His-64 and then finally to Ssk1p-Asp554.