User talk:Chinmay21

Introduction
Like a poor person scavenging for every bit of food in a famine; micro ogranisms living "life on the edge" need to scavage and scrap every bit of nutrients like phosphates in the environment. The pho regulon is a regulon required for regulating phosphate levels in Bacillus subtilis and other bacteria. The regulon is made of 3 two-component regulatory system which interact with each other to control the amount of phosphate in the cell - PhoP/PhoR, ResD/ResE, and a system causing phosphorylation of Spo0A. Two of these systems - PhoP/PhoR and ResD/ResE act positively on pho regulon while Spo0A acts as a repressor. All the regulation is chiefly carried out via phosphorylation of histidine kinase.

Phosphate is an essential nutrient for cells which is normally present at concentrations 2-3 times less than other required ions. Since it is present at such low levels, it is crucial to regulate the phosphate level in the cell. When phophate levels drop below a threshold value, pho regulon is induced and the microbe starts saving phosphate in various ways like stopping synthesis of phosphate rich polymers like teichoic acid and replacing it with non-phosphate polymer teichuronic acid in the cell wall.

Mechanism
The following figure shows a nice illustration of the working of the pho regulon



The Pho regulon system is a primarily a two-component regulatory system – the PhoB - PhoR System. PhoR is a histidine kinase and PhoB is a response regulator protein. PhoR can exist in three states – the active form, inactive form and the deactivated form. The PhoB binds to the DNA binding sites casuing regulation of gene expression of the genes of the Pho regulon system.

PhoR exists in the inactive form when the periplasmic space concentration of inorganic phosphate (Pi) is above 4μM For Pi concentration above 4μM, PhoR is activated and it phosphorylates PhoB. Phosphorylated PhoB then binds then activates or deactivates downstream target genes. When concentration of Pi is within acceptable range, the PhoU regulatory protein deactivates the PhoR.

As pointed out in the introduction, ResD and AbrB system acts as a positive regulator of the Pho regulon. It stimulates PhoR to phosphorylate PhoP by a yet unclear mechanism. ResD also regulates the ResD/ResE two-component regulatory system. It has been found that Pho regulon response is eliminated by ~80% by mutations in the ResD/E system. Another ~20% reduction occurs due to mutations in AbrB. Since the mutations are additive; we see only 5% of the wild type response in strains with mutated AbrB and ResD/ResE; there is a strong case to be made for the model of two separate paths regulating PhoP/R.

The SpoOA protein responsible for causing sporulation is a negative regulator of the Pho regulon. It represses the activity of AbrB and the ResD/E system by an unknown mechanism. It is also possible that this regulation is carried out by intermediate molecules. After some time, if the initial response has not be sufficient and the cell is still in a critical condition, SpoOA shuts down the Pho regulon, and sporulation begins.

The Phosphate Specific Transport System
The inorganic form of phosphate is transported into the cell by the Phosphate Specific Transport (Pst) System. Pst consists of: PstS, PstA, PstB and PstC. All these genes are also a part of pho regulon. PstS is a periplasmic protein and has a very high affinity for Pi. PstA and PstC are transmembrane proteins that transport the Pi into the cell. The two PstB subunits are permeases that release the energy required for carrying Pi into the cell. PstB also helps PhoU in the dephosphorylation of PhoB.

Cross-Regulation
PhoB can also be activated by histidine kinases other than the PhoR. For example, the CreC sensor protein of the CreC-CreB TCS system involved in carbon metabolism can activate PhoB. This helps the cells when there is absence of PhoR. Cross-regulation helps the cells to adapt to changing growth conditions faster.

Genes under the control of pho regulon
The pho regulon exerts a global control on gene expression through out the cell with ~25 genes having already been shown to be directly controlled by the pho regulon. Some of these include genes coding for AMP nucleosidase, Aldolase, Phosphonate transporter subunits, Carbon-phosphorus lyase complex subunits etc. A complete list can be found in references.

Pho Regulon and Virulence
In addition to its role in phosphate homeostasis, the pho regulon also influences the expression of virulence traits due to a large number of cell surface proteins under its control. These effects have been studied by creating mutants of PhoB or the Pst system and in many cases they have shown multiple effects including reduction of virulence. The regulation of cell surface components by the Pho regulon is a major reason of its contribution to bacterial virulence. A strong network of links has also been found to exist between the Pho regulon and some stress responses. Still, we do not know the molecular basis of the interactions between the pho regulon and virulence and hence more comprehensive studies using high throughput genomic and proteomic tools are warranted for this system.

Eukaryotic Homologue
Since many members of the two-component family like histidine kinases are now starting to be found with increasing frequency in lower eukaryotes like yeast, there is a reason to believe that the basic His-to-Asp phosphotransfer mechanism is used by many eukaryotic sensory-response pathways also. Specially in lower eukaryotes like yeast systems analogous to the pho regulon have been observed. . In higher eukaryotes like plants two component regulators have been found for receptors of ethylene, cytokinin signaling etc. However, no such direct homologue for pho regulon has been found so far in higher eukaryotic systems like mammals.