User talk:Raghavendran11/Pho regulon

Phosphate Regulatory System in Enteric bacteria
The Phosphate Regulatory System also known as Pho regulon is a global regulatory circuit involved in bacterial phosphate management. It has been best studied in model cells like Escherichia coli and Bacillus subtilis. Using two-dimensional electrophoresis, it was shown that the PhoB–PhoR system of E. coli regulates by 137 genes, 118 of which are induced and the other 19 are suppressed by phosphorus starvation conditions. More than 30 of these genes have been cloned and sequenced. The following table lists the characterized genes/operons part of the pho regulon, their functions and the cellular locations:

Working
When extracellular inorganic Phosphate(Pi) is in excess, it is transported into the cell by the Pit (phosphate inorganic transport) system. But under phosphate-limiting conditions, Pi is transported by the pst (phosphate-speciﬁc transport) system. Phosphate starvation conditions induce the so called psi (phosphate starvation–inducible) genes which are listed out in the table shown above. Under starvation conditions, the phoE pore protein of the outer membrane facilitates the diffusion of phosphorus-containing compounds through this membrane. When these compounds enter in to periplasm, they cannot penetrate the cytoplasmic membrane due to their large size and hence are hydrolyzed by phoA or appA resulting in the formation of Pi. The Pi then binds to the pstS protein and is transported through the cytoplasmic membrane by the pst system. The pstC and pstA proteins of this system form a channel in the membrane, through which Pi is transported to the cytoplasm. The membrane-bound PstB protein provides energy for the transport process. Phosphate carrying compounds like phosphonates, glycerol-3-phosphate can also be utilized as substrates in phosphate limiting conditions. These are metabolized by the phn and ugpBAECQ operons to give phosphates which are then assimilated by the pho regulon genes.

Mechanism of Signal Transduction
The cytoplasmic protein phoU acts a signal transduction mediator in the pho regulon. When Pi in the medium is in excess, the expression of the Pho regulon genes is repressed due to formation of the repressor complex, which includes the PhoR, PhoU proteins and proteins of the pst system. The complex is formed only when the Pi-binding protein pstS is completely saturated with phosphate. This provokes conformational alterations in the pst system which are transmitted, via the PhoU protein, to PhoR, which transforms to the repressor form PhoRR. The interactions between PhoU and PhoR are essential for the formation of the repressor PhoRR. PhoRR can inactivate PhoB-P by means of its dephosphorylation. Under starvation conditions, when the PstS protein is no longer saturated with Pi, conformational changes in the Pst system eliminate PhoRR from the repressor complex, which leads to the formation of the activator PhoRA which in turn phosphorylates PhoB forming PhoB-P.

Control of Transcription
Initiation of transcription requires the binding of PhoB regulator protein to the promoter region of the pho genes. The promoter region of pho genes have a conservative sequence in the –10 region, which interacts with the &sigma;-70 containing RNA polymerase. The -35 region of all of the Pho regulon genes contain a speciﬁc sequence, the pho box, which is responsible for interaction with the PhoB regulator protein. The phosphorylated PhoB (PhoB-P) recognizes this sequence and interacts with it, thereby initiating gene transcription.

Homologues of Pho regulon
A similar phosphate regulatory network exists in Bacillus subtilis, a gram positive bacteria, however with different cellular localization due to differences in cell wall structure. In yeast Saccharomyces cerevisiae, it has been showed that a phosphate regulatory mechanism exists and inhibition and activation of enzymes involved in Pi assimilation depends on the availability of Phosphate in the medium just like in E. coli. A significant fraction of genes (>10%) are transcribed with cell cycle periodicity. These genes encode critical cell cycle regulators as well as proteins with no direct connection to cell cycle functions. The transcriptional regulators Pho4p and Pho2p are involved in transcription of several genes in the Pho regulon. Genetic evidence with temperature sensitive pho4 and pho2 mutants suggested that Pho4p and Pho2p interact with each other.

Cross Regulation of the Pho Regulon Genes
This type of regulation of the pho genes is independent of extracellular Pi. Essentially, in this mechanism the regulator protein of a two-component regulatory system is controlled by another regulatory system. It is hypothesized that acetyl phosphate, which is an intermediate of phosphorus metabolism that serves as a donor of a phosphate group for ATP, is an inducer of the Pho regulon. Acetyl phosphate controls the Pho regulon due to changes in the proportion between the concentrations of ATP and acetyl phosphate. The expression of the Pho regulon genes is induced when this proportion decreases. There are two phosphate-independent regulatory systems which activate the PhoB protein in the absence of its native kinase, PhoR. Both systems are associated with the central metabolism of bacterial cells and both are regulated by carbon sources.