Pho regulon

Question 1

What is a regulon & how is it different to an operon?

What does the regulon encode?

What is its function?

In what form is phosphate made?

What two proteins are encoded by the regulon which make up the two-component regulatory system?

Answer 1

Collection of genes regulated as a unit to overcome the same condition - each have their own promotor (not controlled by 1 as a unit)

proteins to sense phosphate concentration, activators of gene transcription & genes on transcription to increase phosphate & promote cell survival

Increase [phoshphate] when cellular [phosphate] is low

Inorganic orthophosphate ion PO4 (3-)

Pho R = histidine kinase (phosphate sensing)
Pho B = DNA-binding protein (activate gene transcription)

Question 2

How does PhoR sense the phosphate concentration in the periplasm when its located in the cytoplasm & periplasmic membrane?

What is the final phosphate transporter protein complex?

What else is required for the pathway?

When [phosphate] is sufficient, what is the state of PhoR?

Answer 2

phosphate signal transduction pathway

PstSCAB (phosphate specific transporter)

PhoU - accessory protein which interacts with PstSCAB and PhoR

Inactive

Question 3

When [phosphate] is sufficient, what are the 3 steps?

What is significant about the state of PhoR?

What does this mean about transcription of pho regulon?

Answer 3

1. PstS protein in periplasm binds to phosphate & brings it to periplasmic membrane
2. PstSCAB & PhoU assemble to form channel from periplasm, through periplasmic membrane into cytoplasm
3. when [phosphate] high, channels open & enters cytoplasm

PhoR conformation hasn’t been altered so is inactive

Inactive PhoR (sensing domain) means no transcription of gene

Question 4

When [phosphate] is low, how is PhoR activated? 3 steps

What happens after its activation?

What does this state mean for PhoB?

How does this activate transcription?

How does this activate transcription of all the genes in the regulon?

Answer 4

1. Low phosphate concentration means PstS protein cannot bind to phosphate so doesn’t interact to form PstSCAB across membrane
2. PhoU therefore cannot interact with PstSCAB so undergoes conformation change & therefore PhoR also undergoes change (as both interact)
3. PhoR autophosphorylates itself by phosphorylating histidine residue with ATP to become active histidine kinase

Phosphorylates PhoB (activation)

Phosphorylated PhoB can dimerise & bind to the pho box upstream of the promotor site

Increases affinity of RNA polymerase to promotor through PhoB interaction with sigma factor

All have the same pho box sequences - PhoB can bind to all the sites on all genes