This document summarizes two-component signaling systems in bacteria and plants. It describes how in bacteria, like E. coli, membrane receptors detect extracellular signals and phosphorylate response regulators to control flagella rotation and chemotaxis. A similar system in plants involves cytokinin receptors phosphorylating histidine phosphotransfer proteins and response regulator proteins to control gene expression and cellular responses to cytokinin. The molecular mechanisms of both systems involve a phosphorylation cascade from a sensor kinase to response regulators that control downstream effects.

1. BY
SHARIQ AISHA
DEPARTMENT OF BIORESOURCES
UNIVERSITY OF KASHMIR
rasoolshariqa@gmail.com

2. Two- component signaling system is the most common form of signaling pathway that responds to
extracellular events in bacteria and plants. Protein phosphorylation is the key mechanism for regulating
signal transduction pathways in both prokaryotes and eukaryotes.
• In pioneering studies of chemotaxis in bacteria, Julius Adler showed that Escherichia
coli responds to nutrients in its environment, including sugars and amino acids, by
swimming toward them, propelled by one or a few flagella.
• A family of membrane proteins have binding domains on the outside of the plasma
membrane to which specific attractants/ repellents bind.
• Ligand binding causes an intrinsic kinase activity of the receptor to phosphorylate a
His residue in its cytosolic domain.

3. This first component of the two-component system, the receptor histidine
kinase, then catalyzes transfer of the phosphoryl group from the His residue
to an Asp residue on a second, soluble protein, the response regulator.

4. MOLECULAR MECHANISM OF TWO COMPONENT PHOSPHORYLATION RELAY
SYSTEM
• Genetic studies indicate that four cytoplasmic proteins - CheA, CheW, CheY, and CheZ - are
involved in the intracellular signaling pathway that couples the chemotactic receptors to the
flagellar motor.
• CheY acts at the effector end of the pathway to control the direction of flagellar rotation.
When activated, it binds to the motor, causing it to rotate clockwise and thereby inducing
tumbling; mutants that lack this protein swim constantly without tumbling.
• CheA is a histidine protein kinase. When bound to both an activated chemotactic receptor and
CheW, it phosphorylates itself on a histidine residue and almost immediately transfers the
phosphate to an aspartic acid residue on CheY’

5. • The phosphorylation of CheY activates the protein so that it binds to the flagellar motor
and causes clockwise rotation and tumbling.
• CheZ rapidly inactivates phosphorylated CheY by stimulating its dephosphorylation.

6. • The binding of a repellent to a chemotactic receptor increases the activity of the receptor,
which in turn increases the activity of CheA and thereby the phosphorylation of CheY,
which causes tumbling.
• These phosphorylations occur rapidly: the time required for the tumbling response after
adding a repellent is about 200 milliseconds.
• The binding of an attractant has the opposite effect. It decreases the activity of the receptor,
which decreases the activity of CheA, so that CheY remains dephosphorylated, the motor
continues to rotate counterclockwise, and the bacterium swims smoothly.
• In the absence of any environmental stimulus, the direction of rotation of the disc reverses
every few seconds, producing a characteristic pattern of movement in which smooth
swimming in a straight line is interrupted by abrupt, random changes in direction caused by
tumbling.

8. • There are four types of plasma membrane chemotaxis receptors, each concerned with
the response to a small group of chemicals.
• Type 1 and 2 receptors mediate responses to serine and aspartate, respectively, by
directly binding these amino acids and transducing the binding event into an
intracellular signal.
• Type 3 and 4 receptors mediate responses to sugars and dipeptides, respectively, in a
slightly less direct fashion.

11. CYTOKININ TWO-COMPONENT SIGNALING(IN PLANTS)
• Cytokinins are perceived and transduced by a two- component signaling system.
• Typically these systems are composed of two functional elements: a sensor histidine
kinase, to which a signal binds, and a downstream response regulator, whose activity is
regulated via phosphorylation by the sensor histidine kinase.
TYPES OF TWO-COMPONENT SIGNALING SYSTEM
SIMPLE TWO-COMPONENT SIGNALING SYSTEM
• In simple two-component systems, the input domain is the site where the signal is
sensed. This regulates the activity of the histidine kinase domain, which when activated
autophosphorylates on a conserved His residue. The phosphate is then transferred to an
Asp residue that resides within the receiver domain of a response regulator.
Phosphorylation of this Asp regulates the activity of the output domain of the response
regulator, which in many cases is a transcription factor

13. PHOSPHORELAY TWO-COMPONENT SIGNALING SYSTEM
In the phosphorelay-type two-component signaling system, an extra set of phosphotransfers is
mediated by a histidine phosphotransfer protein (Hpt), called AHP in Arabidopsis. The Arabidopsis
response regulators are called ARRs. H = histidine, D = aspartate.

14. MODEL FOR CYTOKININ SIGNAL TRANSDUCTION VIA MULTISTEP
PHOSPHORELAY SYSTEM

15. STEPS
1. Cytokinin binds to CRE1 (cytokinin response receptor-1), which is likely to occur as a
dimer.Cytokinin binds to an extracellular portion of CRE1 called the CHASE domain.Two
other hybrid sensor kinases (AHK2 and AHK3) containing a CHASE domain are also likely
to act as cytokinin receptors in Arabidopsis.
2. Cytokinin binding to these receptors activates their histidine kinase activity. The phosphate is
transferred to an asparate residue (D) on the fused receiver domains.
3. The phosphate is then transferred to a conserved histidine present in an AHP protein
(Arabidopsis Histidine Phosphotransfer protein).

16. 4. Phosphorylation causes the AHP protein to move into the nucleus, where it transfers the
phosphate to an asparate residue located within the receiver domain of a type-B ARR.
5. The phosphorylation of the type-B ARR activates the output domain to induce
transcription of genes encoding type-A ARRs.
6. The type-A ARRs are likely also to be phosphorylated by the AHP proteins.
7. The phosphorylated type-ARRs interact with various effectors to mediate the changes in
cell function appropriate to cytokinin (indicated in the model as "cytokinin responses").

18. REFRENCES:
 Lehninger Principles of Biochemistry(David and Michael M. Cox )
 Molecular Biology of The Cell (Bruce Alberts)
 Plant Physiology (Lincoln Taiz and Eduardo Zeiger)
 Introduction to Plant Physiology (Willaim G. Hopkins and Norman P. A. Huner)