Two Component Regulatory Systems

Covalent addition of a chemical group (as opposed to binding an entire signal molecule) is widely used to control the activity of both enzymes and DNA-binding proteins. Phosphate is the most common group used, but methyl, acetyl, AMP and ADP-ribose moieties may also be used.

One large class of regulatory systems that use a phosphate group are the two-component regulatory systems. Although often regarded as characteristic of bacteria, they have also been found in lower eukaryotes, including yeast and slime molds. As the name implies, two-component regulatory systems consist of two proteins that cooperate to regulate gene expression. The first component is a DNA-binding regulator protein that only binds DNA when phosphorylated. The second is a trans-membrane sensor kinase that senses a change in the environment and changes shape. This causes the sensor kinase to phosphorylate itself using ATP and then transfer the phosphate group to the DNA-binding regulator (Fig. 9.20).

There are many different two-component regulatory systems in E. coli (see Table 9.02 for examples). Usually the sensor kinase is a membrane protein that senses either physical conditions of some sort (e.g., aeration, osmotic pressure) or a nutrient (e.g., phosphate, nitrate). The DNA-binding form of the regulator may act as an activator or a repressor. The ArcAB system senses aerobic versus anaerobic conditions. Under sensor kinase A protein that phosphorylates itself when it senses a specific signal (often an environmental stimulus, but sometimes an internal signal)

two-component regulatory system A regulatory system consisting of two proteins, a sensor kinase and a DNA-binding regulator

Two-Component Regulatory Systems in E. coli

Stimulus/Function

Sensor

Regulator

Lack of oxygen

ArcB

ArcA

Osmolarity, envelope proteins

EnvZ

OmpR

Osmolarity, potassium transport

KdpD

KdpE

Phosphate deprivation

PhoR

PhoB

Nitrogen metabolism

NtrB

NtrC

Nitrate respiration

NarX

NarL

Nitrate and nitrite respiration

NarQ

NarP

FIGURE 9.21 Four Domain Phosphorelay

The ArcAB two-component regulatory system consists of a membrane sensor, ArcB, and a DNA-binding regulator protein, ArcA. The direction of movement of the phosphate group along the ArcB sensor protein is shown. The phosphate is finally passed from the Arc B sensor to the ArcA regulator.

anaerobic conditions, ArcB phosphorylates itself and then phosphorylates ArcA. The ArcA-P regulator then represses about 20 genes that are only required for aerobic metabolism and activates half a dozen genes needed when oxygen is absent or very low.

Signals are often passed on by adding or removing phosphate groups.

Was this article helpful?

0 0

Post a comment