Activators and Repressors May Be Covalently Modified

Some regulatory proteins do not bind a separate independent signal molecule. Instead, some activators and repressors are chemically modified. Most often this is done by the attachment of a chemical group, usually phosphate (see below). Less commonly, the regulatory protein is altered chemically in some other way, for example, by oxidation or reduction.

Examples of bacterial regulatory proteins that are altered by oxidation or reduction are the activators OxyR and Fnr. OxyR is converted to its active form by hydrogen peroxide or related oxidizing agents that oxidize sulfhydryl groups to disulfides (Fig. 9.19A). It then activates a set of genes involved in protecting bacterial cells against oxidative damage.

In contrast, Fnr is inactive when oxidized and becomes an activator when reduced. In this case, an Fe4S4 iron sulfur cluster in the N-terminal domain of Fnr is reduced under anaerobic conditions. This results in the formation of dimers and a change in shape of the C-terminal DNA-binding domain (Fig. 9.19B). The Fnr activator then activates genes involved in anaerobic respiration, such as those for nitrate reductase, fumarate reductase and formate dehydrogenase.

anaerobic respiration Respiration using other oxidizing agents (e.g. nitrate) instead of oxygen iron sulfur cluster Group of iron and sulfur atoms found in proteins and involved in oxidation/reduction reactions

Two-Component Regulatory Systems 253

FIGURE 9.20 Model of Two-Component Regulatory System

The two-component regulatory system includes a membrane component and a cytoplasmic component. Outside the cell, the sensor domain of the kinase detects an environmental change, which leads to phosphorylation of the transmitter domain. The response regulator protein receives the phosphate group, and as a consequence, changes configuration so as to bind the DNA.

A sensor protein plus a regulator protein often act together as a two component regulatory system.

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