Some Regulatory Proteins May Cause Translational Repression

Just as regulatory proteins may bind to DNA and either promote or hinder transcription, proteins may also bind to specific sequences on a mRNA and regulate its translation. Examples of both positive and negative translational regulation are known. The response of mRNA to iron is used below as an example of translational repression.

Iron is an essential nutrient and is the cofactor for many proteins, such as cytochromes and hemoglobin. However, free iron generates toxic free radicals and is therefore dangerous. Consequently, surplus iron atoms are stored by ferritin and its prokaryotic counterpart, bacterioferritin. Ferritin is a hollow spherical protein consisting of 24 subunits. Up to 5,000 iron atoms may be stored as a hydroxyphosphate complex inside this sphere.

The level of ferritin is regulated in response to the iron supply. In plants, ferritin levels are regulated at the level of transcription. However, in both animals and bacteria, ferritin levels depend on translational regulation. When iron is scarce, translation bacterioferritin The bacterial analog of ferritin, an iron storage protein ferritin An iron storage protein translational repression Form of control in which the translation of a messenger RNA is prevented

Some Regulatory Proteins May Cause Translational Repression 285

FIGURE 11.02 Cleavage of adhE mRNA by RNase III

The ribosome binding site of adhE messenger RNA can not bind to the ribosome due to folding of the pre-mRNA. RNAase III cleaves the adhE pre-mRNA so exposing the ribosome binding site.

In animals, translation of genes involved in iron uptake and storage is controlled by an iron regulatory protein that binds to the mRNA.

of ferritin mRNA is reduced. When iron is plentiful, more ferritin is made. In animals, an RNA-binding protein is responsible, whereas in bacteria control is by antisense RNA (see below).

The 5'-untranslated region (5'-UTR) of ferritin mRNA of animals contains a special recognition sequence known as an iron-responsive element (IRE), which forms a stem loop structure. When iron is scarce, an iron regulatory protein (IRP) binds to the IRE stem and loop and prevents translation. Surplus iron results in the detachment of IRP from the ferritin mRNA, which can then be translated (Fig. 11.03).

5'-untranslated region (5'-UTR) The untranslated sequence between the 5'-end of an mRNA and the start codon iron regulatory protein (IRP) Translational regulator that controls expression of mRNA in animals in response to the level of iron iron-responsive element (IRE) Site on mRNA where the IRP binds

FIGURE 11.03 Regulation of Ferritin mRNA Translation by IRP

The ferritin mRNA has a loop structure (the iron-responsive element or IRE) in the 5' untranslated region. When iron is scarce, the iron regulatory protein (IRP) binds preventing the small subunit of the ribosome from binding to the cap structure of the mRNA to initiate translation. When iron is abundant no IRP is bound and translation occurs.

FIGURE 11.04 Aconitase Activity versus RNA-binding of IRP1

The IRP1 protein acts as an enzyme (aconitase) when iron is plentiful. Upon losing an iron atom from the central Fe4S4 cluster, during situations of iron scarcity, the two major domains open, allowing binding to RNA. The consequence of binding of IRP1 to mRNA is the blockage of transferrin synthesis.

In animals, the enzyme S-aminolevulinic acid synthase catalyses the rate limiting step in the pathway for synthesizing the iron-containing cofactor heme. Not surprisingly, its mRNA contains an iron-responsive element and is also under IRP transla-tional control. The receptor for the iron transport protein transferrin, which is found in blood, is also regulated by IRP binding to an IRE on the mRNA, but in this case mRNA stability is affected.

The free iron concentration in the cytoplasm is directly monitored by the iron regulatory proteins. Although there are several IRPs, the major one, IRP1, is identical to the cytoplasmic enzyme, aconitase. This enzyme has an Fe4S4 cluster that is needed for enzyme activity. When iron is scarce, one of the four iron atoms is lost from the Fe4S4 cluster. The aconitase/IRP1 then loses enzyme activity and changes conformation, exposing its RNA-binding site. Thus when iron is plentiful, aconitase/IRP1 acts as aconitase (an enzyme in the Krebs Cycle that converts citrate to isocitrate) and when iron is scarce, it acts as an RNA-binding translational repressor (Fig. 11.04).

Regulation by translational repression is also used to control the synthesis of ribo-somal proteins in bacteria such as E. coli.The ribosomal proteins are grouped in several operons. For each operon, one of the encoded ribosomal proteins binds to the mRNA and so auto-regulates synthesis of all proteins in the operon. These ribosomal proteins bind to rRNA preferentially and only bind to their own mRNA when there is no rRNA available. This mechanism ensures that the amount of rRNA and ribosomal proteins is balanced. If there is an excess of ribosomal protein over rRNA then translation will be decreased (Fig. 11.05).

aconitase An enzyme of the Krebs cycle that, in animals, also acts as an iron regulatory protein Fe4S4 cluster A group of inorganic iron and sulfur atoms found as a cofactor in several proteins translational repressor A protein that binds to mRNA and prevents its translation

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