MRNA translation, regulatory mechanism

Regulation of expression may occur at both the transcriptional and post-transcriptional levels. The mRNA for GM-CSF contains (in common with those of some other cytokines) conserved regulatory sequences in the 3 untranslated region, which may affect its rate of translation. The gene is constitutively transcribed in monocytes, endothelial cells and fibroblasts, but the mRNA is unstable and so does not accumulate to levels sufficient to allow translation into significant amounts of protein. Activation of these cells results in the increased expression of GM-CSF protein, which arises from both an enhanced rate of transcription (as detected in nuclear runoff experiments) and also an increased stability of the mRNA, perhaps by mechanisms analogous to those described above during activation of G-CSF expression ( 2.2.3.1). 

Iron regulatory proteins (IRPs) regulate the cellular iron level in mammalian cells. IRPs are known as cytosol mRNA binding proteins which control the stability or the translation rate of mRNAs of iron metabolism-related proteins such as TfR, ferritin, and 5-aminolevulinic acid synthetase in response to the availability of cellular iron [19-21] after uptake [5]. The regulatory mechanism involves the interaction between the iron-responsive element (IRE) in the 3 or 5 untranslated regions of the transcripts and cytosolic IRPs (IRP-1 and -2). IRP-1 is an iron-sulfur (Fe-S) protein with aconitase activity containing a cubane 4Fe-4S cluster. When Fe is replete, IRP-1 prevails in a 4Fe-4S form as a holo-form and is an active cytoplasmic aconitase. As shown in Fig. 3, when Fe is deplete, it readily loses one Fe from the fourth labile Fe in the Fe-S cluster to become a 3Fe-4S cluster and in this state has little enzymatic activity [22, 23]. 

HMG-CoA reductase is also subject to translational control by a mevalonate-derived non-sterol regulator (D. Peffley, 1985 M. Nakanishi, 1988). Tliis component of the regulatory mechanism can be observed only when cultured cells are acutely incubated with statins, which block mevalonate formation. Under those conditions, sterols have no effect on HMG-CoA reductase mRNA translation however, mevalonate reduces the HMG-CoA mRNA translation by 80% with no change in mRNA levels. Translational control of hepatic HMG-CoA reductase by dietary cholesterol was shown in an animal model in which polysome-associated HMG-CoA reductase mRNA was analyzed in cholesterol-fed rats (C.M. Chambers, 1997). It was found that cholesterol feeding increased the portion of mRNA associated with translationally inactive monosomes and decreased the portion of mRNA associated with translationally active polysomes. The mechanism of HMG-CoA reductase translational control has not been elucidated. 

Figure 12-1 Constancy of environmental signal (Fe) and gene product (ferritin protein) with variable genetic regulatory mechanisms (DNA transcription or mRNA translation) in animals and plants.

It is noteworthy that free mRNP particles are a temporary untranslatable form of mRNA in the cytoplasm and that proteins associated with the active polysomal mRNP or the repressed mRNP are different [6, 7]. There is now some evidence that ribo-nucleoprotein particles are dynamic structures and that protein exchanges occur between the cytoplasmic mRNA-associated proteins and free proteins. Involvement of mRNA-associated proteins in the regulation of protein synthesis has been considered [7-10] and post-translational modification of these proteins as a regulatory mechanism might be considered. 

Regulatory mechanisms at the level of mRNA translation could also lead to gross metabolic changes. The mechanism of protein synthesis has been exhaustively studied [5], and many components have been implicated. Changes in each of these components—ribosomes, factors involved in the ribosomal binding of mRNA, in the initiation and termination of protein synthesis, and in polypeptide chain elongation, tRNA, and the components responsible for its acylation and subsequent transfer to the polysomal complex—could potentially lead to alteration in the rate, extent, or fidelity of protein synthesis. 

The differential expression of Ctj and Ct2, which controls the synthesis of catalase in corn, is mainly dependent upon the ratio between synthesis and degradation rates (Quail and Scandalios, 1971). It is characteristic that the differential rate of translation is an important regulatory mechanism in hemoglobin synthesis in mammals, wherein the a-chain is translated faster than the j8-chain. Therefore, two different types of mRNA are translated at a different rate inside the same cell (Hunt et al., 1969). 

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