Ribosome translation control

Hinnebusch, A. G. (2000). Mechanism and regulation of initiator methionyl-tRNA binding to ribosomes. In Translational Control of Gene Expression (N. Sonenberg,... 

The synthesis of ribosomal proteins is regulated at the level of translation. Certain ribosomal proteins bind to specific sites on the ribosomal RNAs or their own mRNAs. In the absence of the ribosomal RNAs, they bind to their own mRNAs, which inhibits their translation. This form of translational control regulates the rate of synthesis of ribosomal proteins so that it does not exceed the rate of ribosomal RNA synthesis. 

As was evident in the previous section, the structural characteristics of the mRNA transcripts play a significant role in translation. An overwhelming majority of the structurally dependent mechanisms of translation control dealt with inhibition or improvement of ribosomal access to the initiation sites or altering the rate at which ribosomes scan across the transcripts. This section aims to complement the previous discussion by considering a more fundamental aspect of translation -codon usage. 

Another example of translational control in eukaryotes is the inhibition of yeast GCN4 protein synthesis by stem-loop structures present in the 50 end of the mRNA. GCN4 control, and an analogous situation in bacteria, links amino-acid biosynthesis to ribosome pausing in the 50 end of the mRNA. This mechanism was first described for the tryptophan operon in E. coli and it is often referred to as attenuation. Transcriptional and translational control of the tryptophan biosynthetic enzymes are described in Chapter 28. 

Several mechanisms could explain the presence of the specific mRNA and yet the absence of the specific protein that the mRNA encodes 1. HMG-CoA reductase could be subject to translational control, so that translation of the message and synthesis of HMG-CoA reductase by ribosomes are inhibited by cholesterol. 2. Alternatively, the protein could be synthesized but then rapidly degraded in the cholesterol-fed mice. 

The formation of proteins can be regulated at two levels, either by affecting formation of mRNA (transcription) or its utilization as template (translation). In turn, transcription can be controlled in two ways. One is a control of initiation of transcription (l.e., whether the RNA polymerase forms an initiation complex). The other is a control over how far a polymerase travels and whether it proceeds into a structural gene or not (l.e., whether attenuation of transcription occurs). Translational control would more likely be achieved by controlling ribosome binding to the mRNA, although mechanisms affecting ribosome travel are conceivable. 

To what extent reversible modiflcations of ribosomal constituents are involved in translational control of protein synthesis is uncertain. Although phosphorylation of ribosomal protein S6 increases with cell proliferation, it is not known whether this change is directly related to the accompanying increase in protein synthesis by an effect on the translation rate. 

It makes good sense that initiation is the rate-limiting step in mRNA translation. Once synthesis of a protein chain has been initiated, the remainder of its synthesis follows more or less automatically. The crucial step for control, then, is that involving the attachment of an mRNA molecule to a 40 S ribosomal subunit. Indeed, most cases of translational control concern this step. In general, once mRNA has entered a 40 S initiation complex, formation of the complete encoded polypeptide chain is virtually assured. Hence, this review will concentrate on the properties of mRNA and the translation components involved in the events leading up to this complex. 

As already mentioned, the key event for translational control in terms of differential gene expression can be designated as the step where mRNA binds to the 40 S ribosomal subunit. Since mRNA fails to bind in the presence of all components for initiation except Met-tRNAf, binding of Met-tRNAf to 40 S subunits is a necessary prerequisite for the subsequent binding of mRNA (Darnbrough et al., 1973 Schreier and Staehelin, 1973 Trachsel et al., 1977 Benne and Hershey, 1978). Conversely, the participation of mRNA is not needed for binding of Met-tRNAf to 40 S subunits. 

In order to identify initiation factors involved in mRNA competition, a number of studies have employed reconstituted cell-free systems (Golini et al., 1976 Ray et al., 1983). A major problem with such systems is that certain components may be present in excess, while others may be limiting or partially inactivated, precluding efficient initiation. However, meaningful studies of translation initiation frequency in vitro can only be done in systems where ribosomes can cycle rapidly and repeatedly over mRNA. To date, the only cell-free systems that translate mRNA at high efficiency are the reticulocyte lysate (see Jackson, 1982) and the micrococcal nuclease-treated reticulocyte lysate (Pelham and Jackson, 1976). The latter system offers several advantages over reconstituted cell-free systems. It responds to translational control signals (see below), it is capable of extensive and efficient initiation in conditions more likely to be representative of protein synthesis in intact cells, and except for mRNA, it contains all other components for translation in a proportion much closer to that of the intact cell. 

Several host-virus interactions at the level of translational control may involve eIF-2. The ability of a strong viral RNA template, such as mengovirus RNA, to out-compete host mRNA for eIF-2 (Rosen et al., 1982) will lead to the selective translation of viral mRNA concomitant with a displacement of host mRNA from the ribosomes. 

Kaempfer, R., and Kaufman, J., 1972, Translational control of hemoglobin synthesis by an initiation factor required for recycling of ribosomes and for their binding to messenger RNA, Proc. Natl. Acad. Sci. USA 69 3317. 

The above data led Thach and co-workers to propose a model for translation control based upon competition of cellular and viral mRNAs for a discriminatory factor. They suggested that a host cell-derived discriminatory factor that must be bound to mRNA prior to its recognition by the native 40 S ribosomal subunit exists in two states, bound to mRNA or free. Reducing the rate of elongation with cycloheximide would lead to an increase in the amount of time that the factor is in the free state. This, in turn, would increase the steady state of unbound discriminatory factor resulting in an increase in the probability that a low-affinity mRNA will bind to the factor and be translated. 

Hormonal effects on ribosomal protein synthesis (translation control) rather than at the gene level have been demonstrated in connection with hormonal stimulation (by luteinizing hormone) of the synthesis of steroid hormones by the ovaries (Gorski and Padnos, 1966). The possibility is not ruled out that in phenomena of this type activation may take place throu the "unmasking of messenger RNA as mentioned previously. 

Huang and Ochoa, 1972). The significance of such an interference factor and of changes in IF3 activity to T4 infection is not yet established. An explanation for translation control in T4 infection must account for the fact that "late" proteins of T4 can be synthesized in vitro using late T4 mRNA and ribosomes isolated from uninfected E. coli (Salser et al., I967) and that the time of IF3 inactivation measured so far is too late for an involvement in the host shut-off. 

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