Ribosomes, aminoacyl tRNAs, initiation factors, and elongation factors are all required for the translation of mRNA molecules. The availability of these components affects the rate of translation and therefore influences gene expression. For example, the activation of T lymphocytes (T cells) is critical to the development of immune responses to viruses (see Chapter 22). T cells are normally in the G0 stage of the cell cycle and not actively dividing. On exposure to viral antigens, however, specific T cells become activated and undergo rapid proliferation (Figure 17.14). Activation includes a 7- to 10-fold increase in protein synthesis that causes cells to enter the cell cycle and proliferate. This burst of protein synthesis does not require an increase in mRNA synthesis. Instead, a global increase in protein synthesis is due to the increased availability of initiation factors taking part in translation—initiation factors that allow ribosomes to bind to mRNA and begin translation. This increase in initiation factors leads to more translation from the existing mRNA molecules, increasing the overall amount of protein synthesized. Similarly, insulin stimulates the initiation of overall protein synthesis by increasing the availability of initiation factors. Initiation factors exist in inactive forms and, in response to various cell signals, can be activated by chemical modifications of their structure, such as phosphorylation.
Mechanisms also exist for the regulation of translation of specific mRNAs. The initiation of translation in some mRNAs is regulated by proteins that bind to an mRNA’s 5′ UTR and inhibit the binding of ribosomes, similar to the way in which repressor proteins bind to operators and prevent the transcription of structural genes. The translation of some mRNAs is affected by the binding of proteins to sequences in the 3′ UTR.
Many eukaryotic proteins are extensively modified after translation by the selective cleavage and trimming of amino acids from the ends, by acetylation, or by the addition of phosphate groups, carboxyl groups, methyl groups, or carbohydrates to the protein. These modifications affect the transport, function, and activity of the proteins.
The initiation of translation may be affected by proteins that bind to specific sequences at the 5′ end of mRNA. The availability of ribosomes, tRNAs, initiation and elongation factors, and other components of the translational apparatus may affect the rate of translation. Translation of some mRNAs is regulated by proteins that bind to the 5′ and 3′ untranslated regions of the mRNA.
Now that we have considered the major types of gene regulation in bacteria (Chapter 16) and eukaryotes (this chapter), let’s consider some of the similarities and differences in bacterial and eukaryotic gene control.
These similarities and differences in gene regulation of bacteria and eukaryotes are summarized in Table 17.2.
Characteristic | Bacterial Gene Control | Eukaryotic Gene Control |
---|---|---|
Levels of regulation | Primarily transcription | Many levels |
Cascades of gene regulation | Present | Present |
DNA binding proteins | Important | Important |
Role of chromatin structure | Absent | Important |
Presence of operons | Common | Uncommon |
Negative and positive control | Present | Present |
Initiation of transcription | Relatively simple | Relatively complex |
Enhancers | Less common | More common |
Transcription and translation | Occur simultaneously | Occur separately |
Regulation by small RNAs | Rare | Common |