Ribosomes, aminoacyl tRNAs, initiation factors, and elongation factors are all required for the translation of mRNA molecules, as we saw in Chapter 11. The availability of these components affects the rate of translation and therefore influences gene expression.
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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, in a manner similar to the repressor proteins that bind to operators and prevent the transcription of structural genes in prokaryotes. 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 proteins and therefore have the ability to affect gene expression.
The initiation of translation may be affected by proteins that bind to specific sequences near 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.
A Comparison of Bacterial and Eukaryotic Gene Control
Now that we have considered the major types of gene regulation in bacteria and eukaryotes, let’s consider some of the similarities and differences in bacterial and eukaryotic gene control.
Much of gene regulation in bacterial cells is at the level of transcription (although it does exist at other levels). Gene regulation in eukaryotic cells takes place at multiple levels, including chromatin structure, transcription, mRNA processing, mRNA stability, translation, and posttranslational control.
Complex biochemical and developmental events in bacterial and eukaryotic cells may require a cascade of gene regulation, in which the activation of one set of genes stimulates the activation of another set.
Much of gene regulation in both bacterial and eukaryotic cells is accomplished through proteins that bind to specific sequences in DNA.
Chromatin structure plays a role in eukaryotic (but not bacterial) gene regulation. In general, condensed chromatin represses gene expression; chromatin structure must be altered before transcription can take place. Chromatin structure is altered by chromatin-
In bacterial cells, genes are often clustered in operons and are coordinately expressed by transcription into a single mRNA molecule. In contrast, many eukaryotic genes have their own promoters and are transcribed independently. Coordinated regulation in eukaryotic cells often takes place through common response elements located in the promoters and enhancers of the genes. Different genes that share the same response element are influenced by the same regulatory protein.
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Regulatory proteins that affect transcription exhibit two basic types of control: repressors inhibit transcription (negative control) and activators stimulate transcription (positive control). Both negative control and positive control are found in bacterial and eukaryotic cells.
The initiation of transcription is a relatively simple process in bacterial cells, and regulatory proteins function by blocking or stimulating the binding of RNA polymerase to DNA. In contrast, eukaryotic transcription requires complex machinery that includes RNA polymerase, general transcription factors, and transcriptional activators, which allows transcription to be influenced by multiple factors.
Some eukaryotic transcriptional regulator proteins function at a distance from the gene by binding to enhancers, causing the formation of a loop in the DNA, which brings the promoter and enhancer into close proximity. Some distant-
The greater time lag between transcription and translation in eukaryotic cells than in bacterial cells allows mRNA stability and mRNA processing to play larger roles in eukaryotic gene regulation.
Regulation by small RNAs occurs in both eukaryotes and bacteria.
These similarities and differences in gene regulation in bacteria and eukaryotes are summarized in Table 12.1.
| Characteristic | Bacterial gene control | Eukaryotic gene control |
|---|---|---|
| Levels of regulation | Primarily transcription | Many levels |
| Cascades of gene regulation | Present | Present |
| DNA- |
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 | Common | Common |