Regulatory Control
The transcriptional activity of active genes is regulated by a group of proteins called transcription factors that mediate binding of RNA polymerase to the promoter
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In eukaryotes, RNA polymerase cannot initiate transcription without these factors and hence their levels determine the amount of gene expression
Some transcription factors (called regulatory proteins) may bind to DNA sequences outside of the promoter and interact with the basal transcription factors to mediate RNA polymerase activity
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Activator proteins bind to enhancer sites and increase the rate of transcription (by mediating complex formation)
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Repressor proteins bind to silencer sequences and decrease the rate of transcription (by preventing complex formation)
The presence of certain transcription factors may be tissue-specific, leading to the differentiation and specialisation of cells in these specific tissues
Transcription Factors
Transcription Triggers
The activity of transcription factors can be controlled by cellular signals that may be triggered by internal or external factors
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Certain transcription factors are ligand-activated and require the presence (or absence) of these specific molecular messengers in order to function
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Examples of transcription-inducing ligands include steroid hormones and inducer molecules (such as lactose)
Hormones
Steroid hormones are lipophilic and can freely cross the plasma membrane to induce changes in gene expression
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They bind to receptors in either the cytoplasm or nucleus of the target cell, to form an active receptor-hormone complex
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This activated complex will move into the nucleus and bind directly to DNA, acting as a transcription factor for gene expression
Protein hormones can also induce changes in gene expression, but cannot do so directly as they cannot cross the plasma membrane (they are lipophobic)
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Instead, they bind to transmembrane receptors on the plasma membrane which activate intracellular molecules called second messengers
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Via this process of signal transduction, the second messengers can initiate changes to the pattern of gene expression
Mode of Hormone Action
Inducers
Lactose is a disaccharide that can be used as an inducer molecule in bacteria to regulate the expression of the enzymes responsible for its metabolism
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Lactose can be digested by bacteria to provide a source of energy – the metabolism of lactose is controlled by a group of genes known as the lac operon
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Normally, transcription of the lac operon is inhibited by a repressor protein (which binds to a silencer sequence called the operator)
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When lactose levels are high, lactose binds to the repressor and inactivates it – allowing transcription to proceed
Lac Operon
Gene Expression (no inducer)
Gene Suppression (inducer present)