Gene expression regulation in eukaryotic and prokaryotic cells is important in cellular function, preventing the waste of energy and resources, and allowing control of metabolic pathways.
One example of gene expression regulation in prokaryotes is the lac operon. This method of expression regulation, where the substrate for the metabolic pathway must be present to activate the pathway, allows the expression of the relevant enzymes only when needed. This prevents excess protein production, preventing the waste of amino acids and energy. Lactose binds to the repressor of the operon, causing a conformational change and dissociating it from the DNA. This allows RNA polymerase to transcribe the DNA, allowing the expression of the necessary enzymes. By controlling the expression of these genes, the cell ensures it prioritises the metabolism of glucose ahead of lactose, due to the higher energy present within glucose.
In eukaryotic cells, gene expression regulation is much more complex and less well understood. Expression is regulated by combinations of repressors, enhancers, promoter-proximal elements, distal enhancers, and likely other components that are not yet known. Transcription factors, small proteins that bind to the DNA, are able to increase the transcriptional activity of a given gene, binding to promoter-proximal elements. These are around 200 base pairs before the transcription start site (TSS). ChIP-seq experiments can be used to identify protein binding sites on DNA.
It is important to control gene expression. Enzyme activity may be controlled by the transcriptional activity of its genes. This can allow the suppression of metabolic pathways over time (days to weeks) by preventing synthesis.