Regulation of transcription
Regulation of transcription controls when transcription occurs and how much RNA is created. Transcription of a gene by RNA polymerase can be regulated by at least six mechanisms:
- Specificity factors alter the specificity of RNA polymerase for a given promoter or set of promoters, making it more or less likely to bind to them (i.e. sigma factors used in prokaryotic transcription).
- Repressors bind to non-coding sequences on the DNA strand that are close to or overlapping the promoter region, impeding RNA polymerase's progress along the strand, thus impeding the expression of the gene.
- General transcription factors These transcription factors position RNA polymerase at the start of a protein-coding sequence and then release the polymerase to transcribe the mRNA.
- Activators enhance the interaction between RNA polymerase and a particular promoter, encouraging the expression of the gene. Activators do this by increasing the attraction of RNA polymerase for the promoter, through interactions with subunits of the RNA polymerase or indirectly by changing the structure of the DNA.
- Enhancers are sites on the DNA helix that are bound to by activators in order to loop the DNA bringing a specific promoter to the initiation complex.
- Silencers are regions of DNA that are bound by transcription factors in order to silence gene expression. The mechanism is very similar to that of enhancers.
- Chromatin remodeling through specific use of miRNA molecules presents one method by which euchromatin, typically associated with transcriptional activity, is converted to heterochromatin, reducing transcription. This occurs by means of RNA induced transcriptional silencing complex or "RITS."
Regulatory protein is a term used in genetics to describe a protein involved in regulating gene expression. It is usually bound to a DNA binding site which is sometimes located near the promoter although this is not always the case. Sites of DNA sequence where regulatory proteins bind are called enhancer sequences. Regulatory proteins are often needed to be bound to a regulatory binding site to switch a gene on (activator) or to shut off a gene (repressor). Generally, as the organism grows more sophisticated, its cellular protein regulation becomes more complicated and indeed some human genes can be controlled by many activators and repressors working together.
Prokaryotes vs. eukaryotes
In prokaryotes, regulation of transcription is needed for the cell to quickly adapt to the ever-changing outer environment. The presence or the quantity and type of nutrients determines which genes are expressed; in order to do that, genes must be regulated in some fashion. In prokaryotes, repressors bind to regions called operators that are generally located downstream from and near the promoter (normally part of the transcript). Activators bind to the upstream portion of the promoter, such as the CAP region (completely upstream from the transcript). A combination of activators, repressors and rarely enhancers (in prokaryotes) determines whether a gene is transcribed.
In eukaryotes, transcriptional regulation tends to involve combinatorial interactions between several transcription factors, which allow for a sophisticated response to multiple conditions in the environment. This permits spatial and temporal differences in gene expression. Eukaryotes also make use of enhancers, distant regions of DNA that can loop back to the promoter. A major difference between eukaryotes and prokaryotes is the fact the eukaryotes have a nuclear envelope, which prevents simultaneous transcription and translation. RNA interference also regulate gene expression in most eukaryotes, both by epigenetic modification of promoters and by breaking down mRNA.
- When E. coli bacteria are subjected to heat stress, the σ32 subunit of its RNA polymerase changes such that the enzyme binds to a specialized set of promoters that precede genes for heat-shock response proteins.
- When a bacterium contains a surplus amount of the amino acid tryptophan, the acid binds to a specialized repressor protein (tryptophan repressor). The binding changes the structural conformity of the repressor such that it binds to the operator region for the operon that synthesizes tryptophan, preventing their expression and thus suspending production. This is a form of negative feedback.
- The lac repressor protein, as part of the lac operon in bacteria, blocks the synthesis of enzymes that digest lactose in the absence of lactose. When lactose is present, it is taken up and partly converted to allolactose, which then binds to the repressor, preventing it from binding to the lac operator, allowing transcription of the lac genes.
Inducible vs. repressible systems
Gene regulation can be summarized as how they respond:
- Inducible systems are off unless there is the presence of some molecule (called an inducer) that allows for gene expression. The molecule is said to "induce expression".
- Repressible systems are on except in the presence of some molecule (called a corepressor) that suppresses gene expression. The molecule is said to "repress expression".
The manner in which both systems control expression is dependent on the control mechanisms themselves as well as differences between prokaryotic and eukaryotic cells.
Regulation of transcription machinery
In order for a gene to be expressed, several things must happen. First, there needs to be an initiating signal. This is achieved through the binding of some ligand to a receptor. Activation of g-protein-coupled receptors can have this effect; as can the binding of hormones to intra- or extracellular receptors.
This signal gives rise to the activation of a protein called a transcription factor, and recruits other members of the "transcription machine". Transcription factors generally simultaneously bind DNA as well as an RNA polymerase, as well as other agents necessary for the transcription process (HATs, scaffolding proteins, etc.). Transcription factors, and their cofactors, can be regulated through reversible structural alterations such as phosphorylation or inactivated through such mechanisms as proteolysis.
Transcription is initiated at the promoter site, as an increase in the amount of an active transcription factor binds a target DNA sequence. Other proteins, known as "scaffolding proteins" bind other cofactors and hold them in place. DNA sequences far from the point of initiation, known as enhancers, can aid in the assembly of this "transcription machinery." Histone arms are acetylated, and DNA is transcribed into RNA.
Frequently, extracellular signals induce the expression of immediate early genes (IEGs) such as c-fos, c-jun, or AP-1. These are in and of themselves transcription factors or components thereof, and can further influence gene expression.
Theories of transcriptional regulation
- Michael Carey; Stephen T. Smale (15 June 2001). Transcriptional Regulation in Eukaryotes: Concepts, Strategies, and Techniques. CSHL Press. ISBN 978-0-87969-635-1. Retrieved 25 December 2010.
- Choudhuri S (2004). "Gene Regulation and Molecular Toxicology". Toxicology Mechanisms and Methods 15 (1): 1–23. doi:10.1080/15376520590890686. PMID 20021075.
- Britten R. and E.H. Davidson (1969). "Gene regulation for higher cells: a theory". Science 165 (3891): 349–57. doi:10.1126/science.165.3891.349. PMID 5789433.