A European Informational Website
learn more
Gene expression, or simply expression, is the process by which a gene's DNA sequence is converted into functional proteins.
Any step of gene expression may be modulated, from the transcription step to post-translational modification of a protein. Gene regulation gives the cell control over structure and function, and is the basis for cellular differentiation, morphogenesis and the versatility and adaptability of any organism. Gene regulation may also serve as a susbstrate for evolutionary change, since control of the timing, location, and amount of gene expression can have a profound effect on the functions (actions) the gene in the organism.
Non-protein coding genes (e.g. rRNA genes, tRNA genes) are not translated into protein.
The expression of many genes is known to be regulated after transcription, so an increase in mRNA concentration need not always increase expression. Nevertheless, the expression of many genes at once may be assessed with DNA microarray technology or "tag based" technologies like SAGE or SuperSAGE, which can provide a relative measure of the cellular concentration of different messenger RNAs; often thousands at a time. While the name of this type of assessment is actually a misnomer, it is often referred to as expression profiling. Such expression profiles may be an indicator of response of the cells to certain exposures or events. A more sensitive and more accurate method of assessing the relative expression of individual genes is real-time polymerase chain reaction or RT-PCR. With a carefully constructed standard curve RT-PCR can produce an absolute measurement such as in number of copies of mRNA per nanolitre of homogenized tissue, or in number of copies of mRNA per total poly-adenosine RNA. Protein expression levels can be estimated by a number of means. One method involves fusing the gene sequence of the desired protein to that of another gene which can serve as a reporter protein, such as the green fluorescent protein or the enzyme beta-galactosidase. The expression level of these reporter proteins can be directly quantified using standard techniques. This technique is very powerful, but may be limited by possible changes in the functional behaviour of the expressed fusion construct relative to the natural protein. Less sophisticated methods of measuring protein expression include "Western" blotting, immunoassay, and functional (e.g. biochemical) assays. The pattern of detection of a gene or gene product may be described using terms such as facultative, constituative, circadian, cyclic, housekeeping, or inducible [1].
Regulation of gene expression is the cellular control of the amount and timing of appearance of the functional product of a gene. Any step of gene expression may be modulated, from the DNA-RNA transcription step to post-translational modification of a protein. Gene regulation gives the cell control over structure and function, and is the basis for cellular differentiation, morphogenesis and the versatility and adaptability of any organism.
An expression system consists, minimally, of a source of DNA and the molecular machinery required to transcribe the DNA into mRNA and translate the mRNA into protein using the nutrients and fuel provided. In the broadest sense, this includes every living cell capable of producing protein from DNA. However, an expression system more specifically refers to a laboratory tool, often artificial in some manner, used for assembling the product of a specific gene or genes. It is defined as the "combination of an expression vector, its cloned dna, and the host for the vector that provide a context to allow foreign gene function in a host cell, that is, produce proteins at a high level" [2][3].
In addition to these biological tools, certain naturally observed configurations of DNA (genes, promoters, enhancers, repressors) and the associated machinery itself are reffered to as an expression system, as in the simple repressor 'switch' expression system in Lambda phage. It is these natural expression systems that inspire artificial expression systems, (such as the Tet-on and Tet-off expression systems).
Each expression system has distinct advantages and liabilities, and may be named after the host, the DNA source or the delivery mechanism for the genetic material. For example, common expression systems include bacteria (such as E.coli), yeast (such as S.cerevisiae), plasmid, artificial chromosomes, phage (such as lambda), cell lines, or virus (such as baculovirus, retrovirus, adenovirus).
In the laboratory, the protein encoded by a gene is sometimes expressed in increased quantity. This can come about by increasing the number of copies of the gene or increasing the binding strength of the promoter region.
Often, the DNA sequence for a protein of interest will be cloned or subcloned into a plasmid containing the lac promoter, which is then transformed into the bacterium Escherichia coli. Addition of IPTG (a lactose analog) causes the bacteria to express the protein of interest. However, this strategy does not always yield functional protein, in which case, other organisms or tissue cultures may be more effective. As for example the yeast, Saccharomyces cerevisiae, is often preferred to bacteria for proteins that undergo extensive Posttranslational modification. Nonetheless, bacterial expression has the advantage of easily producing large amounts of protein, which is required for X-ray crystallography or nuclear magnetic resonance experiments for structure determination.
Genes have sometimes been regarded as nodes in a network, with inputs being proteins such as transcription factors, and outputs being the level of gene expression. The node itself performs a function, and the operation of these functions have been interpreted as performing a kind of information processing within cell and determine cellular behaviour.