Gene expression

Gene expression (also protein expression or often simply expression) is the process by which a gene's information is converted into the structures and functions of a cell.

Gene expression is a multi-step process that begins with transcription and translation and is followed by folding, post-translational modification and targeting. The amount of protein that a cell expresses depends on the tissue, the developmental stage of the organism and the metabolic or physiologic state of the cell.

Measurement

Indirectly, the expression of particular genes may be assessed with DNA microarray technology, which can provide a rough measure of the cellular concentration of different mRNAs; 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. (The expression of many genes is known to be regulated after transcription, so an increase in mRNA concentration need not always increase expression.) A more sensitive and more accurate method of relative gene expression measurement is Real-Time PCR. With carefully constructed standard curve it can even produce an absolute measurement (e.g., in number of copies of mRNA per nanolitre of homogenized tissue, or in number of copies of mRNA per total poly-A RNA).

Control of expression

Control of gene expression depends various factors including:

chromosomal activation or deactivation.
Control of initiation of transcription
Processing of RNA (ie. splicing)
Control of RNA transport.
Control of mRNA degradation.
Control of initiation of translation (only in eukaryotes)
Post-translational modifications.

Regulating transcription

Transcription of a gene by RNA polymerase can be regulated by at least three types proteins:

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.
Repressors bind to non-coding sequences on DNA strand impeding RNA polymerase's progress along the strand, thus impeding the expression of the gene.
Activators enhance the interaction between RNA polymerase and a certain promoter, encouraging the expression of the gene.

In prokaryotes, repressors bind to regions called operators that are generally located near the promoter.

Examples:

When E. coli bacteria are subjected to heat stress, the Ã 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 cell 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 genes that help synthesize tryptophan, preventing their expression and thus suspending production. This is a form of negative feedback.
In bacteria, the lac repressor protein blocks the synthesis of enzymes that digest lactose when there is no lactose to feed on. When lactose is present, it binds to the repressor, causing it to detach from the DNA strand.

Overexpression

The protein encoded for by a gene can be expressed in increased quantity. This can come about by:

increasing the number of copies of the gene
increasing the binding strength of the promoter region

Often, the DNA sequence for a protein of interest will be spliced into a plasmid containing the lac promoter and used for transformation of bacteria. Addition of IPTG (a lactose analog) causes the bacteria to produce (express)the protein of interest. It doesn't work with every protein (sometimes yeast do a better job with post-translational modifications), but bacterial expression can sometimes be used to make a lot of protein with minimal fuss, for example for X-ray crystallography or NMR structure determination.

Gene networks and expression

Main article: Gene regulatory network

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, these and the operation of these functions have been interpreted as performing a kind information processing within cell and determine cellular behaviour.

External links

NCBIs Gene Expression Omnibus (http://www.ncbi.nlm.nih.gov/geo/)
Genes and Gene Expression - The Virtual Library of Biochemistry and Cell Biology (http://www.biochemweb.org/genes.shtml)